View source for Biology:Arthropod

{{short description|Phylum of invertebrates with jointed exoskeletons}}
{{Automatic taxobox
| name = Arthropoda
| fossil_range = {{Fossil range|538.8 |0|earliest=555}}<small>[[Earth:Cambrian|Earliest Cambrian]] ([[Earth:Fortunian|Fortunian]])–Recent</small>
| image = <imagemap>
File:Arthropoda collage.png|300px
rect 1 1 400 300 [[Biology:Anomalocaris|Anomalocaris]]
rect 400 1 800 300 [[Biology:Atlantic horseshoe crab|Atlantic horseshoe crab]]
rect 800 1 1200 300 [[Biology:Penaeus monodon|Penaeus monodon]]

rect 1 300 400 600 [[Biology:Isoxys|Isoxys]]
rect 400 300 800 600 [[Biology:Araneus diadematus|Araneus diadematus]]
rect 800 300 1200 600 Chelonibia testudinaria

rect 1 600 400 900 [[Biology:Leanchoilia|Leanchoilia]]
rect 400 600 800 900 [[Biology:Scolopendra cataracta|Scolopendra cataracta]] 
rect 800 600 1200 900 [[Biology:Dicyrtomina|Dicyrtomina]]

rect 1 900 400 1200 [[Biology:Elrathia|Elrathia]]
rect 400 900 800 1200 [[Biology:Juliformia|Juliformia]]
rect 800 900 1200 1200 Carniolan honey bee
</imagemap>
| display_parents = 8
| taxon = Arthropoda
| authority = Gravenhorst, 1843<ref name="Martínez-Muñoz">{{cite journal | last1=Martínez-Muñoz | first1=Carlos A. | title=The correct authorship of Arthropoda—A reappraisal | journal=Integrative Systematics | volume=6 |issue=1 |pages=1–8 | date=2023-05-04 | issn=2628-2380 | doi=10.18476/2023.472723 | s2cid=258497632 | doi-access=free}}</ref><ref>{{cite book |last=Gravenhorst |first=J. L. C. |year=1843 |title=Vergleichende Zoologie |location=Breslau |publisher=Druck und Verlag von Graß, Barth und Comp |url=https://www.biodiversitylibrary.org/item/120240#page/24/mode/1up}}</ref>
| subdivision_ranks = Subphyla, unplaced genera, and classes
| subdivision = {{hidden|title=|content=
*†'''"[[Biology:Dinocaridida|Dinocaridida]]"''' (paraphyletic, sometimes treated as [[Biology:Lobopodia|lobopodia]]ns)
**†[[Biology:Kerygmachelidae|Kerygmachelidae]]
**†''[[Biology:Pambdelurion|Pambdelurion]]'' (Sometimes treated as a lobopodian)
**†''[[Biology:Mieridduryn|Mieridduryn]]'' (Sometimes treated as an opabiniid)
**†''[[Biology:Parvibellus|Parvibellus]]'' (Might actually be a [[Biology:Lobopodia#Siberion and similar taxa|"Siberiid Lobopodian"]]
**†[[Biology:Opabiniidae|Opabiniidae]]
**†[[Biology:Radiodonta|Radiodonta]] (e.g. ''[[Biology:Anomalocaris|Anomalocaris]]'')
**†''[[Biology:Cucumericrus|Cucumericrus]]'' (Sometimes treated as a radiodont)
**†''[[Biology:Caryosyntrips|Caryosyntrips]]'' (Sometimes treated as a radiodont)
*'''[[Biology:Deuteropoda|Deuteropoda]]''' 
*†[[Biology:Isoxyida|Isoxyida]]<ref name=Kylinxia/>
*†''[[Biology:Erratus|Erratus]]''<ref>{{Cite journal |last1=Fu |first1=D. |last2=Legg |first2=D. A. |last3=Daley |first3=A. C. |last4=Budd |first4=G. E. |last5=Wu |first5=Y. |last6=Zhang |first6=X. |year=2022 |title=The evolution of biramous appendages revealed by a carapace-bearing Cambrian arthropod |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |volume=377 |issue=1847 |pages=Article ID 20210034 |doi=10.1098/rstb.2021.0034 |pmid=35125000|pmc=8819368}}</ref>
*†''[[Biology:Fengzhengia|Fengzhengia]]''<ref>{{Cite journal |last1=O'Flynn |first1=Robert J. |last2=Williams |first2=Mark |last3=Yu |first3=Mengxiao |last4=Harvey |first4=Thomas |last5=Liu |first5=Yu |date=2022 |title=A new euarthropod with large frontal appendages from the early Cambrian Chengjiang biota |url=https://palaeo-electronica.org/content/2022/3551-a-new-chengjiang-euarthropod |journal=Palaeontologia Electronica |volume=25 |issue=1 |pages=1–21 |doi=10.26879/1167|s2cid=246779634 |doi-access=free}}</ref>
*†''[[Biology:Kylinxia|Kylinxia]]''<ref name=Kylinxia/>
* '''Class †[[Biology:Megacheira|Megacheira]]''' (possibly paraphyletic)
*†''[[Biology:Kiisortoqia|Kiisortoqia]]''
*†''[[Biology:Bushizheia|Bushizheia]]''
*†[[Biology:Fuxianhuiida|Fuxianhuiida]]
*†[[Biology:Bradoriida|Bradoriida]]
* '''Class †[[Biology:Artiopoda|Artiopoda]]'''
** †[[Biology:Trilobita|Trilobita]]&nbsp;– [[Biology:Trilobite|trilobite]]s
** †[[Biology:Agnostida|Agnostida]] (uncertain if they are trilobites)<ref>{{cite journal | vauthors = Moysiuk J, Caron JB | title = Burgess Shale fossils shed light on the agnostid problem | journal = Proceedings. Biological Sciences | volume = 286 | issue = 1894 | pages = 20182314 | date = January 2019 | pmid = 30963877 | pmc = 6367181 | doi = 10.1098/rspb.2018.2314}}</ref>
** †[[Biology:Nektaspida|Nektaspida]]
** †[[Biology:Aglaspidida|Aglaspidida]]
** †[[Biology:Cheloniellida|Cheloniellida]]
* '''Subphylum [[Biology:Chelicerata|Chelicerata]]'''
** [[Biology:Pycnogonida|Pycnogonida]]&nbsp;– [[Biology:Sea spider|sea spider]]s
** †[[Biology:Megacheira|Megacheira]]?
** †[[Biology:Eurypterida|Eurypterida]]&nbsp;– [[Biology:Eurypterida|sea scorpion]]s
** †[[Biology:Chasmataspidida|Chasmataspidida]]
** [[Biology:Xiphosura|Xiphosura]]&nbsp;– [[Biology:Horseshoe crab|horseshoe crab]]s
** [[Biology:Arachnida|Arachnida]]&nbsp;– [[Biology:Mite|mite]]s, scorpions, [[Biology:Spider|spider]]s, etc.
* †[[Biology:Phosphatocopina|Phosphatocopina]] (stem mandibulate?)<ref>{{Cite journal |last1=Legg |first1=David A. |last2=Sutton |first2=Mark D. |last3=Edgecombe |first3=Gregory D. |date=2013-09-30 |title=Arthropod fossil data increase congruence of morphological and molecular phylogenies |journal=Nature Communications |language=en |volume=4 |issue=1 |pages=2485 |doi=10.1038/ncomms3485 |pmid=24077329 |bibcode=2013NatCo...4.2485L |issn=2041-1723|doi-access=free}}</ref>
* '''Clade [[Biology:Mandibulata|Mandibulata]]'''
** †[[Biology:Hymenocarina|Hymenocarina]]
** †[[Biology:Euthycarcinoidea|Euthycarcinoidea]]
** †[[Biology:Thylacocephala|Thylacocephala]]?<ref name=pulsipher-2022>{{Cite journal |last1=Pulsipher |first1=M. A. |last2=Anderson |first2=E. P. |last3=Wright |first3=L. S. |last4=Kluessendorf |first4=J. |last5=Mikulic |first5=D. G. |last6=Schiffbauer |first6=J. D. |year=2022 |title=Description of ''Acheronauta'' gen. nov., a possible mandibulate from the Silurian Waukesha Lagerstätte, Wisconsin, USA |journal=Journal of Systematic Palaeontology |volume=20 |issue=1 |at=2109216 |doi=10.1080/14772019.2022.2109216 |s2cid=252839113}}</ref>
** '''Subphylum Myriapoda'''
*** [[Biology:Symphyla|Symphyla]]&nbsp;– pseudocentipedes
*** [[Biology:Pauropoda|Pauropoda]]&nbsp;– pauropods
*** [[Biology:Diplopoda|Diplopoda]]&nbsp;– [[Biology:Millipede|millipede]]s
*** [[Biology:Chilopoda|Chilopoda]]&nbsp;– centipedes
** '''Subphylum [[Biology:Pancrustacea|Pancrustacea]]'''
*** '''Superclass [[Biology:Oligostraca|Oligostraca]]'''
**** [[Biology:Ostracoda|Ostracoda]]&nbsp;– seed shrimp
**** [[Biology:Mystacocarida|Mystacocarida]]
**** [[Biology:Branchiura|Branchiura]]&nbsp;– fish lice
**** [[Biology:Pentastomida|Pentastomida]]&nbsp;– tongue worms
*** '''Superclass [[Biology:Multicrustacea|Multicrustacea]]'''
**** †[[Biology:Cyclida|Cyclida]]<ref name=Clark-2020>{{cite journal |url=https://eprints.gla.ac.uk/210399/7/210399.pdf |title=Redescription of ''Americlus rankini'' (Woodward, 1868) (Pancrustacea: Cyclida: Americlidae) and interpretation of its systematic placement, morphology, and paleoecology |first1=Neil D L |last1=Clark |first2=Rodney M |last2=Feldmann |first3=Frederick R |last3=Schram |first4=Carrie E |last4=Schweitzer |journal=Journal of Crustacean Biology |volume=40 |issue=2 |date=2020 |pages=181–193 |doi=10.1093/jcbiol/ruaa001}}</ref>
**** [[Biology:Thecostraca|Thecostraca]]&nbsp;– [[Biology:Barnacle|barnacle]]s
**** Copepoda &nbsp;– [[Biology:Copepod|copepod]]s
**** [[Biology:Malacostraca|Malacostraca]]&nbsp;– isopods, amphipods, [[Biology:Decapoda|decapod]]s, [[Biology:Krill|krill]], etc.
*** '''Clade Allotriocarida'''
**** [[Biology:Cephalocarida|Cephalocarida]]&nbsp;– horseshoe shrimp
**** [[Biology:Branchiopoda|Branchiopoda]]&nbsp;– [[Biology:Brine shrimp|brine shrimp]], tadpole shrimp, water fleas, etc.
**** [[Biology:Remipedia|Remipedia]]&nbsp;– remipedes
**** [[Biology:Hexapoda|Hexapoda]]&nbsp;– [[Biology:Insect|insect]]s, etc.
* ''[[Biology:Incertae sedis|Incertae sedis]]''
** †''[[Biology:Aaveqaspis|Aaveqaspis]]''<ref>{{cite journal |last1=Peel |first1=J.S. |last2=Stein |first2=M. |title=A new Arthropod from the Lower Cambrian Sirius Passet Fossil-Lagerstätten of North Greenland |url=http://www.geology.cz/bulletin/contents/2009/vol84no4/1158_peel.pdf |journal=Bulletin of Geosciences |volume=84 |issue=4 |page=1158}}</ref>
** †''[[Biology:Cambropachycope|Cambropachycope]]''<ref>{{Cite journal |last=Edgecombe |first=Gregory D. |date=2017-09-01 |title=Inferring Arthropod Phylogeny: Fossils and their Interaction with Other Data Sources |journal=Integrative and Comparative Biology |volume=57 |issue=3 |pages=467–476 |doi=10.1093/icb/icx061 |pmid=28957518 |issn=1540-7063|doi-access=free}}</ref>
** †''[[Biology:Camptophyllia|Camptophyllia]]''<ref>{{citation |last1=Garwood |first1=R. |last2=Sutton |first2=M. |date=18 February 2012 |title=The enigmatic arthropod Camptophyllia |journal=Palaeontologia Electronica |doi=10.1111/1475-4983.00174 |doi-access=free |volume=15 |issue=2 |page=12 |url=http://palaeo-electronica.org/content/pdfs/268.pdf/ |url-status=live |access-date=11 June 2012 |archive-url=https://web.archive.org/web/20131202222600/http://palaeo-electronica.org/content/pdfs/268.pdf |archive-date=2 December 2013}}</ref>
**†''[[Biology:Chuandianella|Chuandianella]]''<ref>{{Cite journal |last1=Zhai |first1=Dayou |last2=Williams |first2=Mark |last3=Siveter |first3=David J. |last4=Siveter |first4=Derek J. |last5=Harvey |first5=Thomas H. P. |last6=Sansom |first6=Robert S. |last7=Mai |first7=Huijuan |last8=Zhou |first8=Runqing |last9=Hou |first9=Xianguang |date=2022-02-22 |title=Chuandianella ovata: An early Cambrian stem euarthropod with feather-like appendages |url=https://palaeo-electronica.org/content/2022/3557-chuandianella-ovata |journal=Palaeontologia Electronica |language=English |volume=25 |issue=1 |pages=1–22 |doi=10.26879/1172 |s2cid=247123967 |issn=1094-8074|doi-access=free}}</ref>
** †''[[Biology:Goticaris|Goticaris]]''<ref name=":0">{{Cite journal |last1=Waloszek |first1=Dieter |last2=Müller |first2=Klaus |date=1990-10-01 |title=Upper Cambrian stem-lineage crustaceans and their bearing upon the monophyly of Crustacea and the position of Agnostus |journal=Lethaia |volume=23 |pages=409–427 |doi=10.1111/j.1502-3931.1990.tb01373.x |url=https://www.researchgate.net/publication/280025168}}</ref>
** †[[Biology:Marrellomorpha|Marrellomorpha]]
**†''[[Biology:Parioscorpio|Parioscorpio]]''<ref>{{Cite journal |last1=Van Roy |first1=Peter |last2=Rak |first2=Štěpán |last3=Budil |first3=Petr |last4=Fatka |first4=Oldřich |date=2022-06-13 |title=Redescription of the cheloniellid euarthropod ''Triopus draboviensis'' from the Upper Ordovician of Bohemia, with comments on the affinities of ''Parioscorpio venator'' |journal=Geological Magazine |volume=159 |issue=9 |issn=0016-7568 |doi=10.1017/s0016756822000292 |hdl=1854/LU-8756253 |s2cid=249652930 |pages=1471–1489 |bibcode=2022GeoM..159.1471V}}</ref>
**†''[[Biology:Sarotrocercus|Sarotrocercus]]''<ref>{{Cite journal |last1=Haug |first1=J.T. |last2=Maas |first2=A. |last3=Haug |first3=C. |last4=Waloszek |first4=D. |date=2011-11-01 |title=Sarotrocercus oblitus - Small arthropod with great impact on the understanding of arthropod evolution? |journal=Bulletin of Geosciences |pages=725–736 |doi=10.3140/bull.geosci.1283 |issn=1802-8225|doi-access=free}}</ref>
** †[[Biology:Strabopidae|Strabopida]]<ref>{{Cite journal |last1=Ortega-Hernández |first1=Javier |last2=Legg |first2=David A. |last3=Braddy |first3=Simon J. |date=2013 |title=The phylogeny of aglaspidid arthropods and the internal relationships within Artiopoda |journal=Cladistics |language=en |volume=29 |issue=1 |pages=15–45 |doi=10.1111/j.1096-0031.2012.00413.x |doi-access=free |pmid=34814371 |s2cid=85744103}}</ref>
**†''Wingertshellicus''<ref>{{cite journal |last1=Kühl |first1=Gabrielle |last2=Rust |first2=Jes |year=2009 |title=''Devonohexapodus bocksbergensis'' is a synonym of ''Wingertshellicus backesi'' (Euarthropoda) – no evidence for marine hexapods living in the Devonian Hunsrück Sea |journal=Organisms Diversity & Evolution |doi=10.1016/j.ode.2009.03.002 |volume=9 |issue=3 |pages=215–231|doi-access=free}}</ref>
}}
| diversity = around 1,170,000 species.
| synonyms = Condylipoda <small>Latreille, 1802</small>
}}

'''Arthropods''' ({{IPAc-en|ˈ|ɑːr|θ|r|ə|p|ɒ|d}}, {{ety|grc|''ἄρθρον'' (arthron)|joint||''πούς'' (pous)|foot}} (gen. ποδός)) are [[Biology:Invertebrate|invertebrate]]s in the [[Biology:Phylum|phylum]] '''Arthropoda'''. They possess an [[Biology:Exoskeleton|exoskeleton]] with a [[Biology:Arthropod cuticle|cuticle]] made of [[Chemistry:Chitin|chitin]], often [[Biology:Mineralization|mineralised]] with [[Chemistry:Calcium carbonate|calcium carbonate]], a body with differentiated ([[Biology:Metamerism|metameric]]) [[Biology:Segmentation|segments]], and paired jointed [[Biology:Appendage|appendage]]s. In order to keep growing, they must go through stages of [[Biology:Moulting|moulting]], a process by which they shed their [[Biology:Exoskeleton|exoskeleton]] to reveal a new one. They are an extremely diverse group, with up to 10{{nbsp}}million species.

Haemolymph is the analogue of [[Biology:Blood|blood]] for most arthropods.  An arthropod has an open circulatory system, with a body cavity called a haemocoel through which haemolymph circulates to the interior [[Biology:Organ (anatomy)|organs]]. Like their exteriors, the internal organs of arthropods are generally built of repeated segments. Their [[Biology:Nervous system|nervous system]] is "ladder-like", with paired [[Biology:Anatomical terms of location#Dorsal and ventral|ventral]] [[Biology:Ventral nerve cord|nerve cords]] running through all segments and forming paired ganglia in each segment. Their heads are formed by fusion of varying numbers of segments, and their [[Biology:Brain|brain]]s are formed by fusion of the ganglia of these segments and encircle the [[Biology:Esophagus|esophagus]]. The respiratory and [[Biology:Excretion|excretory]] systems of arthropods vary, depending as much on their environment as on the [[Biology:Subphylum|subphylum]] to which they belong.

Arthropods use combinations of [[Biology:Compound eye|compound eye]]s and [[Biology:Simple eye in invertebrates|pigment-pit]] ocelli for vision. In most species, the ocelli can only detect the direction from which light is coming, and the compound eyes are the main source of information, but the main eyes of [[Biology:Spider|spider]]s are ocelli that can form images and, in a few cases, can swivel to track prey. Arthropods also have a wide range of chemical and mechanical sensors, mostly based on modifications of the many bristles known as [[Biology:Seta|seta]]e that project through their cuticles. Similarly, their reproduction and development are varied; all [[Biology:Terrestrial animal|terrestrial]] species use internal fertilization, but this is sometimes by indirect transfer of the [[Biology:Sperm|sperm]] via an appendage or the ground, rather than by direct injection. Aquatic species use either internal or [[Biology:External fertilization|external fertilization]]. Almost all arthropods lay eggs, with many species giving birth to live young after the eggs have hatched inside the mother; but a few are genuinely viviparous, such as [[Biology:Aphid|aphid]]s. Arthropod hatchlings vary from miniature adults to grubs and [[Biology:Caterpillar|caterpillar]]s that lack jointed limbs and eventually undergo a total [[Biology:Metamorphosis|metamorphosis]] to produce the adult form. The level of maternal care for hatchlings varies from nonexistent to the prolonged care provided by [[Biology:Eusociality#In insects|social insects]].

The evolutionary ancestry of arthropods dates back to the [[Earth:Cambrian|Cambrian]] period. The group is generally regarded as [[Biology:Monophyletic|monophyletic]], and many analyses support the placement of arthropods with [[Biology:Cycloneuralia|cycloneuralia]]ns (or their constituent clades) in a superphylum [[Biology:Ecdysozoa|Ecdysozoa]]. Overall, however, the basal relationships of animals are not yet well resolved. Likewise, the relationships between various arthropod groups are still actively debated. Today, arthropods contribute to the human food supply both directly as food, and more importantly, indirectly as [[Biology:Pollination|pollinators]] of crops. Some species are known to spread severe disease to humans, livestock, and crops.

== Etymology ==

The word ''arthropod'' comes from the [[Social:Greek language|Greek]] {{wikt-lang|grc|ἄρθρον}} ''árthron'', "[[Biology:Joint|joint]]", and {{wikt-lang|grc|πούς}} ''pous'' (gen. ''podos (ποδός)''), i.e. "foot" or "[[Biology:Leg|leg]]", which together mean "jointed leg",<ref name=OnlineEtDict>{{cite web |title=Arthropoda |publisher=Online Etymology Dictionary |url=http://www.etymonline.com/index.php?term=Arthropoda&allowed_in_frame=0 |access-date=2013-05-23 |url-status=live |archive-url=https://web.archive.org/web/20130307075628/http://www.etymonline.com/index.php?term=Arthropoda&allowed_in_frame=0 |archive-date=2013-03-07}}</ref> with the word "arthropodes" initially used in anatomical descriptions by [[Biography:Barthélemy Charles Joseph Dumortier|Barthélemy Charles Joseph Dumortier]] published in 1832.<ref name="Martínez-Muñoz" /> The designation "Arthropoda" appears to have been first used in 1843 by the German zoologist Johann Ludwig Christian Gravenhorst (1777–1857).<ref>{{cite book |last1=Gravenhorst |first1=J. L. C. |date=1843 |title=Vergleichende Zoologie |language=de |trans-title=Comparative Zoology |publisher=Graß, Barth & Comp. |location=Breslau, (Prussia) |page=foldout |url=https://www.biodiversitylibrary.org/page/38963736 |quote=''"Mit gegliederten Bewegungsorganen"'' (with articulated movement organs)}}</ref><ref name="Martínez-Muñoz" /> The origin of the name has been the subject of considerable confusion, with credit often given erroneously to Pierre André Latreille or Karl Theodor Ernst von Siebold instead, among various others.<ref name="Martínez-Muñoz"/> 

In common parlance, terrestrial arthropods are often called bugs.{{refn|group=Note|The Museum of New Zealand notes that "in everyday conversation", ''bug'' "refers to land arthropods with at least six legs, such as insects, spiders, and centipedes".<ref>[https://www.tepapa.govt.nz/discover-collections/read-watch-play/science/your-bug-questions-answered/what-bug-insects-arachnids "What is a bug? Insects, arachnids, and myriapods"] at Museum of New Zealand Te Papa Tongarewa website. Accessed 10 March 2022.</ref>  In a chapter on "Bugs That Are Not Insects", entomologist Gilbert Walbauer specifies centipedes, millipedes, arachnids (spiders, [[Biology:Opiliones|daddy longlegs]], scorpions, [[Biology:Mite|mite]]s, chiggers and ticks) as well as the few terrestrial crustaceans (sowbugs and pillbugs),<ref name="Walbauer5">Gilbert Waldbauer. ''The Handy Bug Answer Book.'' Visible Ink, 1998. [https://archive.org/details/handybuganswerbo00wald/page/5/mode/2up pp. 5–26.] {{ISBN|978-1-57859-049-0}}</ref> but argues that "including legless creatures such as worms, slugs, and snails among the bugs stretches the word too much".<ref name="Walbauer1">Gilbert Waldbauer. ''The Handy Bug Answer Book.'' Visible Ink, 1998. [https://archive.org/details/handybuganswerbo00wald/page/1/mode/2up p. 1.] {{ISBN|978-1-57859-049-0}}</ref>}}  The term is also occasionally extended to colloquial names for freshwater or marine crustaceans (e.g., [[Biology:Balmain bug|Balmain bug]], [[Biology:Moreton Bay bug|Moreton Bay bug]], mudbug) and used by physicians and bacteriologists for disease-causing germs (e.g., [[Biology:Antimicrobial resistance|superbug]]s),<ref name="Walbauer1"/> but entomologists reserve this term for a narrow category of "true bugs", insects of the order [[Biology:Hemiptera|Hemiptera]].<ref name="Walbauer1"/>

== Description ==

Arthropods are [[Biology:Invertebrate|invertebrate]]s with [[Biology:Segmentation|segmented]] bodies and jointed limbs.<ref>{{citation |last=Valentine |first=J. W. |year=2004 |title=On the Origin of Phyla |publisher=[[Organization:University of Chicago Press|University of Chicago Press]] |isbn=978-0-226-84548-7 |page=33 |url=https://books.google.com/books?id=DMBkmHm5fe4C&q=arthropod+synapomorphy}}</ref> The [[Biology:Exoskeleton|exoskeleton]] or [[Biology:Arthropod cuticle|cuticle]]s consists of [[Chemistry:Chitin|chitin]], a polymer of [[Chemistry:N-Acetylglucosamine|N-Acetylglucosamine]].<ref>{{citation |last=Cutler |first=B. |date=August 1980 |title=Arthropod cuticle features and arthropod monophyly |journal=[[Biology:Cellular and Molecular Life Sciences|Cellular and Molecular Life Sciences]] |doi=10.1007/BF01953812 |s2cid=84995596 |volume=36 |issue=8 |page=953}}</ref> The cuticle of many crustaceans, [[Biology:Oribatida|beetle mites]], the clades Penetini and Archaeoglenini inside the beetle subfamily [[Biology:Phrenapatinae|Phrenapatinae]],<ref>[https://books.google.com/books?id=hHW7DwAAQBAJ&dq=Penetini+Archaeoglenini+calcium+carbonate&pg=PT1017 Australian Beetles Volume 2: Archostemata, Myxophaga, Adephaga, Polyphaga]</ref> and millipedes (except for [[Biology:Polyxenida|bristly millipedes]]) is also [[Earth:Biomineralization|biomineralized]] with [[Chemistry:Calcium carbonate|calcium carbonate]]. Calcification of the endosternite, an internal structure used for muscle attachments, also occur in some [[Biology:Opiliones|opiliones]],<ref>{{cite journal |last=Kovoor |first=J. |date=1978 |title=Natural calcification of the prosomatic endosternite in the Phalangiidae (Arachnida:Opiliones). |journal=Calcified Tissue Research |pmid=750069 |doi=10.1007/BF02013269 |s2cid=23119386|volume=26 |issue=3 |pages=267–9}}</ref> and the pupal cuticle of the fly ''[[Biology:Bactrocera dorsalis|Bactrocera dorsalis]]'' contains calcium phosphate.<ref>[https://www.sciencedirect.com/science/article/abs/pii/S0022191019301623 Amorphous calcium phosphate in the pupal cuticle of Bactrocera dorsalis Hendel (Diptera: Tephritidae): A new discovery for reconsidering the mineralization of the insect cuticle]</ref>

=== Diversity ===

[[File:Protaetic cuprea ignicollis 2023-03-22 IZE-066 zoom.jpg|thumb|''[[Biology:Protaetia cuprea|Protaetia cuprea]]'' (copper chafer). Beetles are the largest and most diverse order of arthropods.]]

Arthropoda is the largest animal [[Biology:Phylum|phylum]] with the estimates of the number of arthropod species varying from 1,170,000 to 5 to 10&nbsp;million and accounting for over 80 per cent of all known living animal species.<ref>{{citation |last=Thanukos |first=Anna |title=The Arthropod Story |publisher=[[Organization:University of California, Berkeley|University of California, Berkeley]] |url=http://evolution.berkeley.edu/evolibrary/article/arthropodstory |access-date=2008-09-29 |url-status=live |archive-url=https://web.archive.org/web/20080616122532/http://evolution.berkeley.edu/evolibrary/article/arthropodstory |archive-date=2008-06-16}}</ref><ref>{{Citation |last=Ødegaard |first=Frode |date=December 2000 |title=How many species of arthropods? Erwin's estimate revised |journal=[[Organization:Biological Journal of the Linnean Society|Biological Journal of the Linnean Society]] |doi=10.1006/bijl.2000.0468 |doi-access=free |volume=71 |issue=4 |pages=583–597 |url=http://si-pddr.si.edu/dspace/bitstream/10088/1315/1/Odegaard_2000.pdf |access-date=2010-05-06 |url-status=live |archive-url=https://web.archive.org/web/20101226022738/http://si-pddr.si.edu/dspace/bitstream/10088/1315/1/Odegaard_2000.pdf |archive-date=2010-12-26}}</ref> One arthropod [[Subgroup|sub-group]], the [[Biology:Insect|insect]]s, includes more [[Biology:Species description|described species]] than any other [[Biology:Class|taxonomic class]].<ref name="Thompson1994CoevolutionaryProcess">{{citation |last=Thompson |first=J. N. |year=1994 |title=The Coevolutionary Process |publisher=[[Organization:University of Chicago Press|University of Chicago Press]] |isbn=978-0-226-79760-1 |page=9 |url=https://books.google.com/books?id=AyXPQzEwqPIC&q=arthropod+species+number&pg=PA9}}</ref> The total number of species remains difficult to determine. This is due to the census modeling assumptions projected onto other regions in order to scale up from counts at specific locations applied to the whole world. A study in 1992 estimated that there were 500,000 species of animals and plants in Costa Rica alone, of which 365,000 were arthropods.<ref name="Thompson1994CoevolutionaryProcess"/>

They are important members of marine, freshwater, land and air [[Earth:Ecosystem|ecosystem]]s and one of only two major animal groups that have adapted to life in dry environments; the other is [[Biology:Amniote|amniote]]s, whose living members are reptiles, birds and mammals.<ref name="RuppertFoxBarnes2004P518">Ruppert, Fox & Barnes (2004), pp. 518–522</ref> Both the smallest and largest arthropods are [[Biology:Crustacean|crustacean]]s. The smallest belong to the class [[Biology:Tantulocarida|Tantulocarida]], some of which are less than {{convert|100|um}} long.<ref>{{cite journal |title=Tantulocarida (Crustacea) of the Southern Ocean deep sea, and the description of three new species of ''Tantulacus'' Huys, Andersen & Kristensen, 1992|author=Inga Mohrbeck |author2=Pedro Martínez Arbizu |author3=Thomas Glatzel |journal=Systematic Parasitology |volume=77 |issue=2 |date=October 2010 |pages=131–151 |doi=10.1007/s11230-010-9260-0|pmid=20852984 |s2cid=7325858}}</ref> The largest are species in the class [[Biology:Malacostraca|Malacostraca]], with the legs of the [[Biology:Japanese spider crab|Japanese spider crab]] potentially spanning up to {{convert|4|m|ft}}<ref name="Schmidt-Nielsen1984InsectSize">{{citation |last=Schmidt-Nielsen |first=Knut |year=1984 |title=Scaling: Why is Animal Size So Important? |chapter=The strength of bones and skeletons |publisher={{wipe|Cambridge University Press}} |isbn=978-0-521-31987-4 |pages=[https://archive.org/details/scalingwhyisanim0000schm/page/42 42–55] |url=https://archive.org/details/scalingwhyisanim0000schm/page/42 |chapter-url=https://books.google.com/books?id=8WkjD3L_avQC&q=arthropod+size+range&pg=PA53}}</ref> and the American lobster reaching weights over 20&nbsp;kg (44&nbsp;lbs).

=== Segmentation ===

{{Annotated image |float=left| image=HemimerusHanseni.jpg |caption=Segments and [[Biology:Tagma|tagmata]] of an arthropod<ref name="RuppertFoxBarnes2004P518" /> |width=190 |image-width=80 |height=180
| annotations=
{{Annotation|95|5|Head}}
{{Annotation|90|17|_______________________|color=green}}
{{Annotation|95|37|[[Biology:Thorax|Thorax]]}}
{{Annotation|90|50|_______________________|color=black}}
{{Annotation|95|80|[[Biology:Abdomen|Abdomen]]}}
{{Annotation|90|100|_______________________|color=green}}
}}

[[File:20211214 arthropod biramous arthropodized appendage morphology en.png|thumb|upright=1.35|Structure of a biramous appendage.<ref name=":0a">{{Cite journal |last1=Liu |first1=Yu |last2=Edgecombe |first2=Gregory D. |last3=Schmidt |first3=Michel |last4=Bond |first4=Andrew D. |last5=Melzer |first5=Roland R. |last6=Zhai |first6=Dayou |last7=Mai |first7=Huijuan |last8=Zhang |first8=Maoyin |last9=Hou |first9=Xianguang |date=2021-07-30 |title=Exites in Cambrian arthropods and homology of arthropod limb branches |journal=Nature Communications |language=en |volume=12 |issue=1 |pages=4619 |doi=10.1038/s41467-021-24918-8 |pmid=34330912 |pmc=8324779 |bibcode=2021NatCo..12.4619L |issn=2041-1723}}</ref>]]

The [[Biology:Embryo|embryo]]s of all arthropods are segmented, built from a series of repeated modules. The [[Biology:Most recent common ancestor|last common ancestor]] of living arthropods probably consisted of a series of undifferentiated segments, each with a pair of appendages that functioned as limbs. However, all known living and fossil arthropods have grouped segments into [[Biology:Tagma|tagmata]] in which segments and their limbs are specialized in various ways.<ref name="RuppertFoxBarnes2004P518"/>

The three-part appearance of many [[Biology:Insect|insect]] bodies and the two-part appearance of [[Biology:Spider|spider]]s is a result of this grouping.{{sfnp|Gould|1990|pp=102–106}} There are no external signs of segmentation in [[Biology:Mite|mite]]s.<ref name="RuppertFoxBarnes2004P518" /> Arthropods also have two body elements that are not part of this serially repeated pattern of segments, an [[Biology:Somite|ocular somite]] at the front, where the mouth and eyes originated,<ref name="RuppertFoxBarnes2004P518" /><ref name="linkinghub.elsevier.com">{{Cite journal |last1=Ortega-Hernández |first1=Javier |last2=Janssen |first2=Ralf |last3=Budd |first3=Graham E. |date=2017 |title=Origin and evolution of the panarthropod head – A palaeobiological and developmental perspective |journal=Arthropod Structure & Development |volume=46 |issue=3 |pages=354–379 |doi=10.1016/j.asd.2016.10.011|pmid=27989966 |doi-access=free}}</ref> and a [[Biology:Telson|telson]] at the rear, behind the [[Biology:Anus|anus]].

Originally it seems that each appendage-bearing segment had two separate pairs of appendages: an upper, unsegmented exite and a lower, segmented endopod. These would later fuse into a single pair of biramous appendages united by a basal segment (protopod or basipod), with the upper branch acting as a [[Biology:Gill|gill]] while the lower branch was used for locomotion.<ref>{{cite web |url=http://news.yale.edu/2015/03/11/giant-sea-creature-hints-early-arthropod-evolution |title=Giant sea creature hints at early arthropod evolution |date=2015-03-11 |access-date=2017-01-22 |archive-url=https://web.archive.org/web/20170202020956/http://news.yale.edu/2015/03/11/giant-sea-creature-hints-early-arthropod-evolution |archive-date=2017-02-02|url-status=live}}</ref><ref name="fu2022" /><ref name=":0a" /> The appendages of most crustaceans and some extinct taxa such as trilobites have another segmented branch known as exopods, but whether these structures have a single origin remain controversial.<ref>{{cite journal |last1=Hejnol |first1=Andreas |last2=Scholtz |first2=Gerhard |date=2004-10-01 |title=Clonal analysis of Distal-less and engrailed expression patterns during early morphogenesis of uniramous and biramous crustacean limbs |journal=Development Genes and Evolution |language=en |volume=214 |issue=10 |pages=473–485 |doi=10.1007/s00427-004-0424-2 |pmid=15300435 |s2cid=22426697 |issn=1432-041X}}</ref><ref>{{Cite journal |last1=Wolff |first1=Carsten |last2=Scholtz |first2=Gerhard |date=2008-05-07 |title=The clonal composition of biramous and uniramous arthropod limbs |journal=Proceedings of the Royal Society B: Biological Sciences |volume=275 |issue=1638 |pages= 1023–1028 |doi=10.1098/rspb.2007.1327 |pmc=2600901 |pmid=18252674}}</ref><ref name=":0a" /> In some segments of all known arthropods the appendages have been modified, for example to form gills, mouth-parts, [[Biology:Antenna|antennae]] for collecting information,{{sfnp|Gould|1990|pp=102–106}} or claws for grasping;<ref>{{citation |author1=Shubin, Neil |author2=Tabin, C. |author3=Carroll, Sean |chapter=Fossils, Genes and the Evolution of Animal Limbs |title=Shaking the Tree: Readings from Nature in the History of Life |editor=Gee, H. |publisher=[[Organization:University of Chicago Press|University of Chicago Press]] |year=2000 |isbn=978-0-226-28497-2 |page=110 |chapter-url=https://books.google.com/books?id=M6yF0pU4eCsC&q=arthropod+diversity&pg=PA110}}</ref> arthropods are "like [[Engineering:Swiss Army knife|Swiss Army knives]], each equipped with a unique set of specialized tools."<ref name="RuppertFoxBarnes2004P518" /> In many arthropods, appendages have vanished from some regions of the body; it is particularly common for abdominal appendages to have disappeared or be highly modified.<ref name="RuppertFoxBarnes2004P518" />
[[File:20190913 Arthropoda head segments appendages en.png|center|thumb|upright=4.0|Alignment of anterior body segments and appendages across various arthropod taxa, based on the observations until mid 2010s. Head regions in black.<ref name="linkinghub.elsevier.com"/><ref>{{Cite journal |last1=Dunlop |first1=Jason A. |last2=Lamsdell |first2=James C. |date=2017 |title=Segmentation and tagmosis in Chelicerata |url=https://linkinghub.elsevier.com/retrieve/pii/S1467803916300731 |journal=Arthropod Structure & Development |volume=46 |issue=3 |pages=395–418 |doi=10.1016/j.asd.2016.05.002 |pmid=27240897}}</ref>]]
The most conspicuous specialization of segments is in the head. The four major groups of arthropods&nbsp;– [[Biology:Chelicerata|Chelicerata]] ([[Biology:Sea spider|sea spider]]s, [[Biology:Horseshoe crab|horseshoe crab]]s and [[Biology:Arachnid|arachnid]]s), Myriapoda (symphylan, pauropods, [[Biology:Millipede|millipede]]s and centipedes), [[Biology:Crustacean|Crustacea]] (oligostracans, [[Biology:Copepod|copepod]]s, malacostracans, branchiopods, [[Biology:Hexapoda|hexapod]]s, etc.), and the extinct [[Biology:Trilobita|Trilobita]]&nbsp;– have heads formed of various combinations of segments, with appendages that are missing or specialized in different ways.<ref name="RuppertFoxBarnes2004P518" /> Despite myriapods and hexapods both having similar head combinations, hexapods are deeply nested within crustacea while myriapods are not, so these traits are believed to have evolved separately. In addition, some extinct arthropods, such as ''[[Biology:Marrella|Marrella]]'', belong to none of these groups, as their heads are formed by their own particular combinations of segments and specialized appendages.<ref name="Whittington1971Marrella">{{citation |last=Whittington |first=H. B. |title=Redescription of ''Marrella splendens'' (Trilobitoidea) from the Burgess Shale, Middle Cambrian, British Columbia |journal=Geological Survey of Canada Bulletin |volume=209 |pages=1–24 |year=1971}} Summarised in {{harvp|Gould|1990|pp=107–121}}.</ref>

Working out the evolutionary stages by which all these different combinations could have appeared is so difficult that it has long been known as "The arthropod head problem".<ref>{{cite journal |last=Budd |first=G. E. |date=16 May 2002 |title=A palaeontological solution to the arthropod head problem | journal=Nature |volume=417 |issue=6886 |pages=271–275 |doi=10.1038/417271a |pmid=12015599 |bibcode=2002Natur.417..271B |s2cid=4310080}}</ref> In 1960, R. E. Snodgrass even hoped it would not be solved, as he found trying to work out solutions to be fun.{{refn|group=Note|"It would be too bad if the question of head segmentation ever should be finally settled; it has been for so long such fertile ground for theorizing that arthropodists would miss it as a field for mental exercise."<ref>{{citation |last=Snodgrass |first=R. E. |year=1960 |title=Facts and theories concerning the insect head |journal=Smithsonian Miscellaneous Collections |volume=142 |pages=1–61}}</ref>}}

=== Exoskeleton ===

{{main|Biology:Arthropod exoskeleton}}

[[File:Arthropod cuticle based upon Xvazquez edited to include english legend.PNG|thumb|Illustration of an idealized arthropod exoskeleton.]]

Arthropod exoskeletons are made of [[Biology:Arthropod cuticle|cuticle]], a non-cellular material secreted by the [[Biology:Epidermis (zoology)|epidermis]].<ref name="RuppertFoxBarnes2004P518" /> Their cuticles vary in the details of their structure, but generally consist of three main layers: the epicuticle, a thin outer [[Physics:Wax|wax]]y coat that moisture-proofs the other layers and gives them some protection; the exocuticle, which consists of [[Chemistry:Chitin|chitin]] and chemically hardened [[Biology:Protein|protein]]s; and the endocuticle, which consists of chitin and unhardened proteins. The exocuticle and endocuticle together are known as the procuticle.<ref name="Mechanical Design in Organisms">{{cite book |last1=Wainwright |first1=S. A. |author2=Biggs, W. D. |author3=Gosline, J. M. |name-list-style=amp |title=Mechanical Design in Organisms |publisher=Princeton University Press |year=1982 |isbn=978-0-691-08308-7 |pages=[https://archive.org/details/mechanicaldesign0000unse/page/162 162–163] |url=https://archive.org/details/mechanicaldesign0000unse/page/162}}</ref> Each body segment and limb section is encased in hardened cuticle. The joints between body segments and between limb sections are covered by flexible cuticle.<ref name="RuppertFoxBarnes2004P518" />

The exoskeletons of most aquatic [[Biology:Crustacean|crustacean]]s are [[Earth:Biomineralization|biomineralized]] with [[Chemistry:Calcium carbonate|calcium carbonate]] extracted from the water. Some terrestrial crustaceans have developed means of storing the mineral, since on land they cannot rely on a steady supply of dissolved calcium carbonate.<ref>{{citation |last1=Lowenstam |first1=H. A. |last2=Weiner |first2=S. |title=On biomineralization |publisher=Oxford University Press |year=1989 |isbn=978-0-19-504977-0 |page=111 |url=https://books.google.com/books?id=JbAgy0AAopsC&q=arthropod+biomineralization}}</ref> Biomineralization generally affects the exocuticle and the outer part of the endocuticle.<ref name="Mechanical Design in Organisms"/> Two recent hypotheses about the evolution of biomineralization in arthropods and other groups of animals propose that it provides tougher defensive armor,<ref name="Dzik2007">{{The Rise and Fall of the Ediacaran Biota |author=Dzik, J |chapter=The Verdun Syndrome: simultaneous origin of protective armour and infaunal shelters at the Precambrian–Cambrian transition |pages=405–414 |doi=10.1144/SP286.30 |chapter-url=http://www.paleo.pan.pl/people/Dzik/Publications/Verdun.pdf}}</ref> and that it allows animals to grow larger and stronger by providing more rigid skeletons;<ref name="Cohen2005">{{Citation |author=Cohen, B. L. |year=2005 |title=Not armour, but biomechanics, ecological opportunity and increased fecundity as keys to the origin and expansion of the mineralized benthic metazoan fauna |journal=[[Organization:Biological Journal of the Linnean Society|Biological Journal of the Linnean Society]] |volume=85 |issue=4 |pages=483–490 |url=http://eprints.gla.ac.uk/2933/01/Cohen_2933.pdf |doi=10.1111/j.1095-8312.2005.00507.x |access-date=2008-09-25 |archive-url=https://web.archive.org/web/20081003122813/http://eprints.gla.ac.uk/2933/01/Cohen_2933.pdf |archive-date=2008-10-03 |url-status=live |doi-access=free}}</ref> and in either case a mineral-organic [[Chemistry:Composite material|composite]] exoskeleton is cheaper to build than an all-organic one of comparable strength.<ref name="Cohen2005" /><ref>{{cite book |chapter=Early skeletal fossils |author=Bengtson, S. |editor1=Lipps, J. H. |editor2=Waggoner, B. M. |series=The Paleontological Society Papers |year=2004 |volume=10 |pages=67–78 |title=Neoproterozoic-Cambrian Biological Revolutions |url=http://www.starregister.org/download/18.4e32c81078a8d9249800021554/Bengtson2004ESF.pdf |archive-url=https://web.archive.org/web/20081003122817/http://www.nrm.se/download/18.4e32c81078a8d9249800021554/Bengtson2004ESF.pdf |archive-date=2008-10-03 |doi=10.1017/S1089332600002345}}</ref>

The cuticle may have [[Biology:Seta|seta]]e (bristles) growing from special cells in the epidermis. Setae are as varied in form and function as appendages. For example, they are often used as sensors to detect air or water currents, or contact with objects; aquatic arthropods use feather-like setae to increase the surface area of swimming appendages and to filter food particles out of water; aquatic insects, which are air-breathers, use thick felt-like coats of setae to trap air, extending the time they can spend under water; heavy, rigid setae serve as defensive spines.<ref name="RuppertFoxBarnes2004P518" />

Although all arthropods use muscles attached to the inside of the exoskeleton to flex their limbs, some still use hydraulic pressure to extend them, a system inherited from their pre-arthropod ancestors;<ref>{{citation |author1=Barnes, R. S. K. |author2=Calow, P. |author3=Olive, P. |author4=Golding, D. |author5=Spicer, J. |name-list-style=amp |title=The Invertebrates: A Synthesis |publisher=Blackwell Publishing |year=2001
| isbn=978-0-632-04761-1 |chapter=Invertebrates with Legs: the Arthropods and Similar Groups |page=168 |chapter-url=https://books.google.com/books?id=TBMsbe9efPgC&q=arthropod+hydraulic&pg=PA168}}</ref> for example, all spiders extend their legs hydraulically and can generate pressures up to eight times their resting level.<ref>{{citation |author1=Parry, D. A. |author2=Brown, R. H. J. |name-list-style=amp |title=The hydraulic mechanism of the spider leg |journal=Journal of Experimental Biology |volume=36 |pages=423–433 |year=1959 |url=http://jeb.biologists.org/cgi/reprint/36/2/423.pdf |issue=2 |doi=10.1242/jeb.36.2.423 |access-date=2008-09-25 |archive-url=https://web.archive.org/web/20081003122816/http://jeb.biologists.org/cgi/reprint/36/2/423.pdf |archive-date=2008-10-03 |url-status=live}}</ref>

=== Moulting ===

{{main|Biology:Ecdysis}}

[[File:Cicada climbing out of its exoskeleton while attached to tree.jpg|thumb|upright|[[Biology:Cicada|Cicada]] climbing out of its exuviae while attached to tree|left]]

The exoskeleton cannot stretch and thus restricts growth. Arthropods, therefore, replace their exoskeletons by undergoing [[Biology:Ecdysis|ecdysis]] (moulting), or shedding the old exoskeleton, the [[Biology:Exuviae|exuviae]], after growing a new one that is not yet hardened. Moulting cycles run nearly continuously until an arthropod reaches full size. The developmental stages between each moult (ecdysis) until sexual maturity is reached is called an [[Biology:Instar|instar]]. Differences between instars can often be seen in altered body proportions, colors, patterns, changes in the number of body segments or head width. After moulting, i.e. shedding their exoskeleton, the juvenile arthropods continue in their life cycle until they either pupate or moult again.<ref name="RuppertFoxBarnes2004P523To524">Ruppert, Fox & Barnes (2004), pp. 523–524</ref>

In the initial phase of moulting, the animal stops feeding and its epidermis releases moulting fluid, a mixture of [[Biology:Enzyme|enzyme]]s that digests the endocuticle and thus detaches the old cuticle. This phase begins when the [[Biology:Epidermis (zoology)|epidermis]] has secreted a new epicuticle to protect it from the enzymes, and the epidermis secretes the new exocuticle while the old cuticle is detaching. When this stage is complete, the animal makes its body swell by taking in a large quantity of water or air, and this makes the old cuticle split along predefined weaknesses where the old exocuticle was thinnest. It commonly takes several minutes for the animal to struggle out of the old cuticle. At this point, the new one is wrinkled and so soft that the animal cannot support itself and finds it very difficult to move, and the new endocuticle has not yet formed. The animal continues to pump itself up to stretch the new cuticle as much as possible, then hardens the new exocuticle and eliminates the excess air or water. By the end of this phase, the new endocuticle has formed. Many arthropods then eat the discarded cuticle to reclaim its materials.<ref name="RuppertFoxBarnes2004P523To524"/>

Because arthropods are unprotected and nearly immobilized until the new cuticle has hardened, they are in danger both of being trapped in the old cuticle and of being attacked by [[Earth:Predation|predator]]s. Moulting may be responsible for 80 to 90% of all arthropod deaths.<ref name="RuppertFoxBarnes2004P523To524"/>

=== Internal organs ===

{{Annotated image | float=right | image=Arthropod body struct 01.png
| width=220 | image-width=220 | height=160 
| annotations=
{{Annotation|5|72|{{legend inline|border=none|red|{{=}} heart}}}}
{{Annotation|5|94|{{legend inline|border=none|yellow|{{=}} gut}}}}
{{Annotation|5|116|{{legend inline|border=none|blue|{{=}} brain / ganglia}}}}
{{Annotation|5|138|{{legend inline|border=none|text={{blue|&thinsp;O}}||{{=}} eye}}}}
|caption=Basic arthropod body structure
}}
Arthropod bodies are also segmented internally, and the nervous, muscular, circulatory, and excretory systems have repeated components.<ref name="RuppertFoxBarnes2004P518" /> Arthropods come from a lineage of animals that have a [[Biology:Coelom|coelom]], a membrane-lined cavity between the gut and the body wall that accommodates the internal organs. The strong, segmented limbs of arthropods eliminate the need for one of the coelom's main ancestral functions, as a [[Biology:Hydrostatic skeleton|hydrostatic skeleton]], which muscles compress in order to change the animal's shape and thus enable it to move. Hence the coelom of the arthropod is reduced to small areas around the reproductive and excretory systems. Its place is largely taken by a hemocoel, a cavity that runs most of the length of the body and through which [[Biology:Blood|blood]] flows.<ref name="RuppertFoxBarnes2004P527To528">Ruppert, Fox & Barnes (2004), pp. 527–528</ref>

=== Respiration and circulation ===


[[File:Pone.0028183.g005.png|thumb|Respiration and circulation in a [[Biology:Myodocopida|myodocopid]] ostracod. Simplified transverse section through anterior body and carapace, showing gaseous diffusion through the inner lamella of the carapace (yellow arrows)]]

Arthropods have open [[Biology:Circulatory system|circulatory system]]s. Most have a few short, open-ended [[Biology:Artery|arteries]]. In chelicerates and crustaceans, the blood carries [[Chemistry:Oxygen|oxygen]] to the tissues, while [[Biology:Hexapoda|hexapods]] use a separate system of tracheae. Many crustaceans and a few chelicerates and tracheates use respiratory pigments to assist oxygen transport. The most common respiratory pigment in arthropods is [[Chemistry:Copper|copper]]-based [[Biology:Hemocyanin|hemocyanin]]; this is used by many crustaceans and a few centipedes. A few crustaceans and insects use iron-based [[Biology:Hemoglobin|hemoglobin]], the respiratory pigment used by [[Biology:Vertebrate|vertebrate]]s. As with other invertebrates, the respiratory pigments of those arthropods that have them are generally dissolved in the blood and rarely enclosed in [[Biology:Blood cell|corpuscles]] as they are in vertebrates.<ref name="RuppertFoxBarnes2004P527To528"/>

The heart is a muscular tube that runs just under the back and for most of the length of the hemocoel. It contracts in ripples that run from rear to front, pushing blood forwards. Sections not being squeezed by the heart muscle are expanded either by elastic [[Biology:Ligament|ligament]]s or by small [[Biology:Muscle|muscle]]s, in either case connecting the heart to the body wall. Along the heart run a series of paired ostia, non-return valves that allow blood to enter the heart but prevent it from leaving before it reaches the front.<ref name="RuppertFoxBarnes2004P527To528"/>

Arthropods have a wide variety of respiratory systems. Small species often do not have any, since their high ratio of surface area to volume enables simple diffusion through the body surface to supply enough oxygen. Crustacea usually have gills that are modified appendages. Many arachnids have book lungs.<ref name="Garwood" /> Tracheae, systems of branching tunnels that run from the openings in the body walls, deliver oxygen directly to individual cells in many insects, myriapods and [[Biology:Arachnid|arachnid]]s.<ref>Ruppert, Fox & Barnes (2004), pp. 530, 733</ref>

=== Nervous system ===

[[File:20200213 Remipedia Nectiopoda central nervous system.png|thumb|Central nervous system of a nectiopod remipede, showing the presence of both deutocerebrum (dc) and ventral nerve cord (vnc) organized by segmented ganglia. ]]

Living arthropods have paired main nerve cords running along their bodies below the gut, and in each segment the cords form a pair of ganglia from which [[Biology:Sensory nerve|sensory]] and [[Biology:Motor nerve|motor]] nerves run to other parts of the segment. Although the pairs of ganglia in each segment often appear physically fused, they are connected by [[Biology:Commissure|commissure]]s (relatively large bundles of nerves), which give arthropod nervous systems a characteristic "ladder-like" appearance. The brain is in the head, encircling and mainly ''above'' the esophagus. It consists of the fused ganglia of the acron and one or two of the foremost segments that form the head&nbsp;– a total of three pairs of ganglia in most arthropods, but only two in chelicerates, which do not have antennae or the ganglion connected to them. The ganglia of other head segments are often close to the brain and function as part of it. In insects these other head ganglia combine into a pair of subesophageal ganglia, under and behind the esophagus. Spiders take this process a step further, as ''all'' the [[Biology:Segmental ganglia|segmental ganglia]] are incorporated into the subesophageal ganglia, which occupy most of the space in the cephalothorax (front "super-segment").<ref name="RuppertFoxBarnes2004P531To532">Ruppert, Fox & Barnes (2004), pp. 531–532</ref>

=== Excretory system ===

There are two different types of arthropod excretory systems. In aquatic arthropods, the end-product of biochemical reactions that [[Biology:Metabolism|metabolise]] [[Chemistry:Nitrogen|nitrogen]] is [[Chemistry:Ammonia|ammonia]], which is so toxic that it needs to be diluted as much as possible with water. The ammonia is then eliminated via any permeable membrane, mainly through the gills.<ref name="Garwood"/> All crustaceans use this system, and its high consumption of water may be responsible for the relative lack of success of crustaceans as land animals.<ref name="RuppertFoxBarnes2004P529To530">Ruppert, Fox & Barnes (2004), pp. 529–530</ref> Various groups of terrestrial arthropods have independently developed a different system: the end-product of nitrogen metabolism is [[Chemistry:Uric acid|uric acid]], which can be excreted as dry material; the [[Biology:Malpighian tubule system|Malpighian tubule system]] filters the uric acid and other nitrogenous waste out of the blood in the hemocoel, and dumps these materials into the hindgut, from which they are expelled as [[Biology:Feces|feces]].<ref name="RuppertFoxBarnes2004P529To530" /> Most aquatic arthropods and some terrestrial ones also have organs called nephridia ("little kidneys"), which extract other wastes for excretion as [[Biology:Urine|urine]].<ref name="RuppertFoxBarnes2004P529To530"/>

=== Senses ===

[[File:Tliltocatl albopilosus 1.jpg|thumb|Long bristles (setae) of a ''Tliltocatl albopilosus'' tarantula]]

The stiff [[Biology:Cuticle|cuticle]]s of arthropods would block out information about the outside world, except that they are penetrated by many sensors or connections from sensors to the nervous system. In fact, arthropods have modified their cuticles into elaborate arrays of sensors. Various touch sensors, mostly [[Biology:Seta|seta]]e, respond to different levels of force, from strong contact to very weak air currents. Chemical sensors provide equivalents of [[Biology:Taste|taste]] and [[Biology:Olfaction|smell]], often by means of setae. Pressure sensors often take the form of membranes that function as [[Medicine:Eardrum|eardrum]]s, but are connected directly to nerves rather than to auditory ossicles. The [[Biology:Antenna|antennae]] of most hexapods include sensor packages that monitor [[Physics:Humidity|humidity]], moisture and temperature.<ref name="RuppertFoxBarnes2004P532To537">Ruppert, Fox & Barnes (2004), pp. 532–537</ref>

Most arthropods lack balance and [[Physics:Acceleration|acceleration]] sensors, and rely on their eyes to tell them which way is up. The self-righting behavior of [[Biology:Cockroach|cockroach]]es is triggered when pressure sensors on the underside of the feet report no pressure. However, many malacostracan crustaceans have [[Biology:Statocyst|statocyst]]s, which provide the same sort of information as the balance and motion sensors of the vertebrate inner ear.<ref name="RuppertFoxBarnes2004P532To537" />

The proprioceptors of arthropods, sensors that report the force exerted by muscles and the degree of bending in the body and joints, are well understood. However, little is known about what other internal sensors arthropods may have.<ref name="RuppertFoxBarnes2004P532To537" />

==== Optical ====


[[File:Arthropoda montage (eyes).png|upright=1.35|thumb|Arthropod eyes]]

[[File:Wasp ocelli.JPG|upright=1.15|thumb|right|Head of a [[Biology:Wasp|wasp]] with three ocelli (center), and compound eyes at the left and right]]

Most arthropods have sophisticated visual systems that include one or more usually both of [[Biology:Compound eye|compound eye]]s and pigment-cup ocelli ("little eyes"). In most cases ocelli are only capable of detecting the direction from which light is coming, using the shadow cast by the walls of the cup. However, the main eyes of [[Biology:Spider|spider]]s are pigment-cup ocelli that are capable of forming images,<ref name="RuppertFoxBarnes2004P532To537"/> and those of [[Biology:Jumping spider|jumping spider]]s can rotate to track prey.<ref>Ruppert, Fox & Barnes (2004), pp. 578–580</ref>

Compound eyes consist of fifteen to several thousand independent ommatidia, columns that are usually [[Hexagon|hexagon]]al in [[Cross section (geometry)|cross section]]. Each ommatidium is an independent sensor, with its own light-sensitive cells and often with its own [[Biology:Lens (anatomy)|lens]] and [[Medicine:Cornea|cornea]].<ref name="RuppertFoxBarnes2004P532To537"/> Compound eyes have a wide field of view, and can detect fast movement and, in some cases, the polarization of light.<ref>{{cite journal |url=http://www.suss-microoptics.com/downloads/Publications/Miniaturized_Imaging_Systems.pdf |doi=10.1016/S0167-9317(03)00102-3 |title=Miniaturized imaging systems |last1=Völkel |first1=R. |author2=Eisner, M. |author3=Weible, K. J. |journal=Microelectronic Engineering |volume=67–68 |pages=461–472 |date=June 2003 |archive-url=https://web.archive.org/web/20081001225326/http://www.suss-microoptics.com/downloads/Publications/Miniaturized_Imaging_Systems.pdf |archive-date=2008-10-01}}</ref> On the other hand, the relatively large size of ommatidia makes the images rather coarse, and compound eyes are shorter-sighted than those of birds and mammals&nbsp;– although this is not a severe disadvantage, as objects and events within {{convert|20|cm|in|0|abbr=on}} are most important to most arthropods.<ref name="RuppertFoxBarnes2004P532To537"/> Several arthropods have color vision, and that of some insects has been studied in detail; for example, the ommatidia of bees contain receptors for both green and ultra-violet.<ref name="RuppertFoxBarnes2004P532To537"/>

====Olfaction====

{{further|Biology:Insect olfaction}}

== Reproduction and development ==

{{Multiple image
|image1            = Aphid-giving-birth.jpg
|image2            = Harvestmen mating (44325686201).jpg
|align             = left
|direction         = vertical
|caption1          = [[Biology:Aphid|Aphid]] giving birth to live young from an unfertilized egg
|caption2          = [[Biology:Opiliones|Harvestmen]] mating
}}

A few arthropods, such as [[Biology:Barnacle|barnacle]]s, are hermaphroditic, that is, each can have the organs of both [[Biology:Sex|sex]]es. However, individuals of most species remain of one sex their entire lives.<ref name="RuppertFoxBarnes2004P537To539" /> A few species of [[Biology:Insect|insect]]s and crustaceans can reproduce by [[Biology:Parthenogenesis|parthenogenesis]], especially if conditions favor a "population explosion". However, most arthropods rely on [[Biology:Sexual reproduction|sexual reproduction]], and parthenogenetic species often revert to sexual reproduction when conditions become less favorable.<ref name="Olive2001ReproductionInvertebratesInEncOfLifeSci">{{cite encyclopedia |contribution=Reproduction and LifeCycles in Invertebrates |author=Olive, P. J. W. |title=Encyclopedia of Life Sciences |year=2001 |publisher=John Wiley & Sons |doi= 10.1038/npg.els.0003649 |isbn=978-0-470-01617-6}}</ref>  The ability to undergo [[Biology:Meiosis|meiosis]] is widespread among arthropods including both those that reproduce sexually and those that reproduce [[Biology:Parthenogenesis|parthenogenetically]].<ref>{{Cite journal |last1=Schurko |first1=A. M. |last2=Mazur |first2=D. J. |last3=Logsdon |first3=J. M. |date=February 2010 |title=Inventory and phylogenomic distribution of meiotic genes in Nasonia vitripennis and among diverse arthropods |journal=Insect Molecular Biology |volume=19 |issue=Suppl 1 |pages=165–180 |doi=10.1111/j.1365-2583.2009.00948.x |pmid=20167026 |s2cid=11617147}}</ref> Although meiosis is a major characteristic of arthropods, understanding of its fundamental adaptive benefit has long been regarded as an unresolved problem,<ref>{{Cite journal |last1=Bernstein |first1=H. |last2=Hopf |first2=F. A. |last3=Michod |first3=R. E. |date=1987 |title=The molecular basis of the evolution of sex |journal=Advances in Genetics |volume=24 |pages=323–370 |doi=10.1016/s0065-2660(08)60012-7 |pmid=3324702|isbn=978-0-12-017624-3}}</ref> that appears to have remained unsettled.

Aquatic arthropods may breed by external fertilization, as for example [[Biology:Horseshoe crab|horseshoe crab]]s do,<ref>{{cite web |title=Facts About Horseshoe Crabs and FAQ |url=https://myfwc.com/research/saltwater/crustaceans/horseshoe-crabs/facts/ |access-date=2020-01-19}}</ref> or by internal fertilization, where the ova remain in the female's body and the [[Biology:Sperm|sperm]] must somehow be inserted. All known terrestrial arthropods use internal fertilization. [[Biology:Opiliones|Opiliones]] (harvestmen), [[Biology:Millipede|millipede]]s, and some crustaceans use modified appendages such as [[Biology:Gonopod|gonopod]]s or [[Biology:Opiliones penis|penises]] to transfer the sperm directly to the female. However, most male [[Biology:Terrestrial animal|terrestrial]] arthropods produce spermatophores, waterproof packets of [[Biology:Sperm|sperm]], which the females take into their bodies. A few such species rely on females to find spermatophores that have already been deposited on the ground, but in most cases males only deposit spermatophores when complex [[Biology:Courtship|courtship]] rituals look likely to be successful.<ref name="RuppertFoxBarnes2004P537To539">Ruppert, Fox & Barnes (2004), pp. 537–539</ref>

[[File:Shrimp nauplius.jpg|thumb|right|The nauplius larva of a penaeid shrimp]]

Most arthropods lay eggs,<ref name="RuppertFoxBarnes2004P537To539" /> but scorpions are ovoviviparous: they produce live young after the eggs have hatched inside the mother, and are noted for prolonged maternal care.<ref>{{citation |last=Lourenço |first=Wilson R. |contribution=Reproduction in scorpions, with special reference to parthenogenesis |title=European Arachnology 2000 |editor1=Toft, S. |editor2=Scharff, N. |pages=71–85 |publisher=Aarhus University Press |year=2002 |isbn=978-87-7934-001-5 |url=http://www.european-arachnology.org/proceedings/19th/Lourenco.PDF |access-date=2008-09-28 |archive-url=https://web.archive.org/web/20081003122816/http://www.european-arachnology.org/proceedings/19th/Lourenco.PDF |archive-date=2008-10-03 |url-status=live}}</ref> Newly born arthropods have diverse forms, and insects alone cover the range of extremes. Some hatch as apparently miniature adults (direct development), and in some cases, such as [[Biology:Silverfish|silverfish]], the hatchlings do not feed and may be helpless until after their first moult. Many insects hatch as grubs or [[Biology:Caterpillar|caterpillar]]s, which do not have segmented limbs or hardened cuticles, and [[Biology:Metamorphosis|metamorphose]] into adult forms by entering an inactive phase in which the larval tissues are broken down and re-used to build the adult body.<ref>{{cite journal |last1=Truman |first1=J. W. |last2=Riddiford |first2=L. M. |title=The origins of insect metamorphosis |journal=Nature |volume=401 |issue=6752 |pages=447–452 |date=September 1999 |doi=10.1038/46737 |pmid=10519548 |bibcode=1999Natur.401..447T |s2cid=4327078 |url=http://www.insecta.ufv.br/Entomologia/ent/disciplina/ban%20160/AULAT/aula8/truman.pdf |access-date=2008-09-28 |archive-url=https://web.archive.org/web/20081003122816/http://www.insecta.ufv.br/Entomologia/ent/disciplina/ban%20160/AULAT/aula8/truman.pdf |archive-date=2008-10-03 |url-status=live}}</ref> [[Biology:Dragonfly|Dragonfly]] larvae have the typical cuticles and jointed limbs of arthropods but are flightless water-breathers with extendable jaws.<ref>{{citation |title=Diversity and Adaptations of the Aquatic Insects |last=Smith |first=G. |url=http://faculty.ncf.edu/mccord/pdf/AquaticInsectGeoffSmith.pdf |access-date=2008-09-28 |publisher=[[Organization:New College of Florida|New College of Florida]] |archive-url=https://web.archive.org/web/20081003122816/http://faculty.ncf.edu/mccord/pdf/AquaticInsectGeoffSmith.pdf |archive-date=3 October 2008}}</ref> Crustaceans commonly hatch as tiny [[Biology:Nauplius (larva)|nauplius]] larvae that have only three segments and pairs of appendages.<ref name="RuppertFoxBarnes2004P537To539" />

== Evolutionary history ==

=== Last common ancestor ===

Based on the distribution of shared [[Biology:Plesiomorphy and symplesiomorphy|plesiomorphic]] features in extant and fossil taxa, the last common ancestor of all arthropods is inferred to have been as a modular organism with each module covered by its own [[Biology:Sclerite|sclerite]] (armor plate) and bearing a pair of biramous limbs.<ref name="car-epnla">{{citation |editor=Stefan Koenemann |editor2=Ronald A. Jenner |title=Crustacea and Arthropod Relationships |series=Crustacean Issues |volume=16 |chapter=Early Palaeozoic non-lamellipedian arthropods |last1=Bergström |first1=Jan |first2=Xian-Guang |last2=Hou |isbn=978-0-8493-3498-6 |year=2005 |publisher=Taylor & Francis |location=Boca Raton |doi=10.1201/9781420037548.ch4 |pages=73–93}}</ref> However, whether the ancestral limb was uniramous or biramous is far from a settled debate.
This Ur-arthropod had a ventral mouth, pre-oral antennae and dorsal eyes at the front of the body. It was assumed to have been a non-discriminatory [[Earth:Sediment|sediment]] feeder, processing whatever sediment came its way for food,<ref name="car-epnla" /> but fossil findings hint that the last common ancestor of both arthropods and [[Biology:Priapulida|priapulida]] shared the same specialized mouth apparatus; a circular mouth with rings of teeth used for capturing animal prey.<ref>{{cite web|url=http://www.nhm.ac.uk/our-science/science-news/2016/september/arthropod-ancestor-mouth.html|title=Arthropod ancestor had the mouth of a penis worm |website = Natural History Museum|archive-url=https://web.archive.org/web/20170202073340/http://www.nhm.ac.uk/our-science/science-news/2016/september/arthropod-ancestor-mouth.html |archive-date=2017-02-02|url-status=live|first = Conor |last =McKeever|date =30 September 2016}}</ref>

=== Fossil record ===

[[File:Marrella.png|thumb|''Marrella'', one of the puzzling arthropods from the Burgess Shale ]]

It has been proposed that the [[Earth:Ediacaran|Ediacaran]] animals ''[[Biology:Parvancorina|Parvancorina]]'' and ''[[Biology:Spriggina|Spriggina]]'', from around {{ma|555}}, were arthropods,<ref>{{cite journal |last=Glaessner |first=M. F. |year=1958 |title=New fossils from the base of the Cambrian in South Australia |journal=Transactions of the Royal Society of South Australia |volume=81 |pages=185–188 |url=http://www.samuseum.sa.gov.au/Journals/TRSSA/TRSSA_V081/TRSSA_V081_p185p188.pdf |archive-date=2008-12-16 |archive-url=https://web.archive.org/web/20081216220102/http://www.samuseum.sa.gov.au/Journals/TRSSA/TRSSA_V081/TRSSA_V081_p185p188.pdf}}</ref><ref>{{cite journal |last1=Lin |first1=J. P. |last2=Gon |first2=S. M.|last3=Gehling |first3=J. G. |last4=Babcock |first4=L. E. |last5=Zhao |first5=Y. L. |last6=Zhang |first6=X. L. |last7=Hu |first7=S. X. |last8=Yuan |first8=J. L. |last9=Yu |first9=M. Y. |last10=Peng |first10=J. |title=A ''Parvancorina''-like arthropod from the Cambrian of South China |journal=[[Biology:Historical Biology|Historical Biology]] |volume=18 |issue=1 |pages=33–45 |year=2006 |doi=10.1080/08912960500508689 |s2cid=85821717}}</ref><ref>{{citation |author=McMenamin, M.A.S |year=2003 |title=''Spriggina'' is a trilobitoid ecdysozoan |journal=Abstracts with Programs |volume=35 |issue=6 |pages=105 |url=http://gsa.confex.com/gsa/2003AM/finalprogram/abstract_62056.htm |format=abstract |access-date=2008-10-21 |archive-url=https://web.archive.org/web/20080830080220/http://gsa.confex.com/gsa/2003AM/finalprogram/abstract_62056.htm |archive-date=2008-08-30 |url-status=live}}</ref> but later study shows that their affinities of being origin of arthropods are not reliable.<ref>{{Cite journal |last1=Daley |first1=Allison C. |last2=Antcliffe |first2=Jonathan B. |last3=Drage |first3=Harriet B. |last4=Pates |first4=Stephen |date=2018-05-22 |title=Early fossil record of Euarthropoda and the Cambrian Explosion |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=115 |issue=21 |pages=5323–5331 |doi=10.1073/pnas.1719962115 |pmc=6003487 |pmid=29784780 |bibcode=2018PNAS..115.5323D |doi-access=free}}</ref> Small arthropods with bivalve-like shells have been found in Early Cambrian fossil beds dating {{ma|541|539}} in China and Australia.<ref name="Braun2007">{{cite journal |last1=Braun |first1=A. |author2=Chen, J. |author3=Waloszek, D. |author4=Maas, A. |year=2007 |title=First Early Cambrian Radiolaria |journal=Special Publications |volume=286 |issue=1 |pages=143–149 |url=http://biosys-serv.biologie.uni-ulm.de/Downloadfolder/PDFs%20Team/2007b_Braun_etal.pdf |doi=10.1144/SP286.10 |archive-url=https://web.archive.org/web/20110718095413/http://biosys-serv.biologie.uni-ulm.de/Downloadfolder/PDFs%20Team/2007b_Braun_etal.pdf |archive-date=2011-07-18 |bibcode=2007GSLSP.286..143B |s2cid=129651908}}</ref><ref>{{cite journal |last1=Yuan |first1=X. |last2=Xiao |first2=S. |last3=Parsley |first3=R. L. |last4=Zhou |first4=C.|last5=Chen |first5=Z. |last6=Hu |first6=J. |date=April 2002 |title=Towering sponges in an Early Cambrian Lagerstätte: Disparity between nonbilaterian and bilaterian epifaunal tierers at the Neoproterozoic-Cambrian transition |journal=Geology |volume=30 |issue=4 |pages=363–366|url=https://www.researchgate.net/publication/249520833 |doi=10.1130/0091-7613(2002)030<0363:TSIAEC>2.0.CO;2 |bibcode=2002Geo....30..363Y}}</ref><ref>{{Citation |last1=Skovsted |first1=Christian |last2=Brock |first2=Glenn |last3=Paterson |first3=John |year=2006 |title=Bivalved arthropods from the Lower Cambrian Mernmerna Formation of South Australia and their implications for the identification of Cambrian 'small shelly fossils' |journal=Association of Australasian Palaeontologists Memoirs |volume=32 |pages=7–41 |url=https://www.researchgate.net/publication/303809761|issn=0810-8889}}</ref><ref name=bradoriid>{{Citation |last1=Betts |first1=Marissa |last2=Topper |first2=Timothy |last3=Valentine |first3=James |last4= Skovsted |first4= Christian |last5= Paterson |first5=John |last6=Brock |first6=Glenn |date=January 2014 |title=A new early Cambrian bradoriid (Arthropoda) assemblage from the northern Flinders Ranges, South Australia |journal= Gondwana Research |volume=25 |issue=1 |pages=420–437 |url=https://www.researchgate.net/publication/236901156|doi=10.1016/j.gr.2013.05.007|bibcode=2014GondR..25..420B}}</ref> The earliest Cambrian [[Biology:Trilobite|trilobite]] fossils are about 520&nbsp;million years old, but the class was already quite diverse and worldwide, suggesting that they had been around for quite some time.<ref name="Lieberman1999Trilobites">{{citation |last=Lieberman |first=B. S. |title=Testing the Darwinian legacy of the Cambrian radiation using trilobite phylogeny and biogeography |journal=Journal of Paleontology |volume=73 |issue=2 |date=March 1, 1999 |page=176 |s2cid=88588171 |doi=10.1017/S0022336000027700 |bibcode=1999JPal...73..176L |url=http://jpaleontol.geoscienceworld.org/cgi/content/abstract/73/2/176 |access-date=October 21, 2008 |archive-url=https://web.archive.org/web/20081019012441/http://jpaleontol.geoscienceworld.org/cgi/content/abstract/73/2/176 |archive-date=October 19, 2008 |url-status=live}}</ref> In the [[Earth:Maotianshan Shales|Maotianshan shales]], which date back to 518 million years ago, arthropods such as ''[[Biology:Kylinxia|Kylinxia]]'' and ''[[Biology:Erratus|Erratus]]'' have been found that seem to represent [[Biology:Transitional fossil|transitional fossils]] between stem (e.g. [[Biology:Radiodonta|Radiodonta]] such as ''[[Biology:Anomalocaris|Anomalocaris]]'') and true arthropods.<ref name="phys-Kylinxia">{{cite news |title=A 520-million-year-old, five-eyed fossil reveals arthropod origin |url=https://phys.org/news/2020-11-million-year-old-five-eyed-fossil-reveals-arthropod.html |access-date=8 December 2020 |work=phys.org |language=en}}</ref><!--better source needed, if this is needed at all, the scientific source follows--><ref name="Kylinxia">{{cite journal |last1=Zeng |first1=Han |last2=Zhao |first2=Fangchen |last3=Niu |first3=Kecheng |last4=Zhu |first4=Maoyan |last5=Huang |first5=Diying |title=An early Cambrian euarthropod with radiodont-like raptorial appendages |journal=Nature |date=December 2020 |volume=588 |issue=7836 |pages=101–105 |doi=10.1038/s41586-020-2883-7 |pmid=33149303 |bibcode=2020Natur.588..101Z |s2cid=226248177 |url=https://www.nature.com/articles/s41586-020-2883-7 |access-date=8 December 2020 |issn=1476-4687}}</ref><ref name="fu2022">{{Cite journal |last1=Fu |first1=D. |last2=Legg |first2=D. A. |last3=Daley |first3=A. C. |last4=Budd |first4=G. E. |last5=Wu |first5=Y. |last6=Zhang |first6=X. |year=2022 |title=The evolution of biramous appendages revealed by a carapace-bearing Cambrian arthropod |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |volume=377 |issue=1847 |pages=Article ID 20210034 |doi=10.1098/rstb.2021.0034 |pmid=35125000 |pmc=8819368 |s2cid=246608509}}</ref> Re-examination in the 1970s of the [[Earth:Burgess Shale|Burgess Shale]] fossils from about {{ma|505}} identified many arthropods, some of which could not be assigned to any of the well-known groups, and thus intensified the debate about the [[Earth:Cambrian explosion|Cambrian explosion]].<ref name="Whittington1979Lawn">Whittington, H. B. (1979). Early arthropods, their appendages and relationships. In M. R. House (Ed.), The origin of major invertebrate groups (pp. 253–268). The Systematics Association Special Volume, 12. London: Academic Press.</ref><ref name="Whittington1985BurgessShale">{{citation |title=The Burgess Shale |author=Whittington, H.B. |author2=Geological Survey of Canada |year=1985 |publisher=Yale University Press |isbn=978-0-660-11901-4 |oclc=15630217}}</ref>{{sfnp|Gould|1990|p={{pn|date=January 2024}}}} A fossil of ''[[Biology:Marrella|Marrella]]'' from the Burgess Shale has provided the earliest clear evidence of [[Biology:Moulting|moulting]].<ref>{{cite journal |last1=García-Bellido |first1=D. C. |last2=Collins |first2=D. H. |date=May 2004 |title=Moulting arthropod caught in the act |journal=Nature |volume=429 |pages=40 |doi=10.1038/429040a |pmid=15129272 |issue=6987|bibcode=2004Natur.429...40G |s2cid=40015864 |doi-access=free}}</ref>

[[File:20210310 Kylinxia zhangi.png|thumb|''Kylinxia'' may be a key transitional fossil between stem-arthropods and true arthropods.<ref name="phys-Kylinxia"/>]]

[[File:Yicaris dianensis.jpg|thumb|''Yicaris'' is one of the earliest crustaceans had been discovered.]]

The earliest fossil of likely pancrustacean larvae date from about {{ma|514}} in the [[Earth:Cambrian|Cambrian]], followed by unique taxa like ''[[Biology:Yicaris|Yicaris]]'' and ''Wujicaris''.<ref name=":0b">{{cite journal |last=Hegna |first=Thomas A. |title=The Fossil Record of the Pancrustacea |date=2020-09-10 |url=http://dx.doi.org/10.1093/oso/9780190637842.003.0002 |work=Evolution and Biogeography |pages=21–52 |access-date=2024-01-05 |publisher=Oxford University Press |doi=10.1093/oso/9780190637842.003.0002 |last2=Luque |first2=Javier |last3=Wolfe |first3=Joanna M.}}</ref> The purported pancrustacean/[[Biology:Crustacean|crustacean]] affinity of some cambrian arthropods (e.g. [[Biology:Phosphatocopina|Phosphatocopina]], [[Biology:Bradoriida|Bradoriida]] and [[Biology:Hymenocarina|Hymenocarine]] taxa like waptiids)<ref>{{Cite journal |last=Hou |first=Xian-Guang |last2=Siveter |first2=Derek J. |last3=Aldridge |first3=Richard J. |last4=Siveter |first4=David J. |date=2008-10-10 |title=Collective Behavior in an Early Cambrian Arthropod |url=https://www.science.org/doi/10.1126/science.1162794 |journal=Science |language=en |volume=322 |issue=5899 |pages=224–224 |doi=10.1126/science.1162794 |issn=0036-8075}}</ref><ref>{{citation |last1=Budd |first1=G. E. |author2=Butterfield, N. J. |author3=Jensen, S. |title=Crustaceans and the "Cambrian Explosion" |journal=Science |date=December 2001 |volume=294 |issue=5549 |page=2047 |doi=10.1126/science.294.5549.2047a |pmid=11739918|doi-access=free}}</ref><ref>{{Cite journal |last=Xian‐Guang |first=Hou |last2=Siveter |first2=Derek J. |last3=Aldridge |first3=Richard J. |last4=Siveter |first4=David J. |date=2009 |title=A New Arthropod in Chainlike Associations from the Cnengjiang Lagerstätte (Lower Cambrian), Yunnan, China |journal=Palaeontology |language=en |volume=52 |issue=4 |pages=951–961 |doi=10.1111/j.1475-4983.2009.00889.x |doi-access=free |issn=0031-0239}}</ref> were disputed by subsequent studies, as they might branched before the mandibulate crown-group.<ref name=":0b" /> Within the pancrustacean crown-group, only [[Biology:Malacostraca|Malacostraca]], [[Biology:Branchiopoda|Branchiopoda]] and [[Biology:Pentastomida|Pentastomida]] have Cambrian fossil records.<ref name=":0b"/> Crustacean fossils are common from the [[Earth:Ordovician|Ordovician]] period onwards.<ref>{{cite journal |last1=Zhang |first1=X.-G. |author2=Siveter, D. J. |author3=Waloszek, D. |author4=Maas, A. |title=An epipodite-bearing crown-group crustacean from the Lower Cambrian |journal=Nature |date=October 2007 |volume=449 |issue=7162 |pages=595–598 |doi=10.1038/nature06138 |pmid=17914395|bibcode=2007Natur.449..595Z |s2cid=4329196}}</ref> They have remained almost entirely aquatic, possibly because they never developed [[Biology:Excretory system|excretory system]]s that conserve water.<ref name="RuppertFoxBarnes2004P529To530" /> 

Arthropods provide the earliest identifiable fossils of land animals, from about {{ma|419}} in the Late [[Earth:Silurian|Silurian]],<ref name="Garwood">{{cite journal |first1=Russell J. |last1=Garwood |first2=Greg |last2=Edgecombe |year=2011 |title=Early Terrestrial Animals, Evolution, and Uncertainty |journal=Evolution: Education and Outreach |volume=4 |issue=3 |pages=489–501 |doi=10.1007/s12052-011-0357-y |doi-access=free}}</ref> and terrestrial tracks from about {{ma|450}} appear to have been made by arthropods.<ref>{{cite journal |author=Pisani, D. |author2=Poling, L. L. |author3=Lyons-Weiler M. |author4=Hedges, S. B. |title=The colonization of land by animals: molecular phylogeny and divergence times among arthropods |journal=BMC Biology |year=2004 |volume=2 |doi=10.1186/1741-7007-2-1 |page=1 |pmid=14731304 |pmc=333434 |doi-access=free}}</ref> Arthropods possessed attributes that were easy [[Biology:Exaptation|coopted]] for life on land; their existing jointed exoskeletons provided protection against desiccation, support against gravity and a means of locomotion that was not dependent on water.<ref name="CowenHistLifeEd3P126">{{cite book |author=Cowen, R. |title=History of Life |edition=3rd |publisher=Blackwell Science |isbn=978-0-632-04444-3 |page=126 |year=2000}}</ref> Around the same time the aquatic, scorpion-like [[Biology:Eurypterid|eurypterid]]s became the largest ever arthropods, some as long as {{convert|2.5|m|ftin|abbr=on}}.<ref>{{cite journal |last1=Braddy |first1=S. J. |author2=Markus Poschmann, M. |author3=Tetlie, O. E. |name-list-style=amp |title=Giant claw reveals the largest ever arthropod |journal=[[Biology:Biology Letters|Biology Letters]] |year=2008 |volume=4 |pages=106–109 |doi=10.1098/rsbl.2007.0491 |pmid=18029297 |issue=1 |pmc=2412931}}</ref>

The oldest known [[Biology:Arachnid|arachnid]] is the trigonotarbid ''Palaeotarbus jerami'', from about {{ma|420}} in the [[Earth:Silurian|Silurian]] period.<ref>{{cite journal |last=Dunlop |first=J. A. |title=A trigonotarbid arachnid from the Upper Silurian of Shropshire |journal=Palaeontology |volume=39 |issue=3 |pages=605–614 |date=September 1996 |url=http://palaeontology.palass-pubs.org/pdf/Vol%2039/Pages%20605-614.pdf |archive-url=https://web.archive.org/web/20081216214632/http://palaeontology.palass-pubs.org/pdf/Vol%2039/Pages%20605-614.pdf |archive-date=2008-12-16}}</ref>{{refn|The fossil was originally named ''Eotarbus'' but was renamed when it was realized that a [[Earth:Carboniferous|Carboniferous]] arachnid had already been named ''Eotarbus''.<ref>{{cite journal |author=Dunlop, J. A. |date=1999 |title=A replacement name for the trigonotarbid arachnid ''Eotarbus'' Dunlop |journal=Palaeontology |volume=42 |issue=1 |page=191 |doi=10.1111/1475-4983.00068 |bibcode=1999Palgy..42..191D |s2cid=83825904 |doi-access=free}}</ref>|group=Note}} ''[[Biology:Attercopus|Attercopus]] fimbriunguis'', from {{ma|386}} in the [[Earth:Devonian|Devonian]] period, bears the earliest known silk-producing spigots, but its lack of spinnerets means it was not one of the true [[Biology:Spider|spider]]s,<ref>{{cite journal |last1=Selden |first1=P. A. |author2=Shear, W. A. |title=Fossil evidence for the origin of spider spinnerets |journal=PNAS | doi=10.1073/pnas.0809174106 | pmid=19104044 |pmc=2634869 |volume=105 |issue=52 |date=December 2008 |pages=20781–5 |bibcode=2008PNAS..10520781S |doi-access=free}}</ref> which first appear in the Late [[Earth:Carboniferous|Carboniferous]] over {{ma|299}}.<ref>{{cite journal |last=Selden |first=P. A. |title=Fossil mesothele spiders |journal=Nature |volume=379 |issue=6565 |pages=498–499 |date=February 1996 |s2cid=26323977 |doi=10.1038/379498b0 |bibcode=1996Natur.379..498S}}</ref> The [[Earth:Jurassic|Jurassic]] and [[Earth:Cretaceous|Cretaceous]] periods provide a large number of fossil spiders, including representatives of many modern families.<ref name="VollrathSelden2007BehaviorInEvolutionOfSpiders">{{cite journal |last1=Vollrath |first1=F. |author2=Selden, P. A. |name-list-style=amp |title=The Role of Behavior in the Evolution of Spiders, Silks, and Webs |journal=Annual Review of Ecology, Evolution, and Systematics |date=December 2007 |volume=38 |pages=819–846 |doi=10.1146/annurev.ecolsys.37.091305.110221 |url=http://homepage.mac.com/paulselden/Sites/Website/ARES.pdf |archive-url=https://web.archive.org/web/20081209102852/http://homepage.mac.com/paulselden/Sites/Website/ARES.pdf |archive-date=2008-12-09}}</ref> The oldest known scorpion is ''Dolichophonus,'' dated back to {{ma|436}}.<ref>{{Cite journal |last=Anderson |first=Evan P. |last2=Schiffbauer |first2=James D. |last3=Jacquet |first3=Sarah M. |last4=Lamsdell |first4=James C. |last5=Kluessendorf |first5=Joanne |last6=Mikulic |first6=Donald G. |date=2021 |editor-last=Zhang |editor-first=Xi‐Guang |title=Stranger than a scorpion: a reassessment of Parioscorpio venator, a problematic arthropod from the Llandoverian Waukesha Lagerstätte |journal=Palaeontology |language=en |volume=64 |issue=3 |pages=429–474 |doi=10.1111/pala.12534 |issn=0031-0239}}</ref> Lots of [[Earth:Silurian|Silurian]] and [[Earth:Devonian|Devonian]] scorpions were previously though to be [[Biology:Gill|gill]]-breathing, hence the idea that scorpions were primitively aquatic and evolved air-breathing book lungs later on.<ref>{{cite journal |last=Jeram |first=A. J. |title=Book-lungs in a Lower Carboniferous scorpion |journal=Nature |volume=343 |issue=6256 |pages=360–361 |date=January 1990 |doi=10.1038/343360a0 |bibcode=1990Natur.343..360J |s2cid=4327169}}</ref> However subsequent studies reveal most of them lacking reliable evidence for an aquatic lifestyle,<ref>{{Cite journal |last=Howard |first=Richard J. |last2=Edgecombe |first2=Gregory D. |last3=Legg |first3=David A. |last4=Pisani |first4=Davide |last5=Lozano-Fernandez |first5=Jesus |date=2019-03-01 |title=Exploring the evolution and terrestrialization of scorpions (Arachnida: Scorpiones) with rocks and clocks |journal=Organisms Diversity & Evolution |language=en |volume=19 |issue=1 |pages=71–86 |doi=10.1007/s13127-019-00390-7 |doi-access=free |issn=1618-1077|hdl=1983/9ab6548b-b4de-47b5-b1d0-8008d225c375 |hdl-access=free }}</ref> while exceptional aquatic taxa (e.g. ''Waeringoscorpio'') most likely derived from terrestrial scorpion ancestors.<ref>{{Cite journal |last=Poschmann |first=Markus |last2=Dunlop |first2=Jason A. |last3=Kamenz |first3=Carsten |last4=Scholtz |first4=Gerhard |date=December 2008 |title=The Lower Devonian scorpion ''Waeringoscorpio'' and the respiratory nature of its filamentous structures, with the description of a new species from the Westerwald area, Germany |journal=Paläontologische Zeitschrift |language=en |volume=82 |issue=4 |pages=418–436 |doi=10.1007/BF03184431 |issn=0031-0220}}</ref>

The oldest fossil record of [[Biology:Hexapoda|hexapod]] is obscure, as most of the candidates are poorly preserved and their hexapod affinities had been disputed. An iconic example is the [[Earth:Devonian|Devonian]] ''Rhyniognatha hirsti'', dated at {{ma|396|407}}, its [[Biology:Mandible (insect mouthpart)|mandible]]s are thought to be a type found only in [[Biology:Pterygota|winged insects]], which suggests that the earliest insects appeared in the Silurian period.<ref name="EngelGrim2004">{{cite journal |last1=Engel |first1=M. S. |last2=Grimaldi |first2=D. A. |date=February 2004 |title=New light shed on the oldest insect |journal=Nature |volume=427 |issue=6975 |pages=627–630 |doi=10.1038/nature02291 |pmid=14961119 |bibcode=2004Natur.427..627E |s2cid=4431205 |author-link=Michael S. Engel}}</ref> However later study shows that ''Rhyniognatha'' most likely represent a myriapod, not even a hexapod.<ref name=":0c">{{cite journal |last1=Haug |first1=Carolin |last2=Haug |first2=Joachim T. |date=2017-05-30 |title=The presumed oldest flying insect: more likely a myriapod? |journal=PeerJ |volume=5 |pages=e3402 |doi=10.7717/peerj.3402 |pmc=5452959 |pmid=28584727 |doi-access=free}}</ref> The unequivocal oldest known hexapod and [[Biology:Insect|insect]] is the [[Biology:Springtail|springtail]] ''[[Biology:Rhyniella|Rhyniella]]'', from about {{ma|410}} in the Devonian period, and the [[Biology:Palaeodictyoptera|palaeodictyopteran]] ''Delitzschala bitterfeldensis'', from about {{ma|325}} in the Carboniferous period, respectively.<ref name=":0c"/> The Mazon Creek lagerstätten from the Late Carboniferous, about {{ma|300}}, include about 200 species, some gigantic by modern standards, and indicate that insects had occupied their main modern [[Biology:Ecological niche|ecological niche]]s as [[Earth:Herbivore|herbivore]]s, [[Biology:Detritivore|detritivore]]s and [[Biology:Insectivore|insectivore]]s. Social [[Biology:Termite|termite]]s and [[Biology:Ant|ant]]s first appear in the Early [[Earth:Cretaceous|Cretaceous]], and advanced social bees have been found in Late Cretaceous rocks but did not become abundant until the Middle [[Earth:Cenozoic|Cenozoic]].<ref>{{cite book |last1=Labandeira |first1=C. |last2=Eble |first2=G. J. |contribution=The Fossil Record of Insect Diversity and Disparity |title=Gondwana Alive: Biodiversity and the Evolving Biosphere |editor=Anderson, J. |editor2=Thackeray, F. |editor3=van Wyk, B. |editor4=de Wit, M. |publisher=Witwatersrand University Press |year=2000 |url=http://www.santafe.edu/research/publications/workingpapers/00-08-044.pdf |access-date=2008-10-21 |archive-url=https://web.archive.org/web/20080911075353/http://www.santafe.edu/research/publications/workingpapers/00-08-044.pdf |archive-date=2008-09-11}}</ref>

=== Evolutionary relationships to other animal phyla ===

[[File:Peripatoides indigo2 (cropped).jpg|thumb|left|The velvet worm (Onychophora) is closely related to arthropods<ref name=Budd1996/>]]

From 1952 to 1977, zoologist Sidnie Manton and others argued that arthropods are [[Biology:Polyphyly|polyphyletic]], in other words, that they do not share a common ancestor that was itself an arthropod. Instead, they proposed that three separate groups of "arthropods" evolved separately from common worm-like ancestors: the chelicerates, including [[Biology:Spider|spider]]s and scorpions; the crustaceans; and the [[Biology:Uniramia|uniramia]], consisting of onychophorans, myriapods and [[Biology:Hexapoda|hexapods]]. These arguments usually bypassed [[Biology:Trilobite|trilobite]]s, as the evolutionary relationships of this class were unclear. Proponents of polyphyly argued the following: that the similarities between these groups are the results of [[Biology:Convergent evolution|convergent evolution]], as natural consequences of having rigid, segmented [[Biology:Exoskeleton|exoskeleton]]s; that the three groups use different chemical means of hardening the cuticle; that there were significant differences in the construction of their compound eyes; that it is hard to see how such different configurations of segments and appendages in the head could have evolved from the same ancestor; and that crustaceans have biramous limbs with separate gill and leg branches, while the other two groups have uniramous limbs in which the single branch serves as a leg.<ref>{{cite book |last=Gillott |first=C. |year=1995 |title=Entomology |pages=17–19 |publisher=Springer |isbn=978-0-306-44967-3}}</ref>
<div style="float:right; width:auto; max-width:100%; border:solid 1px silver; padding:2px; margin:2px; font-size:80%;">
<div style="width:auto; border:solid 1px silver; padding:5px">
{{clade
 |label1=&nbsp;
 |1={{clade
    |label1=&nbsp; [[Biology:Onychophora|onychophora]]ns &nbsp;
    |1=<br/>includes ''[[Biology:Aysheaia|Aysheaia]]'' and ''[[Biology:Peripatus|Peripatus]]''<br/> &nbsp;
    |2={{clade
      |label1=&nbsp;
      |1={{clade
          |label1=&nbsp; armored lobopods &nbsp;
          |1=includes ''[[Biology:Hallucigenia|Hallucigenia]]'' and ''[[Biology:Microdictyon|Microdictyon]]''
          |label2=&nbsp; [[Biology:Dinocaridida|dinocarids]] (s.l.) &nbsp;
          |2={{clade
              |label1=&nbsp; anomalocarid- &nbsp;
              |sublabel1=like taxa (s.l.)
              |1=includes modern [[Biology:Tardigrade|tardigrade]]s as<br/>well as extinct animals like<br/>''[[Biology:Kerygmachela|Kerygmachela]]'' and ''[[Biology:Opabinia|Opabinia]]''
              |2={{clade
                  |label1=&nbsp; [[Biology:Anomalocarididae|anomalocarids]] (s.s.) &nbsp;
                  |1=''[[Biology:Anomalocaris|Anomalocaris]]''
                  |label2=&nbsp; arthropods &nbsp;
                  |2=includes living groups and<br/>extinct forms such as [[Biology:Trilobite|trilobite]]s
               }}
           }}
       }}
     }}
  }}
}}
</div>{{center|Simplified summary of Budd's (1996) "broad-scale" cladogram<ref name=Budd1996/>}}</div>

Further analysis and discoveries in the 1990s reversed this view, and led to acceptance that arthropods are [[Biology:Monophyletic|monophyletic]], in other words they are inferred to share a common ancestor that was itself an arthropod.<ref>{{cite web|author=Adrain, J. |date=15 March 1999 |title=''Arthropod Fossils and Phylogeny'', edited by Gregory D. Edgecomb |department=Book review |publisher=Palaeontologia Electronica |url=http://palaeo-electronica.org/1999_1/books/arthropo.htm |url-status=live |access-date=2008-09-28 |archive-url= https://web.archive.org/web/20080908053500/http://palaeo-electronica.org/1999_1/books/arthropo.htm |archive-date=8 September 2008}}
: The book is
{{cite journal |last1=Labandiera |first1=Conrad C. |last2=Edgecombe |first2=Gregory |year=1998 |title=Arthropod Fossils and Phylogeny |journal=PALAIOS |volume=14 |issue=4 |editor=G.D. |publisher=Columbia University Press |pages=347 |bibcode=1999Palai..14..405L |doi=10.2307/3515467 |jstor=3515467}}</ref><ref>{{cite journal |author=Chen, J.-Y. |author2=Edgecombe, G. D. |author3=Ramsköld, L. |author4=Zhou, G.-Q. |date=2 June 1995 |title=Head segmentation in Early Cambrian ''Fuxianhuia'': implications for arthropod evolution |journal=Science |volume=268 |issue=5215 |pages=1339–1343 |doi=10.1126/science.268.5215.1339 |pmid=17778981|bibcode=1995Sci...268.1339C |s2cid=32142337}}</ref> For example, Graham Budd's analyses of ''[[Biology:Kerygmachela|Kerygmachela]]'' in 1993 and of ''[[Biology:Opabinia|Opabinia]]'' in 1996 convinced him that these animals were similar to onychophorans and to various Early [[Earth:Cambrian|Cambrian]] "lobopods", and he presented an "evolutionary family tree" that showed these as "aunts" and "cousins" of all arthropods.<ref name=Budd1996>{{cite journal |author=Budd, G.E. |year=1996 |title=The morphology of ''Opabinia regalis'' and the reconstruction of the arthropod stem-group |journal=Lethaia |volume=29 |issue=1 |pages=1–14 |doi=10.1111/j.1502-3931.1996.tb01831.x}}</ref><ref name=Budd1993>{{cite journal | author=Budd, G. E. |year=1993 |title=A Cambrian gilled lobopod from Greenland |journal=Nature |volume=364 |pages=709–711 |doi=10.1038/364709a0 |issue=6439|bibcode=1993Natur.364..709B |s2cid=4341971}}</ref> These changes made the scope of the term "arthropod" unclear, and Claus Nielsen proposed that the wider group should be labelled "[[Biology:Panarthropoda|Panarthropoda]]" ("all the arthropods") while the animals with jointed limbs and hardened cuticles should be called "Euarthropoda" ("true arthropods").<ref>{{cite book |author=Nielsen, C. |year=2001 |title=Animal Evolution: Interrelationships of the living phyla |edition=2nd |publisher=[[Organization:Oxford University Press|Oxford University Press]] |pages=194–196 |url=https://books.google.com/books?id=UmCg6c0HkqMC&q=nielsen+panarthropoda+euarthropoda&pg=PA194 |isbn=978-0-19-850681-2}}</ref>

A contrary view was presented in 2003, when Jan Bergström and Hou Xian-guang argued that, if arthropods were a "sister-group" to any of the anomalocarids, they must have lost and then re-evolved features that were well-developed in the anomalocarids. The earliest known arthropods ate mud in order to extract food particles from it, and possessed variable numbers of segments with unspecialized appendages that functioned as both gills and legs. Anomalocarids were, by the standards of the time, huge and sophisticated predators with specialized mouths and grasping appendages, fixed numbers of segments some of which were specialized, tail fins, and gills that were very different from those of arthropods. In 2006, they suggested that arthropods were more closely related to lobopods and [[Biology:Tardigrade|tardigrade]]s than to anomalocarids.<ref>{{cite journal |last1=Hou |first1=X.-G. |author2=Bergström, J. |author3=Jie, Y. |year=2006 |title=Distinguishing anomalocaridids from arthropods and priapulids |journal=Geological Journal |volume=41 |issue=3–4 |pages=259–269 |doi=10.1002/gj.1050 |s2cid=83582128}}</ref> In 2014, it was found that tardigrades were more closely related to arthropods than velvet worms.<ref>{{cite press release |title=Misunderstood worm-like fossil finds its place in the Tree of Life |date=17 August 2014 |publisher=Cambridge University |url=http://www.cam.ac.uk/research/news/misunderstood-worm-like-fossil-finds-its-place-in-the-tree-of-life |url-status=live |access-date=24 January 2017 |archive-url=https://web.archive.org/web/20170107150001/http://www.cam.ac.uk/research/news/misunderstood-worm-like-fossil-finds-its-place-in-the-tree-of-life |archive-date=7 January 2017}}</ref>

<div style="width:auto; border:solid 1px silver; padding:2px; margin:2px; font-size:80%;">
{{clade
  |label1=[[Biology:Protostome|Protostome]]s
  |1={{clade
|1=[[Biology:Chaetognatha|Chaetognatha]] <span style="{{MirrorH}}">[[File:Pseudosagitta maxima 31349361.png|70px]]</span>
|2={{clade
       |1=Spiralia ([[Biology:Annelid|annelid]]s, [[Biology:Mollusc|mollusc]]s, [[Biology:Brachiopod|brachiopod]]s, etc.) [[File:Polychaeta (no) 2.jpg|70px]]  <span style="{{MirrorH}}">70px</span>  [[File:LingulaanatinaAA (cropped).JPG|70px]]
       |label2=[[Biology:Ecdysozoa|Ecdysozoa]]
       |2={{clade
            |1=[[Biology:Nematoida|Nematoida]] ([[Biology:Nematode|nematode]]s and close relatives) [[File:CelegansGoldsteinLabUNC 2.jpg|70px]] [[File:Nematomorpha Somiedo (white background).jpg|60px]]
            |2=[[Biology:Scalidophora|Scalidophora]] (priapulids and [[Biology:Kinorhyncha|Kinorhyncha]], and [[Biology:Loricifera|Loricifera]]) [[File:Adult priapulid 2.jpg|50px]]
            |label3=[[Biology:Panarthropoda|Panarthropoda]]
            |3={{clade
                 |1=[[Biology:Onychophora|Onychophora]]ns [[File:Velvet worm.png|80px]]
                 |label2=[[Biology:Tactopoda|Tactopoda]]
                 |2={{clade
                     |1=[[Biology:Tardigrade|Tardigrade]]s [[File:SEM image of Milnesium tardigradum in active state - journal.pone.0045682.g001-2 (white background).png|60px]]
                 |label2=Euarthropoda
                 |2={{clade
                      |1=Chelicerates [[File:Nymphon signatum 194389384 (white background).jpg|90 px]] 70 px [[File:Aptostichus simus Monterey County.jpg|60 px]]
                      |label2=[[Biology:Mandibulata|Mandibulata]] 
                      |2={{clade
                           |1=†Euthycarcinoids <span style="{{MirrorH}}">[[File:Apankura.png|70 px]]</span>
                           |2={{clade
                                |1=Myriapods [[File:Scolopendra japonica アオズムカデ 大阪府 生駒山産.jpg|70 px]] <span style="{{MirrorH}}">[[File:Andrognathus corticarius A.jpg|100 px]]</span>
                                |label2=[[Biology:Pancrustacea|Pancrustacea]]
                                |2={{clade
                                     |1=[[Biology:Crustacean|Crustacean]]s <span style="{{MirrorH}}">[[File:Lobster png by absurdwordpreferred d2xqhvd.png|70 px]]</span>
                                     |2=[[Biology:Hexapoda|Hexapods]] [[File:Platycheirus angustatus (Syrphidae) - (male imago), Elst (Gld), the Netherlands - 2.jpg|60 px]]
                                  }}
                            }}
                       }}
                  }}
             }}
        }}
   }}
}}
}}
|footer=Relationships of Ecdysozoa to each other and to annelids, ''etc''.,<ref name=Telford-Bourlat-etal-2008/>{{Failed verification |date=December 2023|reason=The contents of the cladogram don't match the phylogeny given in the source: the source does not use Tactopoda, does not include Loricifera in Scalidophora, and doesn't even mention Chaetognatha.}} including euthycarcinoids<ref name=Vaccari2004>{{cite journal |author1=Vaccari, N.E. |author2=Edgecombe, G.D. |author3=Escudero, C. |date=29 July 2004 |title=Cambrian origins and affinities of an enigmatic fossil group of arthropods |journal=Nature |volume=430 |issue=6999 |pages=554–557 |doi=10.1038/nature02705 |pmid=15282604 |bibcode=2004Natur.430..554V |s2cid=4419235}}</ref>}}</div>

Higher up the "family tree", the [[Biology:Annelida|Annelida]] have traditionally been considered the closest relatives of the Panarthropoda, since both groups have segmented bodies, and the combination of these groups was labelled Articulata. There had been competing proposals that arthropods were closely related to other groups such as [[Biology:Nematode|nematode]]s, priapulids and [[Biology:Tardigrade|tardigrade]]s, but these remained minority views because it was difficult to specify in detail the relationships between these groups.

In the 1990s, [[Biology:Molecular phylogenetics|molecular phylogenetic]] analyses of [[Biology:DNA|DNA]] sequences produced a coherent scheme showing arthropods as members of a superphylum labelled Ecdysozoa ("animals that moult"), which contained nematodes, priapulids and tardigrades but excluded annelids. This was backed up by studies of the anatomy and development of these animals, which showed that many of the features that supported the Articulata hypothesis showed significant differences between annelids and the earliest Panarthropods in their details, and some were hardly present at all in arthropods. This hypothesis groups annelids with molluscs and [[Biology:Brachiopod|brachiopod]]s in another superphylum, [[Biology:Lophotrochozoa|Lophotrochozoa]].

If the Ecdysozoa hypothesis is correct, then segmentation of arthropods and annelids either has evolved [[Biology:Convergent evolution|convergently]] or has been inherited from a much older ancestor and subsequently lost in several other lineages, such as the non-arthropod members of the Ecdysozoa.<ref>{{cite journal |last1=Schmidt-Rhaesa |first1=A. |author2=Bartolomaeus, T. |author3=Lemburg, C. |author4=Ehlers, U. |author5=Garey, J.R. |title=The position of the Arthropoda in the phylogenetic system |journal=Journal of Morphology |volume=238 |issue=3 |date=January 1999 |doi=10.1002/(SICI)1097-4687(199812)238:3<263::AID-JMOR1>3.0.CO;2-L |pmid=29852696 |pages=263–285 |s2cid=46920478}}</ref><ref name=Telford-Bourlat-etal-2008>{{cite journal |last1=Telford |first1=M.J. |author2=Bourlat, S.J. |author3=Economou, A. |author4=Papillon, D. |author5=Rota-Stabelli, O. |date=January 2008 |title=The evolution of the Ecdysozoa |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |volume=363 |pages=1529–1537 |doi=10.1098/rstb.2007.2243 |pmid=18192181 |issue=1496 |pmc=2614232}}</ref>

===Evolution of fossil arthropods===

{{more|Deuteropoda}}

{{cladogram|title=Arthropod fossil phylogeny<ref name="Edgecombe2020"/>
|{{clade|style=font-size:80%;
|label1=Arthropoda
|1={{clade|state1=double
   |1=[[Biology:Lobopodia#Siberion and similar taxa|giant lobopodians]] †
   |2={{clade|state1=double
      |1=[[Biology:Lobopodia#Gilled lobopodians|gilled lobopodians]] †
      |2={{clade|1=[[Biology:Radiodonta|Radiodonta]] †
         |label2=[[Biology:Deuteropoda|Deuteropoda]]
         |2={{clade
            |1=[[Biology:Chelicerata|Chelicerata]]
            |2=[[Biology:Megacheira|Megacheira]] †
            |3=[[Biology:Artiopoda|Artiopoda]] †
            |4=[[Biology:Isoxyida|Isoxyida]] †
            |5=[[Biology:Mandibulata|Mandibulata]]
            }}
         }}
      }}
   }}
}}
|caption=Summarized [[Biology:Cladogram|cladogram]] of the relationships between extinct arthropod groups. For more, see [[Biology:Deuteropoda|Deuteropoda]].}}

Aside from the four major living groups ([[Biology:Crustacea|crustacea]]ns, chelicerates, myriapods and [[Biology:Hexapoda|hexapod]]s), a number of fossil forms, mostly from the early [[Earth:Cambrian|Cambrian]] period, are difficult to place taxonomically, either from lack of obvious affinity to any of the main groups or from clear affinity to several of them. ''[[Biology:Marrella|Marrella]]'' was the first one to be recognized as significantly different from the well-known groups.<ref name="Whittington1971Marrella"/>

Modern interpretations of the basal, extinct stem-group of Arthropoda recognised the following groups, from most basal to most crownward:<ref name="Ortega-Hernández" /><ref name="Edgecombe2020">{{cite journal |author=Gregory D. Edgecombe |title=Arthropod Origins: Integrating Paleontological and Molecular Evidence |date=2020 |journal=Annu. Rev. Ecol. Evol. Syst. |volume=51 |pages=1–25 |doi=10.1146/annurev-ecolsys-011720-124437 |s2cid=225478171}}</ref>

* The [[Biology:Lobopodia#Siberion and similar taxa|"Giant" or "Siberiid Lobopodians"]], such as ''[[Biology:Jianshanopodia|Jianshanopodia]]'', ''[[Biology:Siberion|Siberion]]'' and ''[[Biology:Megadictyon|Megadictyon]]'', are the most basal [[Biology:Evolutionary grade|grade]] in the total-group Arthropoda.
* The [[Biology:Lobopodia#Gilled lobopodians|"Gilled Lobopodians"]], such as ''[[Biology:Kerygmachela|Kerygmachela]]'', ''[[Biology:Pambdelurion|Pambdelurion]]'' and ''[[Biology:Opabinia|Opabinia]]'', are the second most basal grade.
* The [[Biology:Radiodonta|Radiodonta]], which traditionally known as anomalocaridids come in third position, and are thought to be [[Biology:Monophyly|monophyletic]].
* A possible "upper stem-group" assemblage of more uncertain position<ref name="Edgecombe2020" /> but contained within [[Biology:Deuteropoda|Deuteropoda]]:<ref name="Ortega-Hernández" /> the [[Biology:Fuxianhuiida|Fuxianhuiida]], [[Biology:Megacheira|Megacheira]], and multiple "bivalved forms" including [[Biology:Isoxyida|Isoxyida]]  and [[Biology:Hymenocarina|Hymenocarina]].

The [[Biology:Deuteropoda|Deuteropoda]] is a recently established clade uniting the crown-group (living) arthropods with these possible "upper stem-group" fossils taxa.<ref name="Ortega-Hernández">{{cite journal |last1=Ortega-Hernández |first1=Javier |title=Making sense of 'lower' and 'upper' stem-group Euarthropoda, with comments on the strict use of the name Arthropoda von Siebold, 1848 |journal=Biological Reviews |volume=91 |pages=255–273 |pmid=25528950 |s2cid=7751936 | year=2016 |issue=1 |doi=10.1111/brv.12168}}</ref> The clade is defined by important changes to the structure of the head region such as the appearance of a differentiated deutocerebral appendage pair, which excludes more basal taxa like radiodonts and "gilled lobopodians".<ref name="Ortega-Hernández" />

Controversies remain about the positions of various extinct arthropod groups. Some studies recover Megacheira as closely related to chelicerates, while others recover them as outside the group containing Chelicerate and Mandibulata as stem-group euarthropods.<ref name="Aria-2022">{{Cite journal |last=Aria |first=Cédric |date=2022-04-26 |title=The origin and early evolution of arthropods |journal=Biological Reviews |language=en |volume=97 |issue=5 |pages=1786–1809 |doi=10.1111/brv.12864 |issn=1464-7931 |pmid=35475316 |s2cid=243269510}}</ref> The placement of the [[Biology:Artiopoda|Artiopoda]] (which contains the extinct trilobites and similar forms) is also a frequent subject of dispute.<ref name="Jenner2006ChallengingReceivedWisdoms">{{cite journal |last=Jenner |first=R. A. |title=Challenging received wisdoms: Some contributions of the new microscopy to the new animal phylogeny |journal=Integrative and Comparative Biology |volume=46 |issue=2 |date=April 2006 |pages=93–103 |doi=10.1093/icb/icj014 |pmid=21672726|doi-access=free}}</ref> The main hypotheses position them in the clade [[Biology:Arachnomorpha|Arachnomorpha]] with the Chelicerates. However, one of the newer hypotheses is that the chelicerae have originated from the same pair of appendages that evolved into antennae in the ancestors of [[Biology:Mandibulata|Mandibulata]], which would place trilobites, which had antennae, closer to Mandibulata than Chelicerata, in the clade [[Biology:Antennulata|Antennulata]].<ref name="Aria-2022"/><ref>{{cite web |last=Dunlop |first=Jason A. |title=Fossil Focus: Chelicerata |url=http://www.palaeontologyonline.com/articles/2011/fossil-focus-chelicerata/ |work=Palaeontology Online |volume=1 |issue=Article 1 |pages=1-8 |date=31 January 2011 |access-date=15 March 2018 |archive-date=12 September 2017 |archive-url=https://web.archive.org/web/20170912011805/http://www.palaeontologyonline.com/articles/2011/fossil-focus-chelicerata/ |url-status=live}}</ref> The [[Biology:Fuxianhuiida|fuxianhuiids]], usually suggested to be stem-group arthropods, have been suggested to be Mandibulates in some recent studies.<ref name="Aria-2022"/> The [[Biology:Hymenocarina|Hymenocarina]], a group of bivalved arthropods, previously thought to have been stem-group members of the group, have been demonstrated to be mandibulates based on the presence of mandibles.<ref name="Edgecombe2020"/>

<gallery mode="packed" heights="120" caption="Radiodonts, Opabiniids, Gilled Lobopodians and the more traditional Lobopodians are all examples of basal stem-group arthropod lineages from the Cambrian">
File:20191203 Anomalocaris canadensis.png|''[[Biology:Anomalocaris|Anomalocaris]]'' <br /> ([[Biology:Radiodonta|Radiodonta]])
File:20220213 Opabiniidae Opabiniids.png|''[[Biology:Opabinia|Opabinia]]'' and ''[[Biology:Utaurora|Utaurora]]'' <br /> ([[Biology:Opabiniidae|Opabiniidae]])
File:Mobulavermis adustus.png|''[[Biology:Mobulavermis|Mobulavermis]]'' <br /> ([[Biology:Kerygmachelidae|Kerygmachelidae]])
File:Facivermis.png|''[[Biology:Facivermis|Facivermis]]'' <br /> ([[Biology:Luolishaniidae|Luolishaniidae]])
</gallery><gallery mode="packed" heights="120" caption="Marrellomorphs, megacherians, funxianhuiids and phosphatocopines are some examples of Cambrian arthropods whose classification remains difficult">
Marrella.png|''[[Biology:Marrella|Marrella]]'' <br /> ([[Biology:Marrellomorpha|Marrellomorpha]])
20191027 Leanchoilia superlata.png|''[[Biology:Leanchoilia|Leanchoilia]]'' <br /> ([[Biology:Megacheira|Megacheira]])
20211117 Fuxianhuia protensa.png|''[[Biology:Fuxianhuia|Fuxianhuia]]'' <br /> ([[Biology:Fuxianhuiida|Fuxianhuiida]])
Dabashanella sp.png|''Dabashanella'' <br /> ([[Biology:Phosphatocopina|Phosphatocopina]])
</gallery><gallery mode="packed" heights="120" caption="Other examples of now extinct arthropod groups include">
File:20201229 Acutiramus cummingsi.png|''[[Biology:Acutiramus|Acutiramus]]'' <br /> ([[Biology:Eurypterid|Eurypterida]])
File:Apankura.png|''Apankura'' <br /> ([[Biology:Euthycarcinoidea|Euthycarcinoidea]])
File:TrimerusDelphinocephalus.jpg|''[[Biology:Trimerus|Trimerus]]'' <br /> ([[Biology:Artiopoda|Artiopoda]])
File:Concavicaris georgeorum.png|''Concavicaris'' <br /> ([[Biology:Thylacocephala|Thylacocephala]])
</gallery>

=== Evolution and classification of living arthropods ===


The phylum Arthropoda is typically [[Scientific classification|subdivided]] into four [[Biology:Subphylum|subphyla]], of which one is extinct:<ref>{{ITIS |id=82696 |taxon=Arthropoda |access-date=2006-08-15}}</ref>

# '''[[Biology:Artiopoda|Artiopod]]s''' are an extinct group of formerly numerous marine animals that disappeared in the [[Earth:Permian–Triassic extinction event|Permian–Triassic extinction event]], though they were in decline prior to this killing blow, having been reduced to one order in the [[Earth:Late Devonian extinction|Late Devonian extinction]]. They contain groups such as the [[Biology:Trilobita|trilobites]].
# '''[[Biology:Chelicerata|Chelicerate]]s''' comprise the marine [[Biology:Sea spider|sea spider]]s and [[Biology:Horseshoe crab|horseshoe crab]]s, along with the terrestrial [[Biology:Arachnid|arachnid]]s such as [[Biology:Mite|mite]]s, harvestmen, [[Biology:Spider|spider]]s, scorpions and related organisms characterized by the presence of chelicerae, [[Biology:Appendage|appendage]]s just above/in front of the [[Biology:Arthropod mouthparts|mouthparts]]. Chelicerae appear in scorpions and horseshoe crabs as tiny [[Biology:Claw|claw]]s that they use in feeding, but those of spiders have developed as fangs that inject [[Biology:Venom|venom]].
# '''Myriapods''' comprise [[Biology:Millipede|millipede]]s, centipedes, pauropods and symphylans, characterized by having numerous body segments each of which bearing one or two pairs of legs (or in a few cases being legless). All members are exclusively terrestrial.
# '''[[Biology:Pancrustacea|Pancrustacea]]ns''' comprise [[Biology:Ostracod|ostracod]]s, [[Biology:Barnacle|barnacle]]s, [[Biology:Copepod|copepod]]s, malacostracans, cephalocaridans, branchiopods, remipedes and [[Biology:Hexapoda|hexapod]]s. Most groups are primarily aquatic (two notable exceptions being [[Biology:Woodlouse|woodlice]] and hexapods, which are both purely [[Biology:Terrestrial animal|terrestrial]]) and are characterized by having biramous appendages. The most abundant group of pancrustaceans are the terrestrial hexapods, which comprise [[Biology:Insect|insect]]s, [[Biology:Diplura|diplura]]ns, springtails, and [[Biology:Protura|protura]]ns, with six [[Biology:Thorax (arthropod anatomy)|thoracic]] legs.

The [[Biology:Phylogenetics|phylogeny]] of the major extant arthropod groups has been an area of considerable interest and dispute.<ref>{{cite journal |last1=Carapelli |first1=Antonio |first2=Pietro |last2=Liò |first3=Francesco |last3=Nardi|first4=Elizabeth|last4=van der Wath|first5= Francesco|last5=Frati |date=16 August 2007 |title=Phylogenetic analysis of mitochondrial protein coding genes confirms the reciprocal paraphyly of Hexapoda and Crustacea |journal=BMC Evolutionary Biology |volume=7 |issue=Suppl 2 |doi=10.1186/1471-2148-7-S2-S8 |pmid=17767736 |pages=S8 |pmc=1963475 |doi-access=free}}</ref> Recent studies strongly suggest that Crustacea, as traditionally defined, is [[Biology:Paraphyly|paraphyletic]], with Hexapoda having evolved from within it,<ref name=Regi10/><ref name=vonJennWillDell11>{{cite journal |last1=von Reumont |first1=Bjoern M. |last2=Jenner |first2=Ronald A. |last3=Wills |first3=Matthew A. |last4=Dell'Ampio |first4=Emiliano |last5=Pass |first5=Günther |last6=Ebersberger |first6=Ingo |last7=Meyer |first7=Benjamin |last8=Koenemann |first8=Stefan |last9=Iliffe |first9=Thomas M. |last10=Stamatakis |first10=Alexandros |last11=Niehuis |first11=Oliver |last12=Meusemann |first12=Karen |last13=Misof |first13=Bernhard |year=2011 |title=Pancrustacean phylogeny in the light of new phylogenomic data: support for Remipedia as the possible sister group of Hexapoda |journal=Molecular Biology and Evolution |doi=10.1093/molbev/msr270 |pmid=22049065 |volume=29 |issue=3 |pages=1031–45|doi-access=free}}</ref> so that Crustacea and Hexapoda form a clade, [[Biology:Pancrustacea|Pancrustacea]]. The position of Myriapoda, [[Biology:Chelicerata|Chelicerata]] and Pancrustacea remains unclear {{as of|2012|April|lc=yes}}. In some studies, Myriapoda is grouped with Chelicerata (forming [[Biology:Myriochelata|Myriochelata]]);<ref name="Hassanin">{{cite journal |last=Hassanin |first=Alexandre |title=Phylogeny of Arthropoda inferred from mitochondrial sequences: Strategies for limiting the misleading effects of multiple changes in pattern and rates of substitution |year=2006 |journal=[[Biology:Molecular Phylogenetics and Evolution|Molecular Phylogenetics and Evolution]] |volume=38 |pages=100–116 |doi=10.1016/j.ympev.2005.09.012 |url=http://www.csulb.edu/~dlunderw/entomology/Arthropodphylogeny2006.pdf |pmid=16290034 |issue=1 |access-date=2010-04-16 |archive-url=https://web.archive.org/web/20110110023714/http://www.csulb.edu/~dlunderw/entomology/Arthropodphylogeny2006.pdf |archive-date=2011-01-10 |url-status=live}}</ref><ref>{{cite book |title=The position of crustaceans within Arthropoda &nbsp;– Evidence from nine molecular loci and morphology |last1=Giribet |first1=G. |author2=Richter, S. |author3=Edgecombe, G. D. |author4=Wheeler, W. C. |year=2005 |volume=16 |pages=307–352 |url=http://www.amonline.net.au/palaeontology/pdf/giribet-etal2005.pdf |doi=10.1201/9781420037548.ch13 |series=Crustacean Issues |isbn=978-0-8493-3498-6 |access-date=2006-08-23 |archive-url=https://web.archive.org/web/20060916153440/https://www.amonline.net.au/palaeontology/pdf/giribet-etal2005.pdf |archive-date=2006-09-16 |url-status=live}}</ref> in other studies, Myriapoda is grouped with Pancrustacea (forming [[Biology:Mandibulata|Mandibulata]]),<ref name=Regi10>{{cite journal |last1=Regier |first1=Jerome C. |year=2010 |journal=Nature |volume=463 |pages=1079–1084 |title=Arthropod relationships revealed by phylogenomic analysis of nuclear protein-coding sequences |doi=10.1038/nature08742 |pmid=20147900 |last2=Shultz |first2=J. W. |last3=Zwick |first3=A. |last4=Hussey|first4=A. |last5=Ball |first5=B. |last6=Wetzer |first6=R. |last7=Martin |first7=J. W. |last8=Cunningham |first8=C. W. |issue=7284 |display-authors=0 |bibcode=2010Natur.463.1079R |s2cid=4427443}}</ref> or Myriapoda may be sister to Chelicerata plus Pancrustacea.<ref name=vonJennWillDell11/>

The following cladogram shows the internal relationships between all the living [[Biology:Class|classes]] of arthropods as of late 2010s,<ref name="Lozano-Fernandez">{{cite journal| first1=Jesus| last1=Lozano-Fernandez| first2=Mattia| last2=Giacomelli| first3=James F.| last3=Fleming| first4=Albert| last4=Chen| first5=Jakob| last5=Vinther| first6=Philip Francis| last6=Thomsen| first7=Henrik| last7=Glenner| first8=Ferran| last8=Palero| first9=David A.| last9=Legg| first10=Thomas M.| last10=Iliffe| first11=Davide| last11=Pisani| first12=Jørgen| last12=Olesen| title=Pancrustacean Evolution Illuminated by Taxon-Rich GenomicScale Data Sets with an Expanded Remipede Sampling| journal=Genome Biol. Evol. |volume=11| pages=2055–2070| year=2019| issue=8| doi=10.1093/gbe/evz097| pmid=31270537| pmc=6684935}}</ref><ref>{{cite journal |title=The Phylogeny and Evolutionary History of Arthropods |first1=Gonzalo|last1=Giribet |first2=Gregory |last2=Edgecombe |journal=Current Biology |volume=29 |pages=R592–R602 |date=June 2019 |issue=12 |doi=10.1016/j.cub.2019.04.057 |pmid=31211983 |s2cid=189926344 |doi-access=free}}</ref> as well as the estimated timing for some of the clades:<ref>{{cite journal|title=Phylogenomics resolves the timing and pattern of insect evolution |first=Bernhard |last=Misof |display-authors=0 |journal=Science |date=2014 |volume=346 |issue=6210 |pages=763–767 |doi= 10.1126/science.1257570 |pmid=25378627 |bibcode=2014Sci...346..763M |s2cid=36008925}}</ref>

{{clade|style=font-size:90%;|label1='''Arthropoda'''|1={{clade
|label1=[[Biology:Chelicerata|Chelicerata]]|1={{clade
    |1='''[[Biology:Pycnogonida|Pycnogonida]]''' 
    |label2=Euchelicerata|2={{clade
      |1='''[[Biology:Xiphosura|Xiphosura]]'''
      |2='''[[Biology:Arachnida|Arachnida]]'''
    }}
  }}
|label2=[[Biology:Mandibulata|Mandibulata]]|2={{clade
|label1=Myriapoda|1={{clade
  |1='''[[Biology:Chilopoda|Chilopoda]]'''
  |label2=Progoneata|2={{clade
    |label1=Edafopoda|1={{clade
      |1='''[[Biology:Symphyla|Symphyla]]'''
      |2='''[[Biology:Pauropoda|Pauropoda]]'''
    }}
    |2='''[[Biology:Diplopoda|Diplopoda]]'''
  }}
}}
|label2=[[Biology:Pancrustacea|Pancrustacea]]|2={{clade
|label1=[[Biology:Oligostraca|Oligostraca]]|barbegin1=#bd59a5|1={{clade
  |1='''[[Biology:Ostracoda|Ostracoda]]'''
  |2={{clade
    |1='''[[Biology:Mystacocarida|Mystacocarida]]'''
    |label2=Ichthyostraca|2={{clade
      |1='''[[Biology:Branchiura|Branchiura]]'''
      |2='''[[Biology:Pentastomida|Pentastomida]]'''
    }}
  }}
}}
|label2=Altocrustacea|2={{clade
|label1=[[Biology:Multicrustacea|Multicrustacea]]|bar1=#bd59a5|1={{clade
  |label1=[[Biology:Hexanauplia|Hexanauplia]]|1={{clade
    |1='''Copepoda'''
    |2={{clade
      |1='''[[Biology:Tantulocarida|Tantulocarida]]'''
      |2='''[[Biology:Thecostraca|Thecostraca]]'''
    }}
  }}
  |2='''[[Biology:Malacostraca|Malacostraca]]'''
}}
|label2=Allotriocarida|2={{clade
|1='''[[Biology:Cephalocarida|Cephalocarida]]'''|bar1=#bd59a5
|label2=Athalassocarida|sublabel2={{small|493 mya}}|2={{clade
|1='''[[Biology:Branchiopoda|Branchiopoda]]'''|bar1=#bd59a5
|label2=Labiocarida|2={{clade
|1='''[[Biology:Remipedia|Remipedia]]'''|barend1=#bd59a5
|label2=[[Biology:Hexapoda|Hexapoda]]|sublabel2={{small|470 mya}}|2={{clade
  |label1=Elliplura|barbegin1=#edcb58|1={{clade
    |1='''Collembola'''
    |2='''[[Biology:Protura|Protura]]'''
  }}
  |label2=Cercophora|2={{clade
    |1='''[[Biology:Diplura|Diplura]]'''|barend1=#edcb58
    |2='''[[Biology:Insecta|Insecta]]'''|sublabel2={{small|440 mya}}
}}}}
}}}}
}}}}
}}}}
}}
|grouplabel1={{clade labels|label1=Crustaceans|top1=47%|label2=Entognaths|top2=87%}}
}}

{|class="wikitable"
! Subphyla !! Classes !! Members !! Example species 
|-
| '''[[Biology:Chelicerata|Chelicerata]]''' || [[Biology:Pycnogonida|Pycnogonida]]<br/>[[Biology:Xiphosura|Xiphosura]]<br/>Arachnida || [[Biology:Sea spider|sea spider]]s<br/>[[Biology:Horseshoe crab|horseshoe crab]]s<br/>harvestmen, solifuges, [[Biology:Mite|mite]]s, scorpions, [[Biology:Spider|spider]]s, [[Biology:Tick|tick]]s etc. || [[File:PlatycryptusUndatusFemale.jpg|160px|alt=]]<br/>''Platycryptus undatus''<br/>(Arachnida, [[Biology:Araneae|Araneae]])
|-
| '''Myriapoda''' || [[Biology:Symphyla|Symphyla]]<br/>[[Biology:Pauropoda|Pauropoda]]<br/>Diplopoda<br/>[[Biology:Chilopoda|Chilopoda]] || [[Biology:Symphyla|pseudocentipede]]s<br/>[[Biology:Hexamerocerata|hexamerocerata]]ns, tetrameroceratans<br/>[[Biology:Polyxenida|bristle millipede]]s, [[Biology:Oniscomorpha|pill millipede]]s, [[Biology:Polydesmida|flat-backed millipede]]s, etc.<br/>[[Biology:Scutigeromorpha|scutigeromorph]]s, [[Biology:Lithobiomorpha|lithobiomorph]]s, [[Biology:Scolopendromorpha|Scolopendromorph]]s, etc. || [[File:Archispirostreptus-Gigas-Amphitheatre.jpg|160px|alt=]]<br/>''Archispirostreptus gigas''<br/>(Diplopoda, [[Biology:Spirostreptida|Spirostreptida]]) 
|-
| '''[[Biology:Crustacea|Crustacea]]''' || [[Biology:Ostracoda|Ostracoda]]<br/>[[Biology:Mystacocarida|Mystacocarida]]<br/>[[Biology:Pentastomida|Pentastomida]]<br/>[[Biology:Branchiura|Branchiura]]<br/>[[Biology:Thecostraca|Thecostraca]]<br/>Copepoda<br/>Malacostraca<br/>[[Biology:Cephalocarida|Cephalocarida]]<br/>[[Biology:Branchiopoda|Branchiopoda]]<br/>[[Biology:Remipedia|Remipedia]]<br/> || [[Biology:Ostracoda|seed shrimp]]<br/>[[Biology:Mystacocarida|Mystacocarida]]ns<br/>[[Biology:Pentastomida|tongue worm]]s<br/>fish lice<br/>[[Biology:Barnacle|barnacle]]s, etc.<br/>calanoids, [[Biology:Cyclopoida|cyclopoids]], misophrioids, [[Biology:Siphonostomatoida|siphonostomatoids]], etc.<br/>[[Biology:Mantis shrimp|mantis shrimp]], skeleton shrimp, woodlice, shrimp, [[Biology:Crab|crab]]s, [[Biology:Lobster|lobster]]s, hrill, etc.<br/>[[Biology:Cephalocarida|horseshoe shrimp]]<br/>[[Biology:Fairy shrimp|fairy shrimp]], tadpole shrimp, water fleas, [[Biology:Clam shrimp|clam shrimp]]<br/>[[Biology:Remipedia|remipede]]s<br/>|| [[File:Ocypode-ceratophthalma-horned-ghost-crab-krabi-thailand.jpg|160px|alt=]]<br/>''Ocypode ceratophthalma''<br/>(Malacostraca, [[Biology:Decapoda|Decapoda]]) 
|-
| '''[[Biology:Hexapoda|Hexapoda]]''' || Insecta<br/>[[Biology:Entognatha|Entognatha]] || insects<br/>springtails, etc. || [[File:Saturnia_pavonia_01.jpg|160px|alt=]]<br/>''Saturnia pavonia''<br/>(Insecta, [[Biology:Lepidoptera|Lepidoptera]])
|}

== Interaction with humans ==


[[File:Insect food stall.JPG|thumb|Insects and scorpions on sale in a food stall in [[Place:Bangkok|Bangkok]], Thailand]]

[[Biology:Crustacean|Crustacean]]s such as [[Biology:Crab|crab]]s, [[Biology:Lobster|lobster]]s, crayfish, shrimp, and prawns have long been part of human cuisine, and are now raised commercially.<ref>{{cite book |author1=Wickins, J. F. |author2=Lee, D. O'C. |name-list-style=amp |title=Crustacean Farming: Ranching and Culture |edition=2nd |year=2002 |isbn=978-0-632-05464-0 |publisher=Blackwell |url=http://www.blackwellpublishing.com/book.asp?ref=9780632054640 |access-date=2008-10-03 |archive-url=https://web.archive.org/web/20081206015234/http://www.blackwellpublishing.com/book.asp?ref=9780632054640 |archive-date=2008-12-06 |url-status=live}}</ref> Insects and their grubs are at least as nutritious as meat, and are eaten both raw and cooked in many cultures, though not most European, Hindu, and Islamic cultures.<ref>{{citation |url=http://www.uky.edu/Ag/Entomology/ythfacts/bugfood/bugfood2.htm |access-date=2008-10-03 |title=Bugfood II: Insects as Food!?! |publisher=University of Kentucky Department of Entomology |author=Bailey, S. |archive-url=https://web.archive.org/web/20081216224319/http://www.uky.edu/Ag/Entomology/ythfacts/bugfood/bugfood2.htm |archive-date=2008-12-16 |url-status=live}}</ref><ref>{{citation |url=http://www.uky.edu/Ag/Entomology/ythfacts/bugfood/yf813.htm |access-date=2008-10-03 |title=Bugfood III: Insect Snacks from Around the World |publisher=University of Kentucky Department of Entomology |author=Unger, L.| archive-url= https://web.archive.org/web/20081010124417/http://www.uky.edu/Ag/Entomology/ythfacts/bugfood/yf813.htm| archive-date= 10 October 2008 | url-status= live}}</ref> Cooked [[Biology:Tarantula|tarantula]]s are considered a delicacy in Cambodia,<ref>{{citation |last=Rigby |first=R. |date=September 21, 2002 |title=Tuck into a Tarantula |work=Sunday Telegraph |url=http://www.rhymer.net/cutsE.htm |access-date=2009-08-24 |archive-url=https://web.archive.org/web/20090718173613/http://www.rhymer.net/cutsE.htm |archive-date=July 18, 2009 |url-status=live}}</ref><ref>{{citation |url=http://abc.net.au/news/indepth/featureitems/s664704.htm |archive-url=https://web.archive.org/web/20080603171942/http://www.abc.net.au/news/indepth/featureitems/s664704.htm| archive-date=June 3, 2008 |title=Spiderwomen serve up Cambodia's creepy caviar |date=September 2, 2002 |work=ABC News Online |access-date=2009-08-24}}</ref><ref>{{cite book |last=Ray |first=N. |year=2002 |title=Lonely Planet Cambodia |publisher=Lonely Planet Publications |isbn=978-1-74059-111-9 |page=308}}</ref> and by the  Piaroa Indians of southern Venezuela, after the highly irritant hairs – the spider's main defense system – are removed.<ref>{{citation |last=Weil |first=C. |title=Fierce Food |year=2006 |publisher=Plume |isbn=978-0-452-28700-6 |url=https://archive.org/details/fiercefoodintrep0000weil |access-date=2008-10-03 |archive-url=https://web.archive.org/web/20110511192407/http://www.budgettravel.com/bt-dyn/content/article/2006/10/24/AR2006102400797.html |archive-date=2011-05-11}}</ref> Humans also [[Biology:Entomophagy#Unintentional ingestion|unintentionally eat]] arthropods in other foods,<ref>{{Citation |last=Taylor |first=R. L. |year=1975 |title=Butterflies in My Stomach (or: Insects in Human Nutrition) |publisher=Woodbridge Press Publishing Company, Santa Barbara, California}}</ref> and food safety regulations lay down acceptable contamination levels for different kinds of food material.{{refn|For a mention of insect contamination in an international food quality standard, see sections 3.1.2 and 3.1.3 of Codex 152 of 1985 of the [[Finance:Codex Alimentarius|Codex Alimentarius]]<ref>{{citation |url=http://www.codexalimentarius.net/download/standards/50/CXS_152e.pdf |title=Codex Standard 152 of 1985 (on "Wheat Flour") |author=Codex commission for food hygiene |year=1985 |work=Codex Alimentarius |publisher=Food and Agriculture Organization |access-date=2010-05-08 |archive-url=https://web.archive.org/web/20101231033646/http://www.codexalimentarius.net/download/standards/50/CXS_152e.pdf |archive-date=2010-12-31 |url-status=live}}.</ref>|group=Note}}{{refn|For examples of quantified acceptable insect contamination levels in food see the last entry (on "Wheat Flour") and the definition of "Extraneous material" in ''Codex Alimentarius'',<ref>{{citation |url=http://www.codexalimentarius.net/web/standard_list.do?lang=en |title=Complete list of Official Standards |work=Codex Alimentarius |publisher=Food and Agriculture Organization |access-date=2010-05-08 |archive-url=https://web.archive.org/web/20100131140203/http://www.codexalimentarius.net/web/standard_list.do?lang=en |archive-date=2010-01-31}}</ref> and the standards published by the FDA.<ref>{{citation |publisher=[[Organization:Food and Drug Administration|U. S. Food and Drug Administration]] |url=http://www.cfsan.fda.gov/~dms/dalbook.html |title=The Food Defect Action Levels |access-date=2006-12-16| archive-url=https://web.archive.org/web/20061218113700/http://www.cfsan.fda.gov/~dms/dalbook.html| archive-date=18 December 2006 | url-status=live}}</ref>|group=Note}} The intentional cultivation of arthropods and other small animals for human food, referred to as minilivestock, is now emerging in animal husbandry as an ecologically sound concept.<ref>{{Citation |last=Paoletti|first=M. G. |title=Ecological implications of minilivestock: potential of insects, rodents, frogs, and snails |publisher=Science Publishers |year=2005 |pages=648 |isbn=978-1-57808-339-8 |url=https://books.google.com/books?id=u4eTQgAACAAJ}}</ref> Commercial butterfly breeding provides Lepidoptera stock to butterfly conservatories, educational exhibits, schools, research facilities, and cultural events.

However, the greatest contribution of arthropods to human food supply is by [[Biology:Pollination|pollination]]: a 2008 study examined the 100&nbsp;crops that FAO lists as grown for food, and estimated pollination's economic value as €153&nbsp;billion, or 9.5 per cent of the value of world agricultural production used for human food in 2005.<ref>{{cite journal |author1=Gallai, N. |author2=Salles, J.-M. |author3=Settele, J. |author4=Vaissière, B. E. |title=Economic valuation of the vulnerability of world agriculture confronted with pollinator decline |journal=Ecological Economics |volume=68 |pages=810–821 |date=August 2008 |doi=10.1016/j.ecolecon.2008.06.014 |issue=3 |s2cid=54818498 |url=https://halshs.archives-ouvertes.fr/halshs-01293686/file/Gallai%20et%20al.%202009%20Ecological%20Economics%20Economic%20Valiuation%20of%20Poll.pdf |access-date=2018-11-24 |archive-url=https://web.archive.org/web/20190427184211/https://halshs.archives-ouvertes.fr/halshs-01293686/file/Gallai%20et%20al.%202009%20Ecological%20Economics%20Economic%20Valiuation%20of%20Poll.pdf |archive-date=2019-04-27 |url-status=live}} Free summary at {{cite journal |last1=Gallai |first1=N. |last2=Salles |first2=J. |last3=Settele |first3=J. |last4=Vaissiere |first4=B. |title=Economic value of insect pollination worldwide estimated at 153{{nbsp}}billion euros |url=http://www.eurekalert.org/pub_releases/2008-09/haog-evo091508.php |access-date=2008-10-03 |journal=Ecological Economics |volume=68 |issue=3 |pages=810–821 |year=2009 |doi=10.1016/j.ecolecon.2008.06.014 |s2cid=54818498 |archive-url=https://web.archive.org/web/20081203135146/http://www.eurekalert.org/pub_releases/2008-09/haog-evo091508.php |archive-date=2008-12-03 |url-status=live}}</ref> Besides pollinating, [[Biology:Bee|bee]]s produce [[Chemistry:Honey|honey]], which is the basis of a rapidly growing industry and international trade.<ref>{{citation |title=Apiservices&nbsp;— International honey market&nbsp;— World honey production, imports & exports |url=http://www.beekeeping.com/databases/honey-market/world_honey.htm |access-date=2008-10-03 |archive-url=https://web.archive.org/web/20081206043600/http://www.beekeeping.com/databases/honey-market/world_honey.htm |archive-date=2008-12-06 |url-status=live}}</ref>

The red dye [[Biology:Cochineal|cochineal]], produced from a Central American species of insect, was economically important to the Aztecs and Mayans.<ref>{{citation |title=Time line of fabrics |url=http://threadsintyme.tripod.com/id63.htm |publisher=Threads In Tyme, LTD |archive-url=https://web.archive.org/web/20051028155009/http://threadsintyme.tripod.com/id63.htm |archive-date=October 28, 2005 |access-date=2005-07-14}}</ref> While the region was under {{wipe|Spain}} control, it became Mexico's second most-lucrative export,<ref>{{citation |author=Jeff Behan |title=The bug that changed history |url=http://www.gcrg.org/bqr/8-2/bug.htm |access-date=2006-06-26 |archive-url=https://web.archive.org/web/20060621103349/http://gcrg.org/bqr/8-2/bug.htm |archive-date=21 June 2006}}</ref> and is now regaining some of the ground it lost to synthetic competitors.<ref>{{citation |title=Canary Islands cochineal producers homepage |url=http://www.arrakis.es/~rpdeblas/cochinea.htm |access-date=2005-07-14 |archive-url=https://web.archive.org/web/20050624075803/http://www.arrakis.es/~rpdeblas/cochinea.htm |archive-date=24 June 2005}}</ref> [[Chemistry:Shellac|Shellac]], a resin secreted by a species of insect native to southern Asia, was historically used  in great quantities for many applications in which it has mostly been replaced by synthetic resins, but it is still used in [[Engineering:Woodworking|woodworking]] and as a [[Chemistry:Food additive|food additive]]. The blood of horseshoe crabs contains a clotting agent, Limulus Amebocyte Lysate, which is now used to test that [[Chemistry:Antibiotic|antibiotics]] and kidney machines are free of dangerous [[Biology:Bacteria|bacteria]], and to detect spinal meningitis and some [[Medicine:Cancer|cancer]]s.<ref>{{citation |title=Coast |author=Heard, W. |publisher=[[Organization:University of South Florida|University of South Florida]] |url=http://www.marine.usf.edu/pjocean/packets/f01/f01u5p3.pdf |access-date=2008-08-25 |archive-url=https://web.archive.org/web/20170219095700/http://www.marine.usf.edu/pjocean/packets/f01/f01u5p3.pdf |archive-date=2017-02-19}}</ref> [[Biology:Forensic entomology|Forensic entomology]] uses evidence provided by arthropods to establish the time and sometimes the place of death of a human, and in some cases the cause.<ref>{{Citation |last1=Hall |first1=R. D. |last2=Castner |first2=J. L. |title=Forensic Entomology: the Utility of Arthropods in Legal Investigations |editor=Byrd, J. H. |editor2=Castner, J. L. |publisher=CRC Press |year=2000 |pages=[https://archive.org/details/forensicentomolo0000unse/page/3 3–4] |chapter=Introduction |isbn=978-0-8493-8120-1 |chapter-url=https://books.google.com/books?id=iAtRGA2IfcwC&pg=PA3 |url=https://archive.org/details/forensicentomolo0000unse/page/3}}</ref> Recently insects have also gained attention as potential sources of drugs and other medicinal substances.<ref>{{cite journal|last=Dossey |first=Aaron |title=Insects and their chemical weaponry: New potential for drug discovery |journal=Natural Product Reports |date=December 2010 |volume=27 |issue=12 |pages=1737–1757 |doi=10.1039/C005319H |pmid=20957283}}</ref>

The relative simplicity of the arthropods' body plan, allowing them to move on a variety of surfaces both on land and in water, have made them useful as models for [[Engineering:Robot|robot]]ics. The redundancy provided by segments allows arthropods and biomimetic robots to move normally even with damaged or lost appendages.<ref>{{cite journal |author=Spagna, J. C. |author2=Goldman D. I. |author3=Lin P.-C. |author4=Koditschek D. E. |author5=R. J. Full |date=March 2007 |title=Distributed mechanical feedback in arthropods and robots simplifies control of rapid running on challenging terrain |journal=Bioinspiration & Biomimetics |volume=2 |issue=1 |pages=9–18 |doi=10.1088/1748-3182/2/1/002 |url=http://polypedal.berkeley.edu/twiki/pub/PolyPEDAL/PolypedalPublications/Distributed_BB.pdf |pmid=17671322 |archive-url=https://web.archive.org/web/20120310042430/http://polypedal.berkeley.edu/twiki/pub/PolyPEDAL/PolypedalPublications/Distributed_BB.pdf |archive-date=2012-03-10 |bibcode=2007BiBi....2....9S |s2cid=21564918}}</ref><ref>{{Citation |title=Adaptive Motion of Animals and Machines |chapter=A Turning Strategy of a Multi-legged Locomotion Robot |pages=227–236 |year=2006 |author1=Kazuo Tsuchiya |author2=Shinya Aoi |author3=Katsuyoshi Tsujita |name-list-style=amp |doi=10.1007/4-431-31381-8_20 |isbn=978-4-431-24164-5 |citeseerx=10.1.1.573.1846}}</ref>

{| class="wikitable" style="float:right; margin-left:5px"
|+ Diseases transmitted by insects
! Disease<ref name="Hill1997EconomicImportanceOfInsectsP7To92" /> !! Insect !! Cases per year !! Deaths per year
|-
| [[Medicine:Malaria|Malaria]] || ''[[Biology:Anopheles|Anopheles]]'' mosquito || align="center" |267 M || align="center" |1 to 2 M
|-
| [[Medicine:Dengue fever|Dengue fever]] || ''[[Biology:Aedes|Aedes]]'' mosquito || align="center" | ? || align="center" | ?
|-
| [[Medicine:Yellow fever|Yellow fever]] || ''[[Biology:Aedes|Aedes]]'' mosquito || align="center" |4,432 || align="center" |1,177
|-
| [[Medicine:Filariasis|Filariasis]] || ''[[Biology:Culex|Culex]]'' mosquito || align="center" |250 M || align="center" |unknown
|}

Although arthropods are the most numerous phylum on Earth, and thousands of arthropod species are venomous, they inflict relatively few serious bites and stings on humans. Far more serious are the effects on humans of diseases like [[Medicine:Malaria|malaria]] carried by blood-sucking insects. Other blood-sucking insects infect livestock with diseases that kill many animals and greatly reduce the usefulness of others.<ref name="Hill1997EconomicImportanceOfInsectsP7To92">{{citation |author=Hill, D. |year=1997 |title=The Economic Importance of Insects |pages=77–92 |publisher=Springer |isbn=978-0-412-49800-8}}</ref> [[Biology:Tick|Tick]]s can cause [[Medicine:Tick paralysis|tick paralysis]] and several [[Biology:Parasite|parasite]]-borne diseases in humans.<ref>{{Citation |last1=Goodman |first1=Jesse L. |last2=Dennis |first2=David Tappen |last3=Sonenshine |first3=Daniel E. |title=Tick-borne diseases of humans |url=https://books.google.com/books?id=dKlUARLKT9IC |access-date=2010-03-29 |year=2005 |publisher=ASM Press |isbn=978-1-55581-238-6 |page=114}}</ref> A few of the closely related [[Biology:Mite|mite]]s also infest humans, causing intense itching,<ref>{{citation |title=Parasitic Mites of Humans |author=Potter, M. F. |publisher=University of Kentucky College of Agriculture |url=http://www.ca.uky.edu/entomology/entfacts/ef637.asp |access-date=2008-10-25|url-status=live|archive-date=2009-01-08 |archive-url=https://web.archive.org/web/20090108202742/http://www.ca.uky.edu/entomology/entfacts/ef637.asp}}</ref> and others cause [[Medicine:Allergy|allergic]] diseases, including hay fever, [[Medicine:Asthma|asthma]], and eczema.<ref>{{citation |author1=Klenerman, Paul |author2=Lipworth, Brian |title=House dust mite allergy |url=http://www.netdoctor.co.uk/health_advice/facts/allergyhousedustmite.htm |publisher=NetDoctor |access-date=2008-02-20|archive-url=https://web.archive.org/web/20080211110450/http://www.netdoctor.co.uk/health_advice/facts/allergyhousedustmite.htm| archive-date= 11 February 2008 |url-status=live}}</ref>

Many species of arthropods, principally insects but also mites, are agricultural and forest pests.<ref name="IPM">{{citation |title=Ecological Entomology |last1=Kogan |first1=M. |last2=Croft |first2=B. A. |last3=Sutherst |first3=R. F. |editor1-last= Huffaker |editor1-first=Carl B. |editor2-last=Gutierrez |editor2-first=A. P. |year=1999 |publisher=John Wiley & Sons |isbn=978-0-471-24483-7 |pages=681–736 |chapter-url=https://books.google.com/books?id=aw5Iycas70cC&pg=PA681 |chapter=Applications of ecology for integrated pest management}}</ref><ref>{{citation |title=Insect and Mite Pests in Food: An Illustrated Key |work=Agriculture Handbook Number 655 |last=Gorham |first=J. Richard |year=1991 |publisher=United States Department of Agriculture |pages=1–767 |url=http://www.afpmb.org/pubs/tims/tg27/docs/Insect%20and%20Mite%20Pests%20in%20Food%20Gorham.pdf|access-date=2010-05-06|archive-url=https://web.archive.org/web/20071025222603/http://www.afpmb.org/pubs/tims/tg27/docs/Insect%20and%20Mite%20Pests%20in%20Food%20Gorham.pdf |archive-date=October 25, 2007}}</ref> The mite ''[[Biology:Varroa destructor|Varroa destructor]]'' has become the largest single problem faced by beekeepers worldwide.<ref>{{cite journal |author1=Jong, D. D. |author2=Morse, R. A. |author3=Eickwort, G. C. |name-list-style=amp |title=Mite Pests of Honey Bees |journal=Annual Review of Entomology |volume=27 |pages=229–252 |date=January 1982 |doi=10.1146/annurev.en.27.010182.001305}}</ref> Efforts to control arthropod pests by large-scale use of [[Biology:Pesticide|pesticide]]s have caused long-term effects on human health and on [[Biology:Biodiversity|biodiversity]].<ref>{{Citation |title=Introduction to insect pest management |last1=Metcalf |first1=Robert Lee |last2=Luckmann |first2= William Henry|year=1994 |publisher=Wiley-IEEE |isbn=978-0-471-58957-0 |page=4 |url=https://books.google.com/books?id=pW1dXL2EgnMC}}</ref> Increasing arthropod [[Biology:Pesticide resistance|resistance]] to pesticides has led to the development of [[Earth:Integrated pest management|integrated pest management]] using a wide range of measures including biological control.<ref name="IPM"/> Predatory mites may be useful in controlling some mite pests.<ref name="Shultz2001ArachnidsInEncOfLifeSci">{{citation |contribution=Chelicerata (Arachnids, Including Spiders, Mites and Scorpions) |author=Shultz, J. W. |title=Encyclopedia of Life Sciences |year=2001 |publisher= John Wiley & Sons, Ltd. |doi=10.1038/npg.els.0001605 |isbn=978-0-470-01617-6}}</ref><ref>{{citation |author=Osakabe, M. |title=Which predatory mite can control both a dominant mite pest, ''Tetranychus urticae'', and a latent mite pest, ''Eotetranychus asiaticus'', on strawberry? |journal=Experimental and Applied Acarology |date=March 2002| volume=26 |issue=3–4 |pages=219–230 |pmid=12542009 |s2cid=10823576 |doi=10.1023/A:1021116121604}}</ref>

== See also ==

* [[Biology:Dorsal lobe|Dorsal lobe]]
* Invertebrate paleontology
* Minibeasts

==Notes==

{{Reflist|group=Note}}

==References==

{{Reflist}}

===Bibliography===

* {{cite book |last=Gould |first=S. J. |year=1990 |title=Wonderful Life: The Burgess Shale and the Nature of History |publisher=Hutchinson Radius |isbn=978-0-09-174271-3 |title-link=Wonderful Life: The Burgess Shale and the Nature of History |bibcode=1989wlbs.book.....G}}
* {{cite book |last=Ruppert |first=E. E. |author2=R. S. Fox |author3=R. D. Barnes |year=2004 |title=Invertebrate Zoology |publisher=Brooks/Cole |edition=7th |isbn=978-0-03-025982-1 |ref=refRuppert |url=https://archive.org/details/isbn_9780030259821}}

==External links==


* [http://entomology.ifas.ufl.edu/fasulo/vector/chapter_07.htm Venomous Arthropods] chapter in United States Environmental Protection Agency and [[Organization:University of Florida|University of Florida]]/Institute of Food and Agricultural Sciences National Public Health Pesticide Applicator Training Manual
* [https://web.archive.org/web/20170821005914/http://www.insectlifeforms.com/Phylums488/JOINTED_LEGGED_ANIMALS_ARTHROPODA_6050_488.aspx Arthropods – Arthropoda] Insect Life Forms

{{Animalia}}
{{Arthropods}}
{{Life on Earth}}
{{Taxonbar|from=Q1360}}

[[Category:Arthropods| ]]
[[Category:Animal phyla]]
[[Category:Extant Cambrian first appearances]]

{{Sourceattribution|Arthropod}}