Editing Chelicerata

{{Short description|Subphylum of arthropods}}
{{Good article}}
{{Automatic taxobox
| name = Chelicerata
| fossil_range = {{fossilrange|508|0|earliest=530|latest=0| [[Middle Cambrian]] – [[Holocene|Present]], 508–0 Mya}}<small>Possible [[Fortunian]] record</small>
| image = <imagemap> File:Chelicerata collage 2-3.png|286px
rect 1 1 400 300 [[Sea spider|Pycnogonida]]
rect 400 1 800 300 [[Xiphosura]]
rect 1 300 400 600 [[Eurypterid]]a
rect 400 300 800 600 [[Spider|Araneae]]
rect 1 600 400 900 [[Scorpion]]es
rect 400 600 800 900 [[Mite|Acari]]
</imagemap>
| image_caption = Left to right, top to bottom: ''[[Ammothea hilgendorfi]]'' (Pycnogonida), ''[[Atlantic horseshoe crab|Limulus polyphemus]]'' (Xiphosura), ''[[Eurypterus|Eurypterus remipes]]'' (Eurypterida), ''[[Araneus diadematus]]'' (Araneae), ''[[Buthus occitanus]]'' (Scorpiones), ''[[Trombidium holosericeum]]'' (Acari)
| taxon = Chelicerata
| authority = [[Richard Heymons|Heymons]], 1901
| subdivision_ranks = Groups
| subdivision = *†[[Habeliida]]
*†[[Mollisoniida]]
*†[[Megacheira]]?
*[[Pycnogonida]]
*'''Euchelicerata'''
**†[[Offacolidae]]
***''[[Offacolus]]''
***''[[Dibasterium]]''
***''[[Setapedites]]''
***''[[Bunaia]]''
**†''[[Weinbergina]]''
** [[Prosomapoda]]
| synonyms = *Cheliceriformes <small>Schram and Hedgpeth, 1978</small>
}}

The subphylum '''Chelicerata''' (from [[Neo-Latin]], {{etymology|fr|chélicère|}}, {{etymology|grc|''{{wikt-lang|grc|χηλή}}'' ({{grc-transl|χηλή}})|claw, [[chela (organ)|chela]]||''{{wikt-lang|grc|κέρας}}'' ({{grc-transl|κέρας}})|horn}})<ref>{{cite book |last1=Barnes|first1=R. S. K. |last2=Calow|first2=P. P. |last3=Olive|first3=P. J. W. |title=The Invertebrates: A Synthesis |publisher=[[Wiley (publisher)|John Wiley & Sons]] |year=2009 |edition=3rd |page=174 |isbn=978-1-4443-1233-1}}</ref> constitutes one of the major subdivisions of the phylum [[Arthropod]]a. '''Chelicerates''' include the [[sea spider]]s, [[horseshoe crab]]s, and [[arachnid]]s (including [[Opiliones|harvestmen]], [[scorpion]]s, [[spider]]s, [[Solifugae|solifuges]], [[Tick|ticks]], and [[Mite|mites]], among many others), as well as a number of extinct lineages, such as the [[eurypterid]]s (sea scorpions) and [[chasmataspidid]]s.

Chelicerata  split from [[Mandibulata]] by the mid-[[Cambrian]], as evidenced by [[stem-group]] chelicerates like [[Habeliida]] and ''[[Mollisonia]]'' present by this time.<ref>{{cite journal |last1=Aria |first1=Cédric |last2=Caron |first2=Jean-Bernard |date=September 2019 |title=A middle Cambrian arthropod with chelicerae and proto-book gills |journal=Nature |volume=573 |issue=7775 |pages=586–589 |bibcode=2019Natur.573..586A |doi=10.1038/s41586-019-1525-4 |issn=0028-0836 |pmid=31511691 |s2cid=202550431 |lang=en}}</ref> The surviving marine species include the four species of [[xiphosura]]ns (horseshoe crabs), and possibly the 1,300&nbsp;species of [[Pycnogonida|pycnogonids]] (sea spiders), if the latter are indeed chelicerates. On the other hand, there are over 77,000&nbsp;well-identified species of air-breathing chelicerates, and there may be about 500,000&nbsp;unidentified species.

Like all [[arthropod]]s, chelicerates have [[Segmentation (biology)|segmented]] bodies with jointed limbs, all covered in a [[cuticle]] made of [[chitin]] and [[protein]]s. The chelicerate [[body plan]] consists of two [[Tagma (arthropod anatomy)|tagma]]ta, the [[prosoma]] and the [[opisthosoma]] – excepting the [[Acariformes|mites]], which have lost any visible division between these sections. The [[chelicerae]], which give the group its name, are the only [[appendage]]s that appear before the mouth. In most sub-groups, they are modest pincers used to feed. However, spiders' chelicerae form fangs that most species use to inject [[venom]] into prey. The group has the [[open circulatory system]] typical of arthropods, in which a tube-like heart pumps blood through the [[hemocoel]], which is the major body cavity. Marine chelicerates have gills, while the air-breathing forms generally have both [[book lung]]s and [[Invertebrate trachea|tracheae]]. In general, the [[ganglia]] of living chelicerates' [[central nervous system]]s fuse into large masses in the cephalothorax, but there are wide variations and this fusion is very limited in the [[Mesothelae]], which are regarded as the oldest and most basal group of spiders. Most chelicerates rely on modified [[setae|bristles]] for touch and for information about vibrations, air currents, and chemical changes in their environment. The most active hunting spiders also have very acute eyesight.

Chelicerates were originally predators, but the group has diversified to use all the major feeding strategies: predation, [[parasitism]], [[herbivory]], [[scavenger|scavenging]] and [[detritivory|eating decaying organic matter]]. Although [[harvestmen]] can digest solid food, the guts of most modern chelicerates are too narrow for this, and they generally liquidize their food by grinding it with their chelicerae and [[pedipalps]] and flooding it with digestive [[enzyme]]s. To conserve water, air-breathing chelicerates excrete waste as solids that are removed from their blood by [[Malpighian tubules]], structures that also [[convergent evolution|evolved independently]] in [[insect]]s.<ref name="GarwoodEdgecombe2011">{{cite journal|last1=Garwood|first1=Russell J.|last2=Edgecombe|first2=Gregory D.|title=Early Terrestrial Animals, Evolution, and Uncertainty|journal=Evolution: Education and Outreach|volume=4|issue=3|year=2011|pages=489–501|issn=1936-6426|doi=10.1007/s12052-011-0357-y|doi-access=free}}</ref>

While the marine horseshoe crabs rely on [[external fertilization]], air-breathing chelicerates use internal but usually indirect fertilization. Many species use elaborate [[courtship display|courtship]] rituals to attract mates. Most lay eggs that hatch as what look like miniature adults, but all scorpions and a few species of mites keep the eggs inside their bodies until the young emerge. In most chelicerate species the young have to fend for themselves, but in scorpions and some species of spider the females protect and feed their young.

The [[evolution]]ary origins of chelicerates from the early arthropods have been debated for decades. Although there is considerable agreement about the relationships between most chelicerate sub-groups, the inclusion of the Pycnogonida in this taxon has been questioned, and the exact position of scorpions is still controversial, though they were long considered the most basal of the arachnids.<ref name="Margulis & Schwartz">{{Citation | last1 = Margulis | first1 = Lynn | author-link = Lynn Margulis | last2 = Schwartz | first2 = Karlene | title = Five Kingdoms, An Illustrated Guide to the Phyla of Life on Earth | publisher = W.H. Freeman and Company | year = 1998 | edition = third | isbn = 978-0-7167-3027-9 | url = https://archive.org/details/fivekingdomsillu00marg_0 }}</ref>

Venom has evolved three times in the chelicerates; spiders, scorpions and pseudoscorpions, or four times if the hematophagous secretions produced by ticks are included. In addition there have been unverified descriptions of venom glands in Solifugae.<ref>{{cite journal| pmc=4280546 | pmid=25533518 | doi=10.3390/toxins6123488 | volume=6 | issue=12 | title=Quo vadis venomics? A roadmap to neglected venomous invertebrates | year=2014 | journal=Toxins (Basel) | pages=3488–551 |vauthors=von Reumont BM, Campbell LI, Jenner RA| doi-access=free }}</ref> [[Chemical defenses#Invertebrates|Chemical defense]] has been found in [[Uropygi|whip scorpions]], [[Schizomida|shorttailed whipscorpions]], [[Opiliones|harvestmen]], [[Oribatida|beetle mites]] and [[sea spider]]s.<ref>{{Cite journal |url=https://www.researchgate.net/publication/258638033 |doi=10.1007/BF02059872 |title=Ecdysteroids fromPycnogonum litorale (Arthropoda, Pantopoda) act as chemical defense againstCarcinus maenas (Crustacea, Decapoda) |year=1994 |last1=Tomaschko |first1=K-H |journal=Journal of Chemical Ecology |volume=20 |issue=7 |pages=1445–1455 |pmid=24242643 |bibcode=1994JCEco..20.1445T |s2cid=196623820 }}</ref><ref>{{cite book|chapter-url = https://books.google.com/books?id=pbdpSKHkKDIC&pg=PA382|title =Harvestmen: The Biology of Opiliones|page = 382|chapter = Defense Mechanisms|first1= Pedro|last1 = Gnaspini|first2= Marcos R.|last2= Hara|editor1-first =Ricardo |editor1-last =Pinto-da-Rocha|editor2-first= Glauco|editor2-last = Machado|editor3-first = Gonzalo|editor3-last = Giribet|publisher = Harvard University Press|date = 2007|isbn = 9780674023437}}</ref><ref>{{cite journal| pmid=21898169 | doi=10.1007/s10886-011-0009-2 | volume=37 | issue=9 | title=Tasty but protected--first evidence of chemical defense in oribatid mites | year=2011 | journal=J Chem Ecol | pages=1037–43 |vauthors=Heethoff M, Koerner L, Norton RA, Raspotnig G| bibcode=2011JCEco..37.1037H | s2cid=23628645 }}</ref>

Although the venom of a few spider and scorpion species can be very dangerous to humans, medical researchers are investigating the use of these venoms for the treatment of disorders ranging from [[cancer]] to [[erectile dysfunction]]. The medical industry also uses the blood of horseshoe crabs as a test for the presence of contaminant [[bacteria]]. Mites can cause [[allergies]] in humans, transmit several diseases to humans and their [[livestock]], and are serious agricultural [[Pest (organism)|pest]]s.

==Description==
{{Annotated image/Arthropod head problem | float=right | caption=Formation of anterior segments across arthropod taxa based on previous hypothesis.<ref name="RuppertFoxBarnes2004P518" /> Note the antenna-bearing somite 1 was thought to be lost in Chelicerata.}}
[[File:20190913 Arthropoda head segments appendages en.png|thumb|600px|Formation of anterior segments across arthropod taxa based on gene expression and neuroanatomical observations,<ref name=":1" /><ref name=":3">{{Cite journal|last1=Ortega-Hernández|first1=Javier|last2=Janssen|first2=Ralf|last3=Budd|first3=Graham E.|date=2017-05-01|title=Origin and evolution of the panarthropod head – A palaeobiological and developmental perspective|journal=Arthropod Structure & Development|series=Evolution of Segmentation|volume=46|issue=3|pages=354–379|doi=10.1016/j.asd.2016.10.011|pmid=27989966|issn=1467-8039|doi-access=free|bibcode=2017ArtSD..46..354O }}</ref> Note the chelicera(Ch) and chelifore(Chf) arose from somite 1 and thus correspond to the first antenna(An/An1) of other arthropods.]]

===Segmentation and cuticle===
The Chelicerata are [[arthropod]]s as they have: [[Segmentation (biology)|segmented]] bodies with jointed limbs, all covered in a [[cuticle]] made of [[chitin]] and [[protein]]s; heads that are composed of several segments that fuse during the development of the [[embryo]]; a much reduced [[coelom]]; a [[hemocoel]] through which the [[blood]] circulates, driven by a tube-like heart.<ref name="RuppertFoxBarnes2004P518">{{harvnb|Ruppert|Fox|Barnes|2004|pp=518–522}}</ref> Chelicerates' bodies consist of two [[Tagma (arthropod anatomy)|tagmata]], sets of segments that serve similar functions: the foremost one, called the [[prosoma]] or [[cephalothorax]], and the rear tagma is called the [[opisthosoma]] or [[abdomen]].<ref name="RuppertFoxBarnes2004ChelicerataGen">{{harvnb|Ruppert|Fox|Barnes|2004|pp=554–555}}</ref> However, in the [[Acari]] (mites and ticks) there is no visible division between these sections.<ref name="RuppertFoxBarnes2004Acari" />

The [[prosoma]] is formed in the embryo by fusion of the ocular somite (referred as "acron" in previous literatures), which carries the eyes and [[Labrum (arthropod mouthpart)|labrum]],<ref name=":3" /> with six post-ocular segments (somite 1 to 6),<ref name=":1">{{Cite journal|last1=Dunlop|first1=Jason A.|last2=Lamsdell|first2=James C.|title=Segmentation and tagmosis in Chelicerata|url=https://www.academia.edu/28212892|journal=Arthropod Structure & Development|language=en|volume=46|issue=3|pages=395–418|issn=1467-8039|doi=10.1016/j.asd.2016.05.002|pmid=27240897|year=2017|bibcode=2017ArtSD..46..395D }}</ref> which all have paired appendages. It was previously thought that chelicerates had lost the antennae-bearing somite 1,<ref>{{cite book |author1=Willmer, P. |author2=Willmer, P.G. |title=Invertebrate Relationships: Patterns in animal evolution |publisher=Cambridge University Press |year=1990 |isbn=978-0-521-33712-0 |page=275 |via=Google Books |url=https://books.google.com/books?id=uRrfn_5UNUQC&q=chelicerate+mouth&pg=PA275 |access-date=2008-10-14 |df=dmy-all}}</ref> but later investigations reveal that it is retained and corresponds to a pair of [[chelicera]]e or chelifores,<ref name=":2">{{Cite journal|last1=Telford|first1=Maximilian J.|last2=Thomas|first2=Richard H.|date=1998-09-01|title=Expression of homeobox genes shows chelicerate arthropods retain their deutocerebral segment|journal=Proceedings of the National Academy of Sciences|language=en|volume=95|issue=18|pages=10671–10675|issn=0027-8424|pmid=9724762|pmc=27953|doi=10.1073/pnas.95.18.10671|bibcode=1998PNAS...9510671T|doi-access=free}}</ref> small appendages that often form [[pincer (biology)|pincer]]s. Somite 2 has a pair of [[pedipalp]]s that in most sub-groups perform sensory functions, while the remaining four [[cephalothorax]] segments (somite 4 to 6) have pairs of legs.<ref name=":1" /> In basal forms the ocular somite has a pair of [[compound eye]]s on the sides and four pigment-cup [[ocelli]] ("little eyes") in the middle.<ref name="RuppertFoxBarnes2004ChelicerataGen" /> The mouth is between somite 1 and 2 (chelicerae and pedipalps).

The [[opisthosoma]] consists of thirteen or fewer segments, may or may not end with a [[telson]].<ref name=":1" /> In some taxa such as [[scorpion]] and [[eurypterid]] the opisthosoma divided into two groups, ''mesosoma'' and ''metasoma''.<ref name=":1" /> The abdominal appendages of modern chelicerates are missing or heavily modified<ref name="RuppertFoxBarnes2004ChelicerataGen" /> – for example in [[spider]]s the remaining appendages form [[spinneret (spider)|spinnerets]] that extrude [[spider silk|silk]],<ref name="RuppertFoxBarnes2004Spiders">{{harvnb|Ruppert|Fox|Barnes|2004|pp=571–584}}</ref> while those of [[horseshoe crab]]s (Xiphosura) form [[gill]]s.<ref name="RuppertFoxBarnes2004Xiphosura">{{harvnb|Ruppert|Fox|Barnes|2004|pp=555–559}}</ref><ref name=":1" />

Like all arthropods, chelicerates' bodies and appendages are covered with a tough [[cuticle]] made mainly of chitin and chemically hardened proteins. Since this cannot stretch, the animals must [[molt]] to grow. In other words, they grow new but still soft cuticles, then cast off the old one and wait for the new one to harden. Until the new cuticle hardens the animals are defenseless and almost immobilized.<ref name="RuppertFoxBarnes2004ArthropodCuticle">{{harvnb|Ruppert|Fox|Barnes|2004|pp=521–525}}</ref>
{{Clear}}

===Chelicerae and pedipalps===
Chelicerae and pedipalps are the two pairs of appendages closest to the mouth; they vary widely in form and function and the consistent difference between them is their position in the embryo and corresponding neurons: [[chelicera]]e are deutocerebral and arise from somite 1, ahead of the mouth, while [[pedipalp]]s are tritocerebral and arise from somite 2, behind the mouth.<ref name="RuppertFoxBarnes2004ChelicerataGen" /><ref name=":1" /><ref name=":3" />

The chelicerae ("claw horns") that give the sub-phylum its name normally consist of three sections, and the claw is formed by the third section and a rigid extension of the second.<ref name="RuppertFoxBarnes2004ChelicerataGen" /><ref name="BraddyPoschmannTetlie2008GiantClaw">{{cite journal |author1=Braddy, S.J. |author2=Poschmann, M. Markus |author3=Tetlie, O.E. |name-list-style=amp |title=Giant claw reveals the largest ever arthropod |journal=Biology Letters |year=2008 |volume=4 
|issue=1 |pmid=18029297 |pages=106–109 |pmc=2412931 |doi=10.1098/rsbl.2007.0491 |df=dmy-all}}</ref> However, spiders' have only two sections, and the second forms a fang that folds away behind the first when not in use.<ref name="RuppertFoxBarnes2004Spiders" /> The relative sizes of chelicerae vary widely: those of some fossil [[eurypterid]]s and modern [[harvestmen]] form large claws that extended ahead of the body,<ref name="BraddyPoschmannTetlie2008GiantClaw" /> while [[scorpion]]s' are tiny pincers that are used in feeding and project only slightly in front of the head.<ref name="RuppertFoxBarnes2004Scorpions">{{harvnb|Ruppert|Fox|Barnes|2004|pp=565–569}}</ref>

In basal chelicerates, the pedipalps are unspecialized and subequal to the posterior pairs of walking legs.<ref name=":1" /> However, in sea spider and arachnids, the pedipalps are more or less specialized for sensory<ref name="RuppertFoxBarnes2004ChelicerataGen" /> or prey-catching function<ref name=":1" /> – for example scorpions have pincers<ref name="RuppertFoxBarnes2004Scorpions" /> and male spiders have bulbous tips that act as [[syringe]]s to inject [[sperm]] into the females' reproductive openings when mating.<ref name="RuppertFoxBarnes2004Spiders" />

{{Annotated image | float=right | caption={{{caption|Spider's main organs<ref name="RuppertFoxBarnes2004p571to584">{{cite book 
| author=Ruppert, E. E. | author2=Fox, R. S. | author3=Barnes, R. D. 
| name-list-style=amp | title=Invertebrate Zoology 
| publisher=Brooks / Cole | edition=7th | isbn=0030259827 | date=2004 | pages=571–584
}}</ref>}}} | image=Spider main organs labelled.png | width=350 | image-width=350 | height=520 
| annotations=
{{Annotation|5|260|<span style{{=}}"background-color:blue">&nbsp;&nbsp;&nbsp;</span> Nervous system}}
{{Annotation|175|260|<span style{{=}}"background-color:#207326">&nbsp;&nbsp;&nbsp;</span> Digestive & excretory}}
{{Annotation|191|280|system}}
{{Annotation|5|280|<span style{{=}}"background-color:#b51136">&nbsp;&nbsp;&nbsp;</span> Circulatory system}}
{{Annotation|175|300|<span style{{=}}"background-color:#f5816a">&nbsp;&nbsp;&nbsp;</span> Respiratory system}}
{{Annotation|5|300|<span style{{=}}"background-color:yellow">&nbsp;&nbsp;&nbsp;</span> Reproductive system}}
{{Annotation|5|325|&nbsp;&nbsp;1 [[Chelicera]]}}
{{Annotation|175|325|&nbsp;&nbsp;2 [[Venom]] [[gland]]}}
{{Annotation|5|345|&nbsp;&nbsp;3 [[Brain]]}}
{{Annotation|175|345|&nbsp;&nbsp;4 Pumping [[stomach]]}}
{{Annotation|5|365|&nbsp;&nbsp;5 Forward [[aorta]] branch}}
{{Annotation|175|365|&nbsp;&nbsp;6 Digestive [[cecum]]}}
{{Annotation|5|385|&nbsp;&nbsp;7 [[Heart]]}}
{{Annotation|175|385|&nbsp;&nbsp;8 Midgut}}
{{Annotation|5|405|&nbsp;&nbsp;9 [[Malphigian tubules]]}}
{{Annotation|175|405|10 Stercoral pocket}}
{{Annotation|5|425|11 Rear [[aorta]]}}
{{Annotation|175|425|12 [[Spinneret (spider)|Spinneret]]}}
{{Annotation|5|445|13 [[Silk]] [[gland]]}}
{{Annotation|175|445|14 [[invertebrate trachea|Trachea]]}}
{{Annotation|5|465|15 [[Ovary]] (female)}}
{{Annotation|175|465|16 [[Book lung]]}}
{{Annotation|5|485|17 [[Ventral nerve cord|Nerve cord]]}}
{{Annotation|175|485|18 Legs}}
{{Annotation|5|505|19 [[Pedipalp]]}}
}}

===Body cavities and circulatory systems===
As in all arthropods, the chelicerate body has a very small [[coelom]] restricted to small areas round the reproductive and excretory systems. The main body cavity is a [[hemocoel]] that runs most of the length of the body and through which blood flows, driven by a tubular heart that collects blood from the rear and pumps it forward. Although [[arteries]] direct the blood to specific parts of the body, they have open ends rather than joining directly to [[vein]]s, and chelicerates therefore have open [[circulatory system]]s as is typical for arthropods.<ref name="RuppertFoxBarnes2004ArthropodCirculatory">{{harvnb|Ruppert|Fox|Barnes|2004|pp=527–528}}</ref>

===Respiratory systems===
These depend on individual sub-groups' environments. Modern terrestrial chelicerates generally have both [[book lung]]s, which deliver oxygen and remove waste gases via the blood, and [[Invertebrate trachea|tracheae]], which do the same without using the blood as a transport system.<ref name="RuppertFoxBarnes2004ArachnidaGen">{{harvnb|Ruppert|Fox|Barnes|2004|pp=559–564}}</ref> The living [[horseshoe crab]]s are aquatic and have [[book gill]]s that lie in a horizontal plane. For a long time it was assumed that the extinct [[eurypterid]]s had gills, but the fossil evidence was ambiguous. However, a fossil of the {{convert|45|mm|in}} long eurypterid ''[[Onychopterella]]'', from the Late [[Ordovician]] period, has what appear to be four pairs of vertically oriented book gills whose internal structure is very similar to that of scorpions' book lungs.<ref name="BraddyAldridgeEtAl1999LamellateBookGills">{{citation |author1=Braddy, S.J. |author2=Aldridge, R.J. |author3=Gabbott, S.E. |author4=Theron, J.N. |name-list-style=amp |year=1999 |title=Lamellate book-gills in a late Ordovician eurypterid from the Soom Shale, South Africa: Support for a eurypterid-scorpion clade |journal=[[Lethaia]] |volume=32 |issue=1 |pages=72–74 |doi=10.1111/j.1502-3931.1999.tb00582.x|bibcode=1999Letha..32...72B }}</ref>

===Feeding and digestion===
The guts of most modern chelicerates are too narrow to take solid food.<ref name="RuppertFoxBarnes2004ArachnidaGen" /> All [[scorpion]]s and almost all [[spider]]s are [[predator]]s that "pre-process" food in preoral cavities formed by the [[chelicera]]e and the bases of the [[pedipalp]]s.<ref name="RuppertFoxBarnes2004Spiders" /><ref name="RuppertFoxBarnes2004Scorpions" /> However, one predominantly [[herbivore]] spider species is known,<ref name="MeehanOlsonCurry2008VegetarianJumpingSpider">{{cite conference |author1=Meehan, C.J. |author2=Olson, E.J. |author3=Curry, R.L. |date=21 August 2008 |title=Exploitation of the Pseudomyrmex–Acacia mutualism by a predominantly vegetarian jumping spider (''Bagheera kiplingi'') |conference=93rd ESA Annual Meeting |url=http://eco.confex.com/eco/2008/techprogram/P12401.HTM |access-date=2008-10-10 |df=dmy-all |archive-date=2019-12-01 |archive-url=https://web.archive.org/web/20191201153140/https://eco.confex.com/eco/2008/techprogram/P12401.HTM |url-status=dead }}</ref> and many supplement their diets with [[nectar]] and [[pollen]].<ref name="Jackson2001">{{citation |author=Jackson, R.R. |year=2001 |title=Jumping spiders (Araneae: Salticidae) that feed on nectar |journal=[[Journal of Zoology]] |volume=255 |pages=25–29 |url=http://xnelson.googlepages.com/Jacksonetal2001.pdf |doi=10.1017/S095283690100108X |display-authors=etal |access-date=2008-10-23 |archive-date=2009-03-18 |archive-url=https://web.archive.org/web/20090318020505/http://xnelson.googlepages.com/Jacksonetal2001.pdf |url-status=dead }}</ref> Many of the [[Acari]] (ticks and mites) are blood-sucking [[parasite]]s, but there are many predatory, herbivore and [[scavenger]] sub-groups. All the Acari have a retractable feeding assembly that consists of the chelicerae, pedipalps and parts of the [[exoskeleton]], and which forms a preoral cavity for pre-processing food.<ref name="RuppertFoxBarnes2004Acari">{{harvnb|Ruppert|Fox|Barnes|2004|pp=591–595}}</ref>

[[Harvestmen]] are among the minority of living chelicerates that can take solid food, and the group includes predators, herbivores and scavengers.<ref name="RuppertFoxBarnes2004Opiliones">{{harvnb|Ruppert|Fox|Barnes|2004|pp=588–590}}</ref> [[Horseshoe crab]]s are also capable of processing solid food, and use a distinctive feeding system. Claws at the tips of their legs grab small invertebrates and pass them to a food groove that runs from between the rearmost legs to the mouth, which is on the underside of the head and faces slightly backwards. The bases of the legs form toothed [[gnathobase]]s that both grind the food and push it towards the mouth.<ref name="RuppertFoxBarnes2004Xiphosura" /> This is how the earliest [[arthropod]]s are thought to have fed.<ref name="WonderfulLifeP105">{{cite book |title=Wonderful Life: The Burgess Shale and the Nature of History |location=New York, NY |publisher=W.W. Norton; Hutchinson Radius |page=105 |author=Gould, S.J. |author-link=Stephen Jay Gould |year=1990 |isbn=978-0-09-174271-3 |bibcode=1989wlbs.book.....G }}</ref>

===Excretion===
Horseshoe crabs convert [[nitrogen]]ous wastes to [[ammonia]] and dump it via their gills, and excrete other wastes as [[feces]] via the [[anus]]. They also have [[nephridia]] ("little kidneys"), which extract other wastes for excretion as [[urine]].<ref name="RuppertFoxBarnes2004Xiphosura" /> Ammonia is so toxic that it must be diluted rapidly with large quantities of water.<ref name="RuppertFoxBarnes2004P529To530">{{harvnb|Ruppert|Fox|Barnes|2004|pp=529–530}}</ref> Most terrestrial chelicerates cannot afford to use so much water and therefore convert nitrogenous wastes to other chemicals, which they excrete as dry matter. Extraction is by various combinations of nephridia and [[Malpighian tubules]]. The tubules filter wastes out of the blood and dump them into the hindgut as solids, a system that has evolved [[convergent evolution|independently]] in [[insect]]s and several groups of [[arachnid]]s.<ref name="RuppertFoxBarnes2004ArachnidaGen" />

===Nervous system===
{| align="right" class="wikitable" width="40%" style="margin-left:2px"
! &nbsp; !! Cephalothorax ganglia fused into brain !! Abdominal ganglia fused into brain
|- align="center"
! [[Horseshoe crabs]]
| All || First two segments only
|- align="center"
! [[Scorpion]]s
| All || None
|- align="center"
! [[Mesothelae]]
| First two pairs only || None
|- align="center"
! Other [[arachnid]]s
| All || All
|}
Chelicerate nervous systems are based on the standard arthropod model of a pair of [[Ventral nerve cord|nerve cord]]s, each with a [[ganglion]] per segment, and a [[arthropod brain|brain]] formed by fusion of the ganglia just behind the mouth with those ahead of it.<ref name="RuppertFoxBarnes2004P531To532">{{harvnb|Ruppert|Fox|Barnes|2004|pp=531–532}}</ref> If one assume that chelicerates lose the first segment, which bears [[Antenna (biology)|antenna]]e in other arthropods, chelicerate brains include only one pair of pre-oral ganglia instead of two.<ref name="RuppertFoxBarnes2004ChelicerataGen" /> However, there is evidence that the first segment is indeed available and bears the cheliceres.<ref>{{cite journal |author1=Mittmann, B. |author2=Scholtz, G. |year=2003 |title=Development of the nervous system in the "head" of Limulus polyphemus (Chelicerata: Xiphosura): Morphological evidence for a correspondence between the segments of the chelicerae and of the (first) antennae of Mandibulata |journal=[[Dev Genes Evol]] |volume=213 |issue=1 |pages=9–17 |doi=10.1007/s00427-002-0285-5 |pmid=12590348 |s2cid=13101102 |df=dmy-all}}</ref><ref name=":2" />

There is a notable but variable trend towards fusion of other ganglia into the brain. The brains of [[horseshoe crabs]] include all the ganglia of the [[prosoma]] plus those of the first two opisthosomal segments, while the other opisthosomal segments retain separate pairs of ganglia.<ref name="RuppertFoxBarnes2004Xiphosura" /> In most living [[arachnid]]s, except [[scorpion]]s if they are true arachnids, ''all'' the ganglia, including those that would normally be in the opisthosoma, are fused into a single mass in the prosoma and there are no ganglia in the opisthosoma.<ref name="RuppertFoxBarnes2004ArachnidaGen" /> However, in the [[Mesothelae]], which are regarded as the most basal living spiders, the ganglia of the opisthosoma and the rear part of the prosoma remain unfused,<ref name="CoddingtonLevi1991">{{cite journal |author1=Coddington, J.A. |author2=Levi, H.W. |s2cid=55647804 |year=1991 |title=Systematics and Evolution of Spiders (Araneae) |journal=[[Annu. Rev. Ecol. Syst.]] |volume=22 |pages=565–592 |doi=10.1146/annurev.es.22.110191.003025}}</ref> and in scorpions the ganglia of the cephalothorax are fused but the abdomen retains separate pairs of ganglia.<ref name="RuppertFoxBarnes2004ArachnidaGen" />

===Senses===
As with other arthropods, chelicerates' [[cuticle]]s 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, spiders and other arthropods have modified their cuticles into elaborate arrays of sensors. Various touch and vibration sensors, mostly bristles called [[seta]]e, respond to different levels of force, from strong contact to very weak air currents. Chemical sensors provide equivalents of [[taste]] and [[Olfaction|smell]], often by means of setae.<ref name="RuppertFoxBarnes2004P532To537">{{harvnb|Ruppert|Fox|Barnes|2004|pp=532–537}}</ref>

Living chelicerates have both [[compound eye]]s (only in [[horseshoe crab]]s, as the compound eye in the other clades has been reduced to a cluster of no more than five pairs of [[ocelli]]), mounted on the sides of the head, plus pigment-cup ocelli ("little eyes"), mounted in the middle. These median ocelli-type eyes in chelicerates are assumed to be [[Homology (biology)|homologous]] with the crustacean nauplius eyes and the insect ocelli.<ref>{{cite journal | pmc=4450993 | pmid=26034575 | doi=10.1186/s13227-015-0010-x | volume=6 | title=Differential expression of retinal determination genes in the principal and secondary eyes of Cupiennius salei Keyserling (1877) | journal=Evodevo | page=16 |vauthors=Samadi L, Schmid A, Eriksson BJ| year=2015 | doi-access=free }}</ref> The eyes of horseshoe crabs can detect movement but not form images.<ref name="RuppertFoxBarnes2004Xiphosura" /> At the other extreme, [[jumping spider]]s have a very wide field of vision,<ref name="RuppertFoxBarnes2004Spiders" /> and their main eyes are ten times as acute as those of [[dragonfly|dragonflies]],<ref name="HarlandJackson2000EightLeggedCats">{{cite journal |author1=Harland, D.P. |author2=Jackson, R.R. |year=2000 |title="Eight-legged cats" and how they see - a review of recent research on jumping spiders (Araneae: Salticidae) |journal=Cimbebasia |volume=16 |pages=231–240 |url=http://www.cogs.susx.ac.uk/ccnr/Papers/Downloads/Harland_Cimb2000.pdf |access-date=2008-10-11 |df=dmy-all |archive-url=https://web.archive.org/web/20060928164131/http://www.cogs.susx.ac.uk/ccnr/Papers/Downloads/Harland_Cimb2000.pdf |archive-date=2006-09-28 }}</ref> able to see in both colors and UV-light.<ref>{{cite web |url=http://www.nbcnews.com/id/49454132/ns/technology_and_science-science/t/their-eight-eyes-jumping-spiders-are-true-visionaries/ |archive-url=https://archive.today/20130629234221/http://www.nbcnews.com/id/49454132/ns/technology_and_science-science/t/their-eight-eyes-jumping-spiders-are-true-visionaries/ |url-status=dead |archive-date=June 29, 2013 |title=With their eight eyes, jumping spiders are true visionaries|website=[[NBC News]] |date=2012-10-17}}</ref>

===Reproduction===
[[Image:Vaejovis cashi 222065762.jpg|200px|right|thumb| Female [[scorpion]] ''[[Vaejovis|Vaejovis cashi]]'' carrying its young (white)]]
[[Horseshoe crab]]s use [[external fertilization]]; the [[sperm]] and [[ovum|ova]] meet outside the parents' bodies. Despite being aquatic, they spawn on land in the [[intertidal zone]] on the beach.<ref>{{cite journal | url=https://www.jstor.org/stable/1542294 | jstor=1542294 | title=Nest-Site Selection in the Horseshoe Crab, Limulus polyphemus | last1=Penn | first1=Dustin | last2=Brockmann | first2=H. Jane | journal=Biological Bulletin | year=1994 | volume=187 | issue=3 | pages=373–384 | doi=10.2307/1542294 | pmid=29281397 }}</ref> The female digs a depression in the wet sand, where she will release her eggs. The male, usually more than one, then releases his sperm onto them.<ref>{{Cite news |last=Brown |first=David |date=2018-05-02 |title=Millions of horseshoe crabs spawn on the shores of Delaware Bay each year. Here's how to see them. |language=en-US |newspaper=Washington Post |url=https://www.washingtonpost.com/lifestyle/travel/millions-of-horseshoe-crabs-spawn-on-the-shores-of-delaware-bay-each-year-heres-how-to-see-them/2018/04/26/496a8efa-3daf-11e8-974f-aacd97698cef_story.html |access-date=2023-07-15}}</ref> Their [[trilobite]]-like [[larva]]e look rather like miniature adults as they have full sets of appendages and eyes, but initially they have only two pairs of book-gills and gain three more pairs as they [[molt]].<ref name="RuppertFoxBarnes2004Xiphosura" />

Also the sea spiders have external fertilization. The male and female release their sperm and eggs into the water where fertilization occurs. The male then collects the eggs and carries them around under his body.<ref>{{Cite journal |last=Fornshell |first=John A. |year=2015 |title=Larval stages of two deep sea pycnogonids |url=https://kmkjournals.com/upload/PDF/IZ/IZ%20Vol%2012/invert12_2_197_205_Fornshell.pdf |journal=Invertebrate Zoology |volume=12 |issue=2 |pages=197–205|doi=10.15298/invertzool.12.2.05 }}</ref>

Being air-breathing animals, although many mites have become secondarily aquatic,<ref>{{Cite book |last1=Resh |first1=Vincent H. |url=https://books.google.com/books?id=Ip57QSMCRk4C&dq=5000+species+of+arachnid+mites+adopt+a+secondary+aquatic+existence&pg=PA62 |title=Encyclopedia of Insects |last2=Cardé |first2=Ring T. |date=2003-04-04 |publisher=Elsevier |isbn=978-0-08-054605-6 |language=en|page=62|access-date=11 March 2025|via=Google Books}}</ref> the [[arachnid]]s use [[internal fertilization]]. Except for [[Opiliones]] and some mites, where the male has a penis used for direct fertilization,<ref>{{Cite book |last1=Watling |first1=Les |url=https://books.google.com/books?id=yqWkmpmE2twC&dq=exception+harvestmen+(Opiliones)+mites+(Acari)+males+copulate&pg=PA369 |title=Functional Morphology and Diversity |last2=Thiel |first2=Martin |date=2013-01-16 |publisher=OUP USA |isbn=978-0-19-539803-8 |language=en|pages=368–369|via=Google Books|access-date=11 March 2025}}</ref> fertilization in arachnids is indirect. Indirect fertilization happens in two ways: the male deposit his [[spermatophore]] (package of sperm) on the ground, which is then picked up by the female, or the male stores his sperm in appendages modified into sperm transfer organs, such as the [[pedipalp]]s in male spiders, which are inserted into the female genital openings during copulation.<ref name="RuppertFoxBarnes2004Spiders" /> [[Courtship display|Courtship]] rituals are common, especially in species where the male risks being eaten before mating. Most arachnids lay eggs, but all scorpions and some [[mite]]s are [[Viviparity|viviparous]], giving birth to live young (even more mites are ovoviviparous, but most are oviparous).<ref>{{cite journal | url=https://www.jstor.org/stable/4534928 | jstor=4534928 | last1=Benton | first1=T. G. | title=Reproduction and Parental Care in the Scorpion, Euscorpius flavicaudis | journal=Behaviour | year=1991 | volume=117 | issue=1/2 | pages=20–28 | doi=10.1163/156853991X00102 | url-access=subscription }}</ref><ref>{{cite book | url=https://books.google.com/books?id=QRvLBAAAQBAJ&dq=%22viviparous+mites%22&pg=PA365 | title=Embryology and Phylogeny in Annelids and Arthropods: International Series of Monographs in Pure and Applied Biology Zoology | isbn=9781483187020 | last1=Anderson | first1=D. T. | date=22 October 2013 | publisher=Elsevier }}</ref><ref>{{cite journal | url=https://www.sciencedirect.com/science/article/abs/pii/S1467803916301086 | doi=10.1016/j.asd.2016.09.001 | title=Scorpion katoikogenic ovariuterus – Much more alike to apoikogenic type than it seemed to be | year=2016 | last1=Jędrzejowska | first1=Izabela | last2=Szymusiak | first2=Kamil | last3=Mazurkiewicz-Kania | first3=Marta | last4=Garbiec | first4=Arnold | journal=Arthropod Structure & Development | volume=45 | issue=5 | pages=488–495 | pmid=27645113 | bibcode=2016ArtSD..45..488J | url-access=subscription }}</ref><ref>{{Cite book |last1=Auerbach |first1=Paul S. |url=https://books.google.com/books?id=O2EgDQAAQBAJ&dq=mites+oviparous,+some+are+ovoviviparous,+and+a+few+are+viviparous&pg=PA963 |edition=7th |title=Auerbach's Wilderness Medicine E-Book |last2=Cushing |first2=Tracy A. |last3=Harris |first3=N. Stuart |date=2016-09-21 |publisher=Elsevier Health Sciences |isbn=978-0-323-39609-7 |language=en}}</ref> Female pseudoscorpions carry their eggs in a brood pouch on the belly, where the growing embryos feeds on a nutritive fluid provided by the mother during development, and are therefore [[Matrotrophy|matrotrophic]].<ref>{{cite journal | pmc=9018881 | year=2022 | last1=Garbiec | first1=A. | last2=Christophoryová | first2=J. | last3=Jędrzejowska | first3=I. | title=Spectacular alterations in the female reproductive system during the ovarian cycle and adaptations for matrotrophy in chernetid pseudoscorpions (Pseudoscorpiones: Chernetidae) | journal=Scientific Reports | volume=12 | issue=1 | page=6447 | doi=10.1038/s41598-022-10283-z | pmid=35440674 | bibcode=2022NatSR..12.6447G }}</ref>

Levels of parental care for the young range from zero to prolonged. Scorpions carry their young on their backs until the first [[molt]], and in a few semi-social species the young remain with their mother.<ref>{{cite book |author=Lourenço, W.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 |df=dmy-all |archive-date=2008-10-03 |archive-url=https://web.archive.org/web/20081003122816/http://www.european-arachnology.org/proceedings/19th/Lourenco.PDF |url-status=dead }}</ref> Some spiders care for their young, for example a [[wolf spider]]'s brood cling to rough bristles on the mother's back,<ref name="RuppertFoxBarnes2004Spiders" /> and females of some species respond to the "begging" behavior of their young by giving them their prey, provided it is no longer struggling, or even [[Regurgitation (digestion)|regurgitate]] food.<ref name="Foelix1996SpidersReproduction">{{cite book |title=Biology of Spiders |author=Foelix, R.F. |publisher=Oxford University Press US |year=1996 |isbn=978-0-19-509594-4 |chapter=Reproduction |pages=[https://archive.org/details/biologyofspiders00foel_0/page/176 176–212] |via=Google Books |chapter-url=https://books.google.com/books?id=XUgyqxNKhyAC&q=%22Biology+of+Spiders%22+Foelix&pg=PP1 |access-date=2008-10-08 |df=dmy-all |url=https://archive.org/details/biologyofspiders00foel_0/page/176 }}</ref>

==Evolutionary history==
===Fossil record===
There are large gaps in the chelicerates' [[fossil]] record because, like all [[arthropod]]s, their [[exoskeleton]]s are [[organic matter|organic]] and hence their fossils are rare except in a few [[lagerstätte]]n where conditions were exceptionally suited to preserving fairly soft tissues. The [[Burgess shale]] animals like ''[[Sidneyia]]'' from about {{ma|505}} have been classified as chelicerates, the latter because its appendages resemble those of the [[Xiphosura]] (horseshoe crabs). However, [[cladistic]] analyses that consider wider ranges of characteristics place neither as chelicerates. There is debate about whether ''[[Fuxianhuia]]'' from earlier in the [[Cambrian]] period, about {{ma|525}}, was a chelicerate. Another Cambrian fossil, ''[[Kodymirus]]'', was originally classified as an [[aglaspid]] but may have been a [[eurypterid]] and therefore a chelicerate. If any of these was closely related to chelicerates, there is a gap of at least 43&nbsp;million years in the record between true chelicerates and their nearest not-quite chelicerate relatives.<ref name="old">{{citation
| author=Wills, M.A.
| title=How good is the fossil record of arthropods? An assessment using the stratigraphic congruence of cladograms
| journal=Geological Journal | volume=36 | issue=3–4 | pages=187–210 | year=2001
| doi=10.1002/gj.882
| s2cid=86279530
| doi-access=free
| bibcode=2001GeolJ..36..187W
}}</ref>
[[File:20191003 Mollisonia plenovenatrix side.png|thumb|Reconstruction of ''Mollisonia plenovenatrix'', the oldest known arthropod with confirmed chelicerae]]
''[[Sanctacaris]]'', member of the family [[Sanctacarididae]] from the Burgess Shale of [[Canada]], represents the oldest occurrence of a confirmed chelicerate, Middle Cambrian in age.<ref name="san">{{cite journal|url=https://www.researchgate.net/publication/266683582|title=''Sanctacaris uncata'': the oldest chelicerate (Arthropoda)|first=David A.|last=Legg|journal=Naturwissenschaften|year=2014|volume=101|issue=12|pages=1065–1073|doi=10.1007/s00114-014-1245-4|pmid=25296691|bibcode=2014NW....101.1065L|s2cid=15290784}}</ref> Although its chelicerate nature has been doubted for its pattern of [[tagmosis]] (how the segments are grouped, especially in the head),<ref name="old" /> a restudy in 2014 confirmed its phylogenetic position as the oldest chelicerate.<ref name="san" /> Another fossil of the site, ''[[Mollisonia]]'', is considered a basal chelicerate and it has the oldest known chelicerae and proto-[[book gills]].<ref>{{Cite journal |last1=Aria |first1=Cédric |last2=Caron |first2=Jean-Bernard |date=2019 |title=A middle Cambrian arthropod with chelicerae and proto-book gills |url=https://www.nature.com/articles/s41586-019-1525-4 |journal=Nature |language=en |volume=573 |issue=7775 |pages=586–589 |doi=10.1038/s41586-019-1525-4 |pmid=31511691 |bibcode=2019Natur.573..586A |s2cid=256819092 |issn=1476-4687|url-access=subscription }}</ref>
[[File:Lunataspis aurora MM I-4000A.jpg|left|thumb|232x232px|Holotype of the [[xiphosura]]n ''Lunataspis aurora'']]
The eurypterids have left few good fossils and one of the earliest confirmed eurypterid, ''[[Pentecopterus|Pentecopterus decorahensis]]'', appears in the Middle [[Ordovician]] period {{ma|467.3}}, making it the oldest eurypterid.<ref name=":0">{{citation
|last1=Lamsdell
|first1=James C.
|last2=Briggs
|first2=Derek E. G.
|last3=Liu
|first3=Huaibao
|last4=Witzke
|first4=Brian J.
|last5=McKay
|first5=Robert M.
|year=2015
|title=The oldest described eurypterid: a giant Middle Ordovician (Darriwilian) megalograptid from the Winneshiek Lagerstätte of Iowa
|journal=[[BMC Evolutionary Biology]]
|volume=15|issue=1
|
pages=169
|doi=10.1186/s12862-015-0443-9
|pmc=4556007
|pmid=26324341
|doi-access=free
|bibcode=2015BMCEE..15..169L
}}</ref>
Until recently the earliest known [[xiphosura]]n fossil dated from the Late Llandovery stage of the [[Silurian]] {{ma|436|428}},<ref>{{citation
|author1=Moore, R.A. |author2=Briggs, D.E.G. |author3=Braddy, S.J. |author4=Anderson, L.I. |author5=Mikulic, D.G. |author6=Kluessendorf, J. |name-list-style=amp | title=A new synziphosurine (Chelicerata, Xiphosura) from the late Llandovery (Silurian) Waukesha Lagerstaette, Wisconsin, USA
| journal=Journal of Paleontology | date=March 2005| volume=79 | issue=2
| pages=242–250
| doi=10.1666/0022-3360(2005)079<0242:ANSCXF>2.0.CO;2
|bibcode=2005JPal...79..242M |s2cid=56570105 | issn=0022-3360
 }}</ref> but in 2008 an older specimen described as ''[[Lunataspis|Lunataspis aurora]]'' was reported from about {{ma|445}} in the Late [[Ordovician]].<ref>{{citation
|author1=Rudkin, D.M. |author2=Young, G.A. |author3=Nowlan, G.S. |s2cid=62891048 |name-list-style=amp | title=The Oldest Horseshoe Crab: a New Xiphosurid from Late Ordovician Konservat-Lagerstätten Deposits, Manitoba, Canada
| journal=Palaeontology | volume=51 | issue=1 | date=January 2008
| doi=10.1111/j.1475-4983.2007.00746.x
| pages=1–9
| doi-access=free |bibcode=2008Palgy..51....1R }}</ref>

The oldest known [[arachnid]] is the [[trigonotarbid]] ''[[Palaeotarbus]] jerami'', from about {{ma|420}} in the [[Silurian]] period, and had a triangular [[cephalothorax]] and segmented abdomen, as well as eight legs and a pair of [[pedipalp]]s.<ref>{{citation |author=Dunlop, J.A. |title=A trigonotarbid arachnid from the Upper Silurian of Shropshire |date=September 1996 |journal=Palaeontology |volume=39 |issue=3 |pages=605–614 |url=http://palaeontology.palass-pubs.org/pdf/Vol%2039/Pages%20605-614.pdf |access-date=2008-10-12 |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 |url-status=usurped}} The fossil was originally named ''[[Eotarbus]]'' but was renamed when it was realized that a [[Carboniferous]] arachnid had already been named ''[[Eotarbus]]'': {{citation |author=Dunlop, J.A. |title=A replacement name for the trigonotarbid arachnid ''Eotarbus'' Dunlop |journal=Palaeontology |volume=42 |issue=1 |page=191 |year=1999 |bibcode=1999Palgy..42..191D |doi=10.1111/1475-4983.00068 |s2cid=83825904 |doi-access=free}}</ref>

''[[Attercopus]] fimbriunguis'', from {{ma|386}} in the [[Devonian]] period, bears the earliest known silk-producing spigots, and was therefore hailed as a spider,<ref name="VollrathSelden2007BehaviorInEvolutionOfSpiders">{{citation 
 |author1=Vollrath, F. 
 |author2=Selden, P.A. 
 |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 
 |access-date=2008-10-12 
 |url-status=dead 
 |archive-url=https://web.archive.org/web/20081209102852/http://homepage.mac.com/paulselden/Sites/Website/ARES.pdf 
 |archive-date=2008-12-09 
 }}</ref> but it lacked [[spinneret (spider)|spinnerets]] and hence was not a true spider.<ref>{{citation
|author1=Selden, P.A. |author2=Shear, W.A. | title=Fossil evidence for the origin of spider spinnerets
| journal=PNAS | date=July 2008 | doi = 10.1073/pnas.0809174106 | pmid=19104044 | pmc=2634869 | volume=105 |issue=52 | pages=20781–5|bibcode=2008PNAS..10520781S |doi-access=free }}</ref> Rather, it was likely sister group to the spiders, a clade which has been named Serikodiastida.<ref name=Garw>{{cite journal|title=Three-dimensional reconstruction and the phylogeny of extinct chelicerate orders|first1=Russell J.|last1=Garwood|first2=Jason A.|last2=Dunlop|year=2014|journal=PeerJ|volume=2|pages=e641|doi=10.7717/peerj.641|pmid=25405073|pmc=4232842 |doi-access=free }}</ref> Close relatives of the group survived through to the [[Cretaceous]] Period.<ref name="WangDunlop2018">{{cite journal|last1=Wang|first1=Bo|last2=Dunlop|first2=Jason A.|last3=Selden|first3=Paul A.|last4=Garwood|first4=Russell J.|last5=Shear|first5=William A.|last6=Müller|first6=Patrick|last7=Lei|first7=Xiaojie|title=Cretaceous arachnid Chimerarachne yingi gen. et sp. nov. illuminates spider origins|journal=Nature Ecology & Evolution|volume=2|issue=4|year=2018|pages=614–622|issn=2397-334X|doi=10.1038/s41559-017-0449-3|pmid=29403075|bibcode=2018NatEE...2..614W |s2cid=4239867|url=https://www.research.manchester.ac.uk/portal/en/publications/cretaceous-arachnid-chimerarachne-yingi-gen-et-sp-nov-illuminates-spider-origins(b82d83bd-6081-4187-acfc-fb7358c33358).html}}</ref> Several [[Carboniferous]] spiders were members of the [[Mesothelae]], a basal group now represented only by the [[Liphistiidae]],<ref name="VollrathSelden2007BehaviorInEvolutionOfSpiders" /> and fossils suggest taxa closely related to the spiders, but which were not true members of the group were also present during this Period.<ref name="GarwoodDunlop2016">{{cite journal|last1=Garwood|first1=Russell J.|last2=Dunlop|first2=Jason A.|last3=Selden|first3=Paul A.|last4=Spencer|first4=Alan R. T.|last5=Atwood|first5=Robert C.|last6=Vo|first6=Nghia T.|last7=Drakopoulos|first7=Michael|title=Almost a spider: a 305-million-year-old fossil arachnid and spider origins|journal=Proceedings of the Royal Society B: Biological Sciences|volume=283|issue=1827|year=2016|pages=20160125|issn=0962-8452|doi=10.1098/rspb.2016.0125|pmid=27030415|pmc=4822468|doi-access=free}}</ref>

The Late [[Silurian]] ''[[Proscorpius]]'' has been classified as a scorpion, but differed significantly from modern scorpions: it appears wholly aquatic since it had [[gill]]s rather than [[book lung]]s or [[Invertebrate trachea|trachea]]e; its mouth was completely under its head and almost between the first pair of legs, as in the extinct [[eurypterid]]s and living [[horseshoe crab]]s.<ref name="Weygoldt1998EvolutionAndSystematicsOfChelicerata" /> Fossils of terrestrial scorpions with [[book lung]]s have been found in Early [[Devonian]] rocks from about {{ma|402}}.<ref>{{citation
| author=Shear, W.A., Gensel, P.G. and Jeram, A.J.
| title=Fossils of large terrestrial arthropods from the Lower Devonian of Canada
| journal=Nature | volume=384 | pages=555–557 | date=December 1996| doi=10.1038/384555a0 | issue=6609
| bibcode=1996Natur.384..555S
| s2cid=4367636
}}</ref> The oldest species of scorpion found as of 2021 is ''[[Dolichophonus|Dolichophonus loudonensis]]'', which lived during the Silurian, in present-day Scotland.<ref>{{Cite journal|last1=Anderson|first1=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|title=Stranger than a scorpion: a reassessment of Parioscorpio venator, a problematic arthropod from the Llandoverian Waukesha Lagerstätte|url=https://onlinelibrary.wiley.com/doi/abs/10.1111/pala.12534|journal=Palaeontology|language=en|volume=64|issue=3|pages=429–474|doi=10.1111/pala.12534|bibcode=2021Palgy..64..429A |s2cid=234812878|issn=1475-4983|url-access=subscription}}</ref>

===Relationships with other arthropods===
<div style="float:right; width:auto; border:solid 1px silver; padding:2px; margin:2px">
<div style="width:auto; border:solid 1px silver; padding:5px">
{{clade
 |label1=[[Arthropoda]]
 |1={{clade
 |label1=[[Pancrustacea]]
 |1={{clade
 |1=[[Hexapoda]]
 |2=[[Crustacean|Crustacea]]
 }}
 |label2=[[Paradoxopoda]]
 |2={{clade
 |1=[[Myriapoda]]
 |2=Chelicerata
 }}
 }}
}}
</div>&nbsp;A recent view of chelicerate [[phylogeny]]<ref>{{cite book|vauthors=Giribet G, Edgecombe G|chapter= The Arthropoda: A Phylogenetic Framework|date= April 2013|title=Arthropod Biology and Evolution|pages= 17–40|doi=10.1007/978-3-642-36160-9_2|isbn= 978-3-642-36159-3}}</ref></div>
<div style="float:right; width:auto; border:solid 1px silver; padding:2px; margin:2px;margin-left:1em">
<div style="width:auto; border:solid 1px silver; padding:5px">
{{clade
 |label1=[[Arthropoda]]
 |1={{clade
 |1=Chelicerata
 |label2=[[Mandibulata]]
 |2={{clade
 |1=[[Crustacean|Crustacea]]
 |2={{clade
 |1=[[Trilobita]]
 |label2=[[Tracheata]]
 |2={{clade
 |1=[[Hexapoda]]
 |2=[[Myriapoda]]
 }}
 }}
 }}
 }}
}}
</div>&nbsp;A "traditional" view of chelicerate [[phylogeny]]<ref>{{Cite journal|vauthors=Turbeville J, Pfeifer D, Field K, Raff R|title= The phylogenetic status of arthropods, as inferred from 18S rRNA sequences|journal=Molecular Biology and Evolution| volume=8|issue=5|date=September 1991|pages=669–686|doi=10.1093/oxfordjournals.molbev.a040677|pmid= 1766363|doi-access=free}}</ref><ref>{{Cite journal|vauthors=Giribet G, Ribera C|date=2000|title=A Review of Arthropod Phylogeny: New Data Based on Ribosomal DNA Sequences and Direct Character Optimization|journal=Cladistics|volume=16|issue=2|pages=204–231|doi=10.1111/j.1096-0031.2000.tb00353.x|pmid=34902954 |s2cid=84370269}}</ref></div>
The "traditional" view of the arthropod "family tree" shows chelicerates as less closely related to the other major living groups ([[crustacea]]ns; [[hexapoda|hexapods]], which includes [[insect]]s; and [[myriapod]]s, which includes [[centipede]]s and [[millipede]]s) than these other groups are to each other. Recent research since 2001, using both [[molecular phylogenetic]]s (the application of cladistic analysis to [[biochemistry]], especially to organisms' [[DNA]] and [[RNA]]) and detailed examination of how various arthropods' [[nervous system]]s develop in the [[embryo]]s, suggests that chelicerates are most closely related to myriapods, while hexapods and crustaceans are each other's closest relatives. However, these results are derived from analyzing only living arthropods, and including extinct ones such as [[trilobite]]s causes a swing back to the "traditional" view, placing trilobites as the sister-group of the [[Tracheata]] (hexapods plus myriapods) and chelicerates as least closely related to the other groups.<ref name="Jenner2006ChallengingReceivedWisdoms">{{citation
| author=Jenner, 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 | year=2006| pages=93–103 | doi=10.1093/icb/icj014 | pmid=21672726
| doi-access=free}}</ref>
{{Clear}}

===Major sub-groups===
<div style="float:right; width:auto; border:solid 1px silver; padding:2px; margin:2px; font-size:90%">
<div style="width:auto; border:solid 1px silver; padding:5px">
{{clade
  |label1=Chelicerata
  |1={{clade
       |1=[[Xiphosura]] (horseshoe crabs) [[File:Limulus polyphemus (aquarium) (white background).jpg|70 px]]
       |2={{clade
            |1=[[Eurypterida]]'''†''' <span style="{{MirrorH}}">[[File:Eurypterus Paleoart (no background).png|70 px]]</span>
            |2=[[Chasmataspidida]]'''†''' [[File:20200606 Chasmataspis laurencii.png|80px]]
          }}
       |label3=[[Arachnida]]
       |3={{clade
           |1={{clade
                |1=[[Scorpiones]] [[File:Buthus_mariefranceae_(10.3897-zookeys.686.12206)_Figure_1.jpg|65px]] 
                |2=[[Opiliones]] (harvestmen) [[File:Phalangium opilio 2 (Nemo5576) (white background).jpg|120px]]
              }}
           |2={{clade
                |1=[[Pseudoscorpiones]] <span style="{{MirrorH}}">[[File:Neobisium sylvaticum 03.png|70px]]</span>
                |2=[[Solifugae]] (sun spiders) [[File:Ammotrecha itzaana 4414721993.png|80px]]
              }}
           |3=[[Palpigradi]] (microwhip scorpions) [[File:Live Eukoenenia spelaea in its cave habitat (no background).png|70px]]
           |4={{clade
                |1=[[Trigonotarbida]]'''†''' [[File:20201202 Trigonotarbus johnsoni.png|80px]]
                |2={{clade
                     |1=[[Araneae]] ([[spider]]s) [[File:Aptostichus simus Monterey County.jpg|70 px]]
                     |2={{clade
                          |1=[[Haptopoda]]'''†''' [[File:20200823 Plesiosiro madeleyi.png|80px]]
                          |2={{clade
                               |1=[[Amblypygi]] (whip spiders) [[File:Flickr - ggallice - Tailless whip-scorpion, La Muerta.png|120px]]
                               |2={{clade
                                    |1=[[Uropygi]] (whip scorpions) [[File:Whip Scorpion body (9672115742) (white background).png|80px]] 
                                    |2=[[Schizomida]] [[File:Brignolizomus woodwardi 175486060.jpg|80px]]
                                  }}
                             }}
                        }}
                   }}
              }}
           |5={{clade
                |1=[[Ricinulei]] (hooded tickspiders) [[File:Ricinulei from Fernandez & Giribet (2015).png|70px]]
                |2=[[Anactinotrichida]] [[File:Ixodes scapularis P1170301a (white background).png|70px]]
                |3=[[Acariformes]] ([[mite]]s) [[File:Rote Samtmilbe Namibia.png|70px]]
              }}
         }}
     }}
}}
</div>Shultz (2007)'s evolutionary family tree of [[arachnid]]s<ref name="Schultz2007ArachnidPhylogeny" /> – '''†''' marks extinct groups.</div>

It is generally agreed that the Chelicerata contain the [[Class (biology)|classes]] [[Arachnida]] ([[spider]]s, [[scorpion]]s, [[mite]]s, etc.), [[Xiphosura]] ([[horseshoe crab]]s) and [[Eurypterida]] (sea scorpions, extinct).<ref name="Schultz2007ArachnidPhylogeny">{{citation
| author=Schultz, J.W.
| title=A phylogenetic analysis of the arachnid orders based on morphological characters
| journal=Zoological Journal of the Linnean Society | year=2007 | volume=150 | pages=221–265
| doi=10.1111/j.1096-3642.2007.00284.x
| issue=2
| doi-access=free
}}</ref> The extinct [[Chasmataspidida]] may be a sub-group within Eurypterida.<ref name="Schultz2007ArachnidPhylogeny" /><ref>{{citation
|author1=O. Tetlie, E. |author2=Braddy, S.J.
|s2cid=73596575
| title=The first Silurian chasmataspid, ''Loganamaraspis dunlopi'' gen. et sp. nov. (Chelicerata: Chasmataspidida) from Lesmahagow, Scotland, and its implications for eurypterid phylogeny
| journal=Transactions of the Royal Society of Edinburgh: Earth Sciences | year=2003 | volume=94
| pages=227–234 | doi=10.1017/S0263593300000638
| issue=3
}}</ref> The [[Pycnogonida]] ([[sea spider]]s) were traditionally classified as chelicerates, but some features suggest they may be representatives of the earliest arthropods from which the well-known groups such as chelicerates evolved.<ref name="PoschmannDunlop2006NewSeaSpider">{{citation
|author1=Poschmann, M. |author2=Dunlop, J.A.
| title=A New Sea Spider (Arthropoda: Pycnogonida) with a Flagelliform Telson from the Lower Devonian Hunsrück Slate, Germany
| journal=Palaeontology | volume=49 | issue=5 | pages=983–989 | year=2006
| doi=10.1111/j.1475-4983.2006.00583.x
| doi-access=free |bibcode=2006Palgy..49..983P
}}</ref>

However, the structure of "family tree" relationships within the Chelicerata has been controversial ever since the late 19th century. An attempt in 2002 to combine analysis of [[DNA]] features of modern chelicerates and anatomical features of modern and fossil ones produced credible results for many lower-level groups, but its results for the high-level relationships between major sub-groups of chelicerates were unstable, in other words minor changes in the inputs caused significant changes in the outputs of the computer program used (POY).<ref>{{citation
| author=Gonzalo Giribet G., Edgecombe, G.D., Wheeler, W.C., and Babbitt, C.
| s2cid=16833833
| title=Phylogeny and Systematic Position of Opiliones: A Combined Analysis of Chelicerate Relationships Using Morphological and Molecular Data
| journal=Cladistics | volume=18
| issue=1 | pages=5–70 | year=2002
| pmid=14552352 | doi=10.1111/j.1096-0031.2002.tb00140.x
| doi-access=free
}}</ref> An analysis in 2007 using only anatomical features produced the [[cladogram]] on the right, but also noted that many uncertainties remain.<ref name="Shultz2007ArachnidPhylogeny">{{citation
| author=Shultz, J.W.
| title=A phylogenetic analysis of the arachnid orders based on morphological characters
| journal=Zoological Journal of the Linnean Society | year=2007 | volume=150 | pages=221–265
| doi=10.1111/j.1096-3642.2007.00284.x
| issue=2
| doi-access=free
}}</ref> In recent analyses the clade [[Tetrapulmonata]] is reliably recovered, but other ordinal relationships remain in flux.<ref name="WangDunlop2018"/><ref name="GarwoodDunlop2017">{{cite journal|last1=Garwood|first1=Russell J.|last2=Dunlop|first2=Jason A.|last3=Knecht|first3=Brian J.|last4=Hegna|first4=Thomas A.|title=The phylogeny of fossil whip spiders|journal=BMC Evolutionary Biology|volume=17|issue=1|year=2017|issn=1471-2148|doi=10.1186/s12862-017-0931-1|pmid=28431496|doi-access=free|pmc=5399839|page=105|bibcode=2017BMCEE..17..105G }}</ref><ref name="GarwoodDunlop2016"/><ref name="GarwoodDunlop2014">{{cite journal|last1=Garwood|first1=Russell J.|last2=Dunlop|first2=Jason|title=Three-dimensional reconstruction and the phylogeny of extinct chelicerate orders|journal=PeerJ|volume=2|year=2014|pages=e641|issn=2167-8359|doi=10.7717/peerj.641|doi-access=free|pmid=25405073|pmc=4232842}}</ref><ref name="Giribet2018">{{cite journal|last1=Giribet|first1=Gonzalo|title=Current views on chelicerate phylogeny—A tribute to Peter Weygoldt|journal=Zoologischer Anzeiger|volume=273|year=2018|pages=7–13|issn=0044-5231|doi=10.1016/j.jcz.2018.01.004|bibcode=2018ZooAn.273....7G |s2cid=90344977 |url=http://nrs.harvard.edu/urn-3:HUL.InstRepos:37308630|url-access=subscription}}</ref><ref name="SharmaKaluziak2014">{{cite journal|last1=Sharma|first1=Prashant P.|last2=Kaluziak|first2=Stefan T.|last3=Pérez-Porro|first3=Alicia R.|last4=González|first4=Vanessa L.|last5=Hormiga|first5=Gustavo|last6=Wheeler|first6=Ward C.|last7=Giribet|first7=Gonzalo|title=Phylogenomic Interrogation of Arachnida Reveals Systemic Conflicts in Phylogenetic Signal|journal=Molecular Biology and Evolution|volume=31|issue=11|year=2014|pages=2963–2984|issn=1537-1719|doi=10.1093/molbev/msu235|pmid=25107551|doi-access=free}}</ref><ref name="BallesterosSharma2019">{{cite journal|last1=Ballesteros|first1=Jesús A|last2=Sharma|first2=Prashant P|last3=Halanych|first3=Ken|title=A Critical Appraisal of the Placement of Xiphosura (Chelicerata) with Account of Known Sources of Phylogenetic Error|journal=Systematic Biology|volume=68|issue=6|year=2019|pages=896–917|issn=1063-5157|doi=10.1093/sysbio/syz011|pmid=30917194|doi-access=free}}</ref>

The position of scorpions is particularly controversial. Some early fossils such as the Late [[Silurian]] ''[[Proscorpius]]'' have been classified by paleontologists as scorpions, but described as wholly aquatic as they had [[gill]]s rather than [[book lung]]s or [[Invertebrate trachea|trachea]]e. Their mouths are also completely under their heads and almost between the first pair of legs, as in the extinct [[eurypterid]]s and living [[horseshoe crab]]s.<ref name="Weygoldt1998EvolutionAndSystematicsOfChelicerata" /> This presents a difficult choice: classify ''Proscorpius'' and other aquatic fossils as something other than scorpions, despite the similarities; accept that "scorpions" are not monophyletic but consist of separate aquatic and terrestrial groups;<ref name="Weygoldt1998EvolutionAndSystematicsOfChelicerata" /> or treat scorpions as more closely related to eurypterids and possibly horseshoe crabs than to spiders and other [[arachnid]]s,<ref name="BraddyAldridgeEtAl1999LamellateBookGills" /> so that either scorpions are not arachnids or "arachnids" are not monophyletic.<ref name="Weygoldt1998EvolutionAndSystematicsOfChelicerata">{{citation
| author=Weygoldt, P. | title=Evolution and systematics of the Chelicerata
| journal=Experimental and Applied Acarology | volume=22 | issue=2 | date=February 1998 | pages=63–79
| doi=10.1023/A:1006037525704
| s2cid=35595726
}}</ref> [[Cladistic]] analyses have recovered ''[[Proscorpius]]'' within the scorpions,<ref name="Garw"/> based on reinterpretation of the species' breathing apparatus.<ref>{{cite journal |author1=Jason A. Dunlop |author2=O. Erik Tetlie |author3=Lorenzo Prendini |s2cid=53521811 |doi=10.1111/j.1475-4983.2007.00749.x |title=Reinterpretation of the Silurian scorpion ''Proscorpius osborni'' (Whitfield): integrating data from Palaeozoic and recent scorpions |year=2008 |journal=Palaeontology |volume=51 |issue=2 |pages=303–320|doi-access=free |bibcode=2008Palgy..51..303D }}</ref> This is reflected also in the reinterpretation of ''[[Palaeoscorpius]]'' as a terrestrial animal.<ref>{{cite journal |author1=G. Kühl |author2=A. Bergmann |author3=J. Dunlop |author4=R. J. Garwood |author5=J. Rust |doi=10.1111/j.1475-4983.2012.01152.x|title=Redescription and palaeobiology of ''Palaeoscorpius devonicus'' Lehmann, 1944 from the Lower Devonian Hunsrück Slate of Germany |year=2012 |journal=Palaeontology |volume=55 |issue=4 |pages=775–787|doi-access=free |bibcode=2012Palgy..55..775K }}</ref>

A 2013 phylogenetic analysis<ref>{{Cite journal|last=Lamsdell|first=James C.|date=2013-01-01|title=Revised systematics of Palaeozoic 'horseshoe crabs' and the myth of monophyletic Xiphosura|journal=Zoological Journal of the Linnean Society|language=en|volume=167|issue=1|pages=1–27|doi=10.1111/j.1096-3642.2012.00874.x|issn=0024-4082|doi-access=free}}</ref> (the results presented in a cladogram below) on the relationships within the Xiphosura and the relations to other closely related groups (including the eurypterids, which were represented in the analysis by genera ''Eurypterus'', ''Parastylonurus'', ''[[Rhenopterus]]'' and ''[[Stoermeropterus]]'') concluded that the Xiphosura, as presently understood, was [[paraphyletic]] (a group sharing a [[last common ancestor]] but not including all descendants of this ancestor) and thus not a valid phylogenetic group. Eurypterids were recovered as closely related to arachnids instead of xiphosurans, forming the group [[Sclerophorata]] within the clade [[Dekatriata]] (composed of sclerophorates and [[Chasmataspidida|chasmataspidids]]). This work suggested it is possible that Dekatriata is synonymous with Sclerophorata as the reproductive system, the primary defining feature of sclerophorates, has not been thoroughly studied in chasmataspidids. Dekatriata is in turn part of the [[Prosomapoda]], a group including the [[Xiphosurida]] (the only monophyletic xiphosuran group) and other stem-genera. A recent phylogenetic analysis of the chelicerates places the Xiphosura within the Arachnida as the sister group of Ricinulei,<ref name="BallesterosSharma2019" /><ref>{{cite journal |last1=Sharma |first1=Prashant P. |last2=Gavish-Regev |first2=Efrat |title=The Evolutionary Biology of Chelicerata |journal=Annual Review of Entomology |date=28 January 2025 |volume=70 |issue=1 |pages=143–163 |doi=10.1146/annurev-ento-022024-011250 |pmid=39259983 |issn=1545-4487}}</ref> but others still retrieve a monophyletic arachnida.<ref name="Lozano-FernandezTanner2019">{{cite journal|last1=Lozano-Fernandez|first1=Jesus|last2=Tanner|first2=Alastair R.|last3=Giacomelli|first3=Mattia|last4=Carton|first4=Robert|last5=Vinther|first5=Jakob|last6=Edgecombe|first6=Gregory D.|last7=Pisani|first7=Davide|title=Increasing species sampling in chelicerate genomic-scale datasets provides support for monophyly of Acari and Arachnida|journal=Nature Communications|volume=10|issue=1|year=2019|page=2295|issn=2041-1723|doi=10.1038/s41467-019-10244-7|pmid=31127117|pmc=6534568|bibcode=2019NatCo..10.2295L|doi-access=free}}</ref>

{{clade|{{clade
 |1=†''[[Fuxianhuia]]''
 |2={{clade
 |label1=†[[Antennulata]]
 | 1={{clade
 | 1=†''[[Emeraldella]]''
 | 2=†[[Trilobitomorpha]]
 | 3=†''[[Sidneyia]]''
}} 
 | 2={{clade
 |label1=†[[Megacheira]]
 | 1={{clade
 | 1=†''[[Yohoia]]''
 | 2={{clade
 | 1=†''[[Alalcomenaeus]]''
 | 2=†''[[Leanchoilia]]''
}} }}
 |label2=Chelicerata
 | 2={{clade
 |label1=[[Pycnogonida]]
 | 1={{clade
 | 1=†''[[Palaeoisopus]]''
 | 2={{clade
 | 1=''[[Pycnogonum]]''
 | 2=†''[[Haliestes]]''
 }} }}
 |label2=Euchelicerata
 | 2={{clade
 | 1=†''[[Offacolus]]''
 |label2=[[Prosomapoda]]
 | 2={{clade
 | 1=†''[[Weinbergina]]''
 | 2={{clade
 | 1={{clade
 | 1=†''[[Venustulus]]''
 | 2=†''[[Camanchia]]''
}}
 | 2={{clade
 | 1=†''[[Legrandella]]''
 | 2={{clade
 |label1=[[Xiphosura]]
 | 1={{clade
 | 1=†''[[Kasibelinurus]]''
 | 2={{clade
 | 1=†''[[Willwerathia]]''
 |label2=[[Xiphosurida]]
 | 2={{clade
 | 1=†''[[Lunataspis]]''
 | 2={{clade
 | 1=†[[Belinurina]]
 | 2=[[Limulina]]
}} }} }} }}
 |label2=[[Planaterga]]
 | 2={{clade
 | 1={{clade
 | 1=†''[[Pseudoniscus]]''
 | 2=†''[[Cyamocephalus]]''
}}
 | 2=†''[[Pasternakevia]]''
 | 3={{clade
 | 1=†''[[Bunodes]]''
 | 2=†''[[Limuloides]]''
}}
 | 4=†''[[Bembicosoma]]''
 |label5=[[Dekatriata]]
 | 5={{clade
 | 1=†[[Chasmataspidida]]
 |label2=[[Sclerophorata]]
 | 2={{clade
 | 1=[[Arachnida]]
 | 2=†[[Eurypterida]]
}} }} }} }} }} }} }} }} }} }} }} }}|label1=[[Arachnomorpha]]|style=font-size:80%; line-height:80%}}

==Diversity==
Although well behind the insects, chelicerates are one of the most diverse groups of animals, with over 77,000 living species that have been described in scientific publications.<ref name="Shultz2001ArachnidsInEncOfLifeSci">{{citation
| contribution=Chelicerata (Arachnids, Including Spiders, Mites and Scorpions) | author=Shultz, J.W.
| s2cid=85601266
| title=Encyclopedia of Life Sciences | year=2001 | publisher= John Wiley & Sons, Ltd.
| doi=10.1038/npg.els.0001605
| isbn=978-0470016176
}}</ref> Some estimates suggest that there may be 130,000 undescribed species of spider and nearly 500,000 undescribed species of mites and ticks.<ref>{{citation
| title=Numbers of Living Species in Australia and the World | date=September 2005
| publisher=Department of the Environment and Heritage, Australian Government
| url=http://www.environment.gov.au/biodiversity/abrs/publications/other/species-numbers/2009/pubs/nlsaw-2nd-complete.pdf
| access-date=2010-03-29
}}</ref> While the earliest chelicerates and the living [[Pycnogonida]] (if they are chelicerates<ref name="PoschmannDunlop2006NewSeaSpider" />) and [[Xiphosura]] are marine animals that breathe dissolved [[oxygen]], the vast majority of living species are air-breathers,<ref name="Shultz2001ArachnidsInEncOfLifeSci" /> although a few spider species build "[[diving bell]]" webs that enable them to live under water.<ref>{{citation
 |author1=Schütz, D. 
 |author2=Taborsky, M. 
 |title=Adaptations to an aquatic life may be responsible for the reversed sexual size dimorphism in the water spider, ''Argyroneta aquatica'' 
 |journal=Evolutionary Ecology Research 
 |year=2003 
 |volume=5 
 |issue=1 
 |pages=105–117 
 |url=http://www.zoology.unibe.ch/behav/pdf_files/Schuetz_EvolEcolRes03.pdf 
 |access-date=2008-10-11 
 |url-status=dead 
 |archive-url=https://web.archive.org/web/20081216214632/http://www.zoology.unibe.ch/behav/pdf_files/Schuetz_EvolEcolRes03.pdf 
 |archive-date=2008-12-16 
}}</ref> Like their ancestors, most living chelicerates are carnivores, mainly on small [[invertebrate]]s. However, many species feed as [[parasite]]s, [[herbivore]]s, [[scavenger]]s and [[detritivore]]s.<ref name="RuppertFoxBarnes2004Acari" /><ref name="RuppertFoxBarnes2004Opiliones" /><ref name="Shultz2001ArachnidsInEncOfLifeSci" />
{| class="wikitable" style="text-align:center"
| align="center" colspan="3" | Diversity of living chelicerates
|-
! Group !! Described species<ref name="Shultz2001ArachnidsInEncOfLifeSci" /><ref>Pinto-da-Rocha, R., G. Machado, G. Giribet. 2007. Harvestmen: The Biology of Opiliones. Harvard University Press. Cambridge, MA.</ref><ref>{{Cite journal |last1=Gloor |first1=Daniel |last2=Nentwig |first2=Wolfgang |last3=Blick |first3=Theo |last4=Kropf |first4=Christian |date=2017 |title=World Spider Catalog |url=http://wsc.nmbe.ch/ |language=en |doi=10.24436/2}}</ref> !! Diet
|-
! [[Pycnogonida]] (sea-spiders)
| 500 <!-- use Shultz in ELS for num of species, as in other articles - hope ELS consistent-->|| Carnivorous<ref name="Shultz2001ArachnidsInEncOfLifeSci" />
|-
![[Araneae]] (spiders)
| 50,300 || Carnivorous;<ref name="Shultz2001ArachnidsInEncOfLifeSci" /> 1 [[Bagheera kiplingi|herbivore]]<ref name="MeehanOlsonCurry2008VegetarianJumpingSpider" />
|-
![[Acari]] (mites and ticks)
| 32,000 || Carnivorous, parasitic, herbivore, [[detritivore]]<ref name="RuppertFoxBarnes2004Acari" /><ref name="Shultz2001ArachnidsInEncOfLifeSci" />
|-
![[Opiliones]] (harvestmen)
| 6,500 || Carnivorous, herbivore, detritivore<ref name="RuppertFoxBarnes2004Opiliones" />
|-
![[Pseudoscorpiones]] (false scorpions)
| 3,200 || Carnivorous<ref>{{citation
| title=Pseudoscorpion - Penn State Entomology Department Fact Sheet | publisher=Pennsylvania State University
| url=http://www.ento.psu.edu/extension/factsheets/pseudoscorpion.htm | access-date=2008-10-26 }}</ref>
|-
![[Scorpiones]] (scorpions)
| 1,400 || Carnivorous<ref name="RuppertFoxBarnes2004Scorpions" />
|-
![[Solifugae]] (sunspiders)
| 900 || Carnivorous, [[omnivorous]]<ref name="RuppertFoxBarnes2004Solifugae">{{harvnb|Ruppert|Fox|Barnes|2004|pp=586–588}}</ref>
|-
![[Schizomida]] (small whipscorpions)
| 180 ||Carnivorous<ref name="Beccaloni">{{cite book |last=Beccaloni |first=Jan |date=2009 |title=Arachnids |location=Berkeley, CA |publisher=University of California Press |page=135|isbn=978-0-520-26140-2}}</ref>
|-
![[Amblypygi]] (whipspiders)
| 100 ||Carnivorous<ref name="Chapin">{{cite journal |last1=Chapin |first1=KJ |last2=Hebets |first2=EA |year=2016 |title=Behavioral ecology of amblypygids |url=http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1056&context=bioscihebets |journal=Journal of Arachnology |volume=44 |issue=1 |pages=1–14 |doi=10.1636/V15-62.1 |s2cid=29923727|url-access=subscription }}</ref>
|-
![[Uropygi]] (whipscorpions)
| 90 || Carnivorous<ref>{{citation
 |author = Harvey, M.S.
 |title = The Neglected Cousins: What do we Know about the Smaller Arachnid Orders?
 |journal = Journal of Arachnology
 |volume = 30
 |pages = 357–372
 |year = 2002
 |url = http://www.americanarachnology.org/JoA_Congress/JoA_v30_n2/arac-30-02-357.pdf
 |access-date = 2008-10-26
 |doi = 10.1636/0161-8202(2002)030[0357:TNCWDW]2.0.CO;2
 |issn = 0161-8202
 |issue = 2
 |s2cid = 59047074
 |archive-url = https://web.archive.org/web/20101213135221/http://www.americanarachnology.org/JoA_Congress/JoA_v30_n2/arac-30-02-357.pdf
 |archive-date = 2010-12-13
 |url-status = dead
}}</ref>
|-
![[Palpigradi]] (micro whipscorpions)
| 60 ||&nbsp;
|-
! [[Xiphosura]] (horseshoe crabs)
| 4 || Carnivorous<ref name="Shultz2001ArachnidsInEncOfLifeSci" />
|-
! [[Ricinulei]]
| 60 || Carnivorous<ref>{{cite journal |author=J. A. L. Cooke |year=1967 |title=Observations on the biology of Ricinulei (Arachnida) with descriptions of two new species of ''Cryptocellus'' |journal=[[Journal of Zoology]] |volume=151 |issue=1 |pages=31–42 |doi=10.1111/j.1469-7998.1967.tb02864.x}}</ref>
|}

==Interaction with humans==
[[Image:Yellow mite (Tydeidae) Lorryia formosa 2 edit.jpg|thumb|200px|A microscopic mite ''[[Lorryia formosa]]''.]]
In the past, [[Indigenous peoples of the Americas|Native American]]s ate the flesh of [[horseshoe crabs]], and used the tail spines as spear tips and the shells to bail water out of their canoes. More recent attempts to use horseshoe crabs as food for [[livestock]] were abandoned when it was found that this gave the meat a bad taste. Horseshoe crab blood contains a clotting agent, ''[[limulus amebocyte lysate]]'', which is used to test antibiotics and kidney machines to ensure that they are free of dangerous [[bacteria]], and to detect [[spinal meningitis]] and some [[cancer]]s.<ref>{{citation
| title=Coast
| author=Heard, W.
| url=http://www.marine.usf.edu/pjocean/packets/f01/f01u5p3.pdf
| access-date=2008-08-25
| publisher=University of South Florida
| isbn=978-1-59874-147-6
| year=2008
| archive-url=https://web.archive.org/web/20170219095700/http://www.marine.usf.edu/pjocean/packets/f01/f01u5p3.pdf
| archive-date=2017-02-19
| url-status=dead
}}</ref>

Cooked [[tarantula]] spiders are considered a delicacy in [[Cambodia]],<ref>{{citation
| author=Ray, 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.<ref>{{citation 
 |author=Weil, C. 
 |title=Fierce Food 
 |year=2006 
 |publisher=Plume 
 |isbn=978-0-452-28700-6 
 |url=https://archive.org/details/fiercefoodintrep0000weil 
 |url-status=dead 
 |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> Spider [[venom]]s may be a less polluting alternative to conventional [[pesticide]]s as they are deadly to insects but the great majority are harmless to [[vertebrate]]s.<ref>{{citation
| title=Spider Venom Could Yield Eco-Friendly Insecticides | date=3 May 2004
| publisher=National Science Foundation (USA)
| url=https://www.nsf.gov/discoveries/disc_summ.jsp?cntn_id=100676&org=NSF | access-date=2008-10-11
}}</ref> Possible medical uses for spider venoms are being investigated, for the treatment of [[cardiac arrhythmia]],<ref>{{citation
| author=Novak, K. | title=Spider venom helps hearts keep their rhythm
| journal=Nature Medicine | volume=7 | issue=155 | year=2001
| pmid=11175840 | doi=10.1038/84588
| pages=155
| s2cid=12556102
| doi-access=free}}</ref> [[Alzheimer's disease]],<ref>{{citation
| author1=Lewis, R.J.
| author2=Garcia, M.L.
| title=Therapeutic potential of venom peptides
| journal=Nature Reviews Drug Discovery
| volume=2
| issue=10
| pages=790–802
| date=October 2003
| pmid=14526382
| doi=10.1038/nrd1197
| s2cid=1348177
| doi-access=free
}}</ref> [[stroke]]s,<ref>{{citation
 |author = Bogin, O.
 |title = Venom Peptides and their Mimetics as Potential Drugs
 |journal = Modulator
 |issue = 19
 |date = Spring 2005
 |url = http://www.alomone.com/System/UpLoadFiles/DGallery/Docs/Venom%20Peptides%20and%20their%20Mimetics%20as%20Potential%20Drugs.pdf
 |access-date = 2008-10-11
 |archive-url = https://web.archive.org/web/20081209094652/http://www.alomone.com/System/UpLoadFiles/DGallery/Docs/Venom%20Peptides%20and%20their%20Mimetics%20as%20Potential%20Drugs.pdf
 |archive-date = 2008-12-09
 |url-status = dead
}}</ref> and [[erectile dysfunction]].<ref>{{citation
|author1=Andrade, E. |author2=Villanova, F. |author3=Borra, P. | title=Penile erection induced ''in vivo'' by a purified toxin from the Brazilian spider ''Phoneutria nigriventer''
| journal=British Journal of Urology International | volume=102 | issue=7 | pages=835–837
| doi=10.1111/j.1464-410X.2008.07762.x
| pmid=18537953
| date= June 2008
|s2cid=24771127 |display-authors=etal| doi-access=free }}</ref>

Because spider silk is both light and very strong, but large-scale harvesting from spiders is impractical, work is being done to produce it in other organisms by means of [[genetic engineering]].<ref name=Futurism>{{cite web |url=https://futurism.com/bacterial-factories-bulletproof-fabric |title=Scientists gene-hacked bacteria to make bullet-proof spider silk |last=Robitzski |first=Dan |website=futurism.com |date=2019-04-02 |access-date=2019-06-08}}</ref> Spider silk proteins have been successfully produced in [[transgenic]] goats' milk,<ref>{{citation
 |author = Hinman, M.B., Jones J.A., and Lewis, R.W.
 |title = Synthetic spider silk: a modular fiber
 |journal = Trends in Biotechnology
 |volume = 18
 |issue = 9
 |date = September 2000
 |pmid = 10942961
 |pages = 374–379
 |doi = 10.1016/S0167-7799(00)01481-5
 |url = http://www.tech.plym.ac.uk/sme/FailureCases/Natural_Structures/Synthetic_spider_silk.pdf
 |access-date = 2008-10-19
 |archive-url = https://web.archive.org/web/20081216214633/http://www.tech.plym.ac.uk/sme/FailureCases/Natural_Structures/Synthetic_spider_silk.pdf
 |archive-date = 2008-12-16
 |url-status = dead
|citeseerx = 10.1.1.682.313
 }}</ref>
tobacco leaves,<ref>{{citation
|author1=Menassa, R. |author2=Zhu, H. |author3=Karatzas, C.N. |author4=Lazaris, A. |author5=Richman, A. |author6=Brandle, J. |name-list-style=amp | title=Spider dragline silk proteins in transgenic tobacco leaves: accumulation and field production
| journal=Plant Biotechnology Journal | volume=2 | issue=5 | pages=431–438 | date=June 2004
| pmid=17168889
| doi=10.1111/j.1467-7652.2004.00087.x
| doi-access=free }}</ref>
[[silkworm]]s,<ref>{{cite journal |last1=Kojima |first1=Katsura |last2=Tamada |first2=Yasushi |last3=Nakajima |first3=Ken-ichi |last4=Sezutsu |first4=Hideki |last5=Kuwana |first5=Yoshihiko |title=High-Toughness Silk Produced by a Transgenic Silkworm Expressing Spider (Araneus ventricosus) Dragline Silk Protein |journal=[[PLOS ONE]] |volume=9 |issue=8 |pages=e105325 |doi=10.1371/journal.pone.0105325 |issn=1932-6203 |pmc=4146547 |pmid=25162624 |date=2014-08-27 |bibcode=2014PLoSO...9j5325K |doi-access=free }}</ref><ref>{{cite web |url=https://phys.org/news/2018-08-gene-technique-silkworms-spider-silk.html |title=Gene editing technique allows silkworms to produce spider silk |last=Yirka |first=Bob |website=[[Phys.org]] |date=2018-08-07 |access-date=2019-06-08}}</ref><ref name=kraiglabs>{{cite web |url=https://www.kraiglabs.com/spider-silk/ |title=Spider Silk {{!}} Kraig Biocraft Laboratories |website=[[Kraig Biocraft Laboratories]] |date=13 October 2014 |access-date=2019-06-08}}</ref>
and bacteria,<ref name=Futurism /><ref>{{cite web |url=https://phys.org/news/2018-08-scientists-bacteria-biosynthetic-silk-threads.html |title=Engineering scientists use bacteria to create biosynthetic silk threads stronger and more tensile than before |last=Jefferson |first=Brandie |website=phys.org |date=2018-08-21 |access-date=2019-06-08}}</ref><ref>{{cite news |url=https://www.sciencenews.org/article/bacteria-can-be-coaxed-making-toughest-kind-spider-silk |title=Bacteria can be coaxed into making the toughest kind of spider silk |last=Rehm |first=Jeremy |website=[[Science News]] |date=2019-05-01 |access-date=2019-06-08}}</ref> and [[Recombinant DNA|recombinant]] spider silk is now available as a commercial product from some biotechnology companies.<ref name=kraiglabs />

In the 20th century, there were about 100 reliably reported deaths from spider bites,<ref>{{citation
| author=Diaz, J.H.
| title=The Global Epidemiology, Syndromic Classification, Management, and Prevention of Spider Bites
| journal=American Journal of Tropical Medicine and Hygiene | volume=71 | issue=2 | date= August 1, 2004 | pages=239–250
| url=http://www.ajtmh.org/cgi/content/abstract/71/2/239 | access-date=2008-10-11
| pmid=15306718
| doi=10.4269/ajtmh.2004.71.2.0700239
| doi-access=free
}}</ref> compared with 1,500 from [[jellyfish]] stings.<ref name="WilliamsonFennerEtAl1996VenomousMarine Animals">{{citation
|author1=Williamson, J.A. |author2=Fenner, P.J. |author3=Burnett, J.W. |author4=Rifkin, J. |name-list-style=amp | title=Venomous and Poisonous Marine Animals: A Medical and Biological Handbook
| publisher=UNSW Press | year=1996 | isbn=978-0-86840-279-6 | pages=65–68
| url=https://books.google.com/books?id=YsZ3GryFIzEC&q=mollusc+venom+fatal&pg=PA75 | access-date=2008-10-03
}}</ref> Scorpion stings are thought to be a significant danger in less-developed countries; for example, they cause about 1,000 deaths per year in [[Mexico]], but only one every few years in the USA. Most of these incidents are caused by accidental human "invasions" of scorpions' nests.<ref>{{citation
|title=Scorpion Sting |author1=Cheng, D. |author2=Dattaro, J.A. |author3=Yakobi, R. |name-list-style=amp | publisher=WebMD
| url=http://www.emedicine.com/med/topic2081.htm | access-date=2008-10-25
}}</ref> On the other hand, medical uses of scorpion venom are being investigated for treatment of brain cancers and bone diseases.<ref>{{citation
| title='Scorpion venom' attacks tumours | work=BBC News
| url=http://news.bbc.co.uk/2/hi/health/5214784.stm | access-date=2008-10-25
 | date=2006-07-30}}</ref><ref>{{citation
| title=Scorpion venom blocks bone loss | publisher=Harvard University
| url=http://harvardscience.harvard.edu/medicine-health/articles/scorpion-venom-blocks-bone-loss
| access-date=2008-10-25
}}</ref>

[[Tick]]s are parasitic, and some transmit micro-organisms and [[parasite]]s that can cause diseases in humans, while the saliva of a few species can directly cause [[tick paralysis]] if they are not removed within a day or two.<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= 29 March 2010 |year=2005 |publisher=ASM Press |isbn=978-1-55581-238-6 |page=114 }}</ref>

A few of the closely related mites also infest humans, some causing intense itching by their bites, and others by burrowing into the skin. Species that normally infest other animals such as [[rodent]]s may infest humans if their normal hosts are eliminated.<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
}}</ref> Three species of mite are a threat to [[honey bee]]s and one of these, ''[[Varroa destructor]]'', has become the largest single problem faced by [[beekeeper]]s worldwide.<ref>{{citation
|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> Mites cause several forms of allergic diseases, including [[hay fever]], [[asthma]] and [[eczema]], and they aggravate [[atopic dermatitis]].<ref>{{citation |url=http://www.netdoctor.co.uk/health_advice/facts/allergyhousedustmite.htm |title=House dust mite allergy |publisher=NetDoctor |author1=Klenerman, Paul |author2=Lipworth, Brian |author3=authors |access-date=2008-02-20 |archive-date=2008-02-11 |archive-url=https://web.archive.org/web/20080211110450/http://www.netdoctor.co.uk/health_advice/facts/allergyhousedustmite.htm |url-status=dead }}</ref> Mites are also significant crop pests, although [[predator]]y mites may be useful in controlling some of these.<ref name="Shultz2001ArachnidsInEncOfLifeSci" /><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 & Applied Acarology | year=2002| volume=26 | issue=3–4 | pages=219–230
| doi=10.1023/A:1021116121604
| pmid=12542009
| s2cid=10823576
}}</ref>
{{Clear}}

==References==
{{Reflist|30em}}

== Bibliography ==
*{{Citation |last1=Ruppert |first1=E. E. |last2=Fox |first2=R. S. |last3=Barnes |first3=R. D. |title=Invertebrate Zoology |publisher=[[Brooks/Cole]] |edition=7th |isbn=978-0-03-025982-1 |year=2004 |url=https://archive.org/details/isbn_9780030259821 }}

{{Arthropods}}
{{Taxonbar|from=Q1359}}
{{Authority control}}

[[Category:Chelicerates| ]]
[[Category:Animal subphyla]]
[[Category:Late Ordovician first appearances]]
[[Category:Extant Ordovician first appearances]]
[[Category:Taxa named by Richard Heymons]]