Editing Chelicerata (section)

{{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.