Editing Chelicerata (section)

==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>