Editing Arachnid

{{Short description|Class of arthropods}}
{{Other uses}}
{{Redirect|Arachnida|the arachnida curve|Sectrix of Maclaurin}}
{{Automatic taxobox
| fossil_range = {{Fossil range|435|0}}<small>[[Llandovery epoch|Early Silurian]] – [[Holocene|present]]</small>
| image = <imagemap>
File:Arachnida collage.png|300px|center
rect 1 1 400 300 [[Spider|Araneae]]
rect 400 1 800 300 [[Amblypygi]]
rect 800 1 1200 300 [[Uropygi]]

rect 1 300 400 600 [[Schizomida]]
rect 400 300 800 600 [[Scorpion]]es
rect 800 300 1200 600 [[Pseudoscorpion]]es

rect 1 600 400 900 [[Solifugae]]
rect 400 600 800 900 [[Ricinulei]]
rect 800 600 1200 900 [[Opiliones]]

rect 1 900 400 1200 [[Palpigradi]]
rect 400 900 800 1200 [[Acariformes]]
rect 800 900 1200 1200 [[Parasitiformes]]
</imagemap>
| image_upright = 1.3
| image_caption = Representatives of the 12 extant orders of arachnids
| display_parents = 2
| taxon = Arachnida
| authority = [[Lamarck]], 1801
| subdivision_ranks = Subdivisions
| subdivision = * [[Solifugae]]&nbsp;– camel spiders
* [[Opiliones]]&nbsp;– harvestmen
* [[Ricinulei]]&nbsp;- "hooded tickspiders"
* [[Acari]] (likely polyphyletic)
** [[Acariformes]]&nbsp;– various mites
** [[Parasitiformes]]&nbsp;– various mites and ticks
* †[[Phalangiotarbida]] 
* [[Palpigradi]]&nbsp;– micro-whipscorpions
* [[Arachnopulmonata]]
** [[Panscorpiones]]<ref>{{Cite journal |last1=Gainett |first1=Guilherme |last2=Klementz |first2=Benjamin C. |last3=Setton |first3=Emily V. W. |last4=Simian |first4=Catalina |last5=Iuri |first5=Hernán A. |last6=Edgecombe |first6=Gregory D. |last7=Peretti |first7=Alfredo V. |last8=Sharma |first8=Prashant P. |date=July 2024 |title=A plurality of morphological characters need not equate with phylogenetic accuracy: A rare genomic change refutes the placement of Solifugae and Pseudoscorpiones in Haplocnemata |url=https://onlinelibrary.wiley.com/doi/10.1111/ede.12467 |journal=Evolution & Development |language=en |volume=26 |issue=4 |pages=e12467 |doi=10.1111/ede.12467 |pmid=38124251 |issn=1520-541X|hdl=11086/552527 |hdl-access=free }}</ref>
*** [[Scorpion]]es&nbsp;– scorpions
*** [[Pseudoscorpion]]es&nbsp;– pseudoscorpions
** Pantetrapulmonata
*** †''[[Douglassarachne]]''
***[[Ricinulei]]?
***†[[Trigonotarbida]] 
*** [[Tetrapulmonata]]
**** †''[[Xenarachne]]''
**** [[Schizotarsata]]
***** †[[Haptopoda]] 
***** [[Pedipalpi]]
****** [[Amblypygi]]&nbsp;– whip spiders
****** [[Schizomida]]&nbsp;– short-tailed whipscorpions
****** [[Uropygi]]&nbsp;– vinegaroons
**** [[Serikodiastida]]
***** †''[[Idmonarachne]]''
***** †[[Uraraneida]] 
***** [[Spider|Araneae]]&nbsp;– spiders
}}
'''Arachnids''' are [[arthropod]]s in the [[Class (biology)|class]] '''Arachnida''' ({{IPAc-en|ə|ˈ|r|æ|k|n|ɪ|d|ə}}) of the subphylum [[Chelicerata]]. Arachnida includes, among others, [[spider]]s, [[scorpion]]s, [[tick]]s, [[mite]]s, [[pseudoscorpion]]s, [[opiliones|harvestmen]], [[Solifugae|camel spiders]], [[Amblypygi|whip spiders]] and [[Uropygi|vinegaroons]].<ref>{{cite book |editor1-first=Joel |editor1-last=Cracraft |editor2-first=Michael |editor2-last=Donoghue |title=Assembling the Tree of Life|url=https://archive.org/details/assemblingtreeli00crac_335 |url-access=limited |publisher=[[Oxford University Press]] |date=2004 |page=[https://archive.org/details/assemblingtreeli00crac_335/page/n314 297] |name-list-style=amp}}</ref>

Adult arachnids have eight [[Arthropod leg|legs]] attached to the  [[cephalothorax]]. In some species the frontmost pair of legs has converted to a sensory function, while in others, different [[appendage]]s can grow large enough to take on the appearance of extra pairs of legs.

Almost all [[Extant taxon|extant]] arachnids are [[terrestrial animal|terrestrial]], living mainly on land. However, some inhabit freshwater environments and, with the exception of the [[pelagic zone]], marine environments as well. They comprise over 110,000 named [[species]], of which 51,000 are species of spiders.<ref>{{Cite book|last=Brabazon|first=Anthony|title=Foraging-Inspired Optimisation Algorithms|publisher=Springer International Publishing|year=2018|isbn=9783319591568|pages=237}}</ref><ref name=":0">{{Cite journal |last=Agnarsson |first=Ingi |date=2023 |title=Grand challenges in research on arachnid diversity, conservation, and biogeography |journal=Frontiers in Arachnid Science |volume=2 |doi=10.3389/frchs.2023.1101141 |doi-access=free }}</ref>

The term is derived from the [[Ancient Greek|Greek]] word {{lang|grc|[[wikt:ἀράχνη#Ancient Greek|ἀράχνη]]}} (''aráchnē'', 'spider'), from the myth of the hubristic human weaver [[Arachne]], who was turned into a spider.<ref>{{cite encyclopedia |title=Arachnid |encyclopedia=[[Oxford English Dictionary]] |date=1989 |edition=2nd}}</ref>

== Morphology ==
[[File:Spider-characteristics.png|thumb|left|Basic characteristics of arachnids include four pairs of [[arthropod leg|legs]] (1) and a body divided into two [[tagma (biology)|tagmata]]: the cephalothorax (2) and the abdomen (3)]]Almost all adult arachnids have eight legs, unlike adult [[insect]]s which all have six legs. However, arachnids also have two further pairs of appendages that have become adapted for feeding, defense, and sensory perception. The first pair, the [[chelicera]]e, serve in feeding and defense. The next pair, the [[pedipalp]]s, have been adapted for feeding, locomotion, and/or [[reproduction|reproductive]] functions. In scorpions, pseudoscorpions, and [[ricinulei]]ds the pedipalps end in a pair of pinchers, while in whip scorpions, [[Schizomida]], [[Amblypygi]], and most harvestmen, they are [[raptorial]] and used for prey capture.<ref>{{Cite book |last1=Schierwater |first1=Bernd |url=https://books.google.com/books?id=Bk4vEAAAQBAJ&dq=%2522pincers+for+prey+capture+in+scorpions%252C+pseudoscorpions%252C+and+Ricinulei%2522&pg=PA428 |title=Invertebrate Zoology: A Tree of Life Approach |last2=DeSalle |first2=Rob |date=2021-07-08 |publisher=CRC Press |isbn=978-1-4822-3582-1 |language=en}}</ref> In [[Solifugae]], the palps are quite leg-like, so that these animals appear to have ten legs. The [[larva]]e of mites and Ricinulei have only six legs; a fourth pair usually appears when they [[ecdysis|moult]] into [[nymph (biology)|nymph]]s. However, mites are variable: as well as eight, there are adult mites with six or, like in [[Eriophyoidea]], even four legs.<ref name=schmidt93>{{cite book |first=Günther |last=Schmidt |date=1993 |title=Giftige und gefährliche Spinnentiere |trans-title=Poisonous and dangerous arachnids |language=de |publisher=Westarp Wissenschaften |isbn=978-3-89432-405-6 |page=75}}</ref><ref>{{cite journal | doi=10.11158/saa.22.8.2 | title=Morphological support for a clade comprising two vermiform mite lineages: Eriophyoidea (Acariformes) and Nematalycidae (Acariformes) | year=2017 | last1=Bolton | first1=Samuel J. | last2=Chetverikov | first2=Philipp E. | last3=Klompen | first3=Hans | journal=Systematic and Applied Acarology | volume=22 | issue=8 | page=1096 | s2cid=90899467 | doi-access=free }}</ref> While the adult males in some members of [[Podapolipidae]] have six legs, the adult females have only a single pair.<ref>{{Cite book |last=Dhooria |first=Manjit Singh |url=https://books.google.com/books?id=ktS4DQAAQBAJ&dq=Podapolipidae+pair+of+legs&pg=PA49 |title=Fundamentals of Applied Acarology |date=2016-12-14 |publisher=Springer |isbn=978-981-10-1594-6 |language=en}}</ref>

Arachnids are further distinguished from insects by the fact they do not have [[antenna (biology)|antennae]] or [[insect wing|wings]]. Their body is organized into two [[tagma (biology)|tagmata]], called the [[prosoma]] and [[opisthosoma]], also referred to as the [[cephalothorax]] and [[Abdomen#Arthropoda|abdomen]]. However, there are questions about the validity of the latter terms. While the term ''cephalothorax'' implies a fused [[cephalon (arthropod anatomy)|cephalon]] (head) and [[Thorax (arthropod anatomy)|thorax]], there is currently neither fossil nor embryological evidence that arachnids ever had a separate thorax-like division. Likewise, the 'abdomen' of many arachnids contains organs atypical of an abdomen, such as a heart and respiratory organs.<ref name="Shultz2009">{{cite book |last1=Shultz |first1=Stanley |last2=Shultz |first2=Marguerite |title=The Tarantula Keeper's Guide |date=2009 |publisher=Barron's |location=Hauppauge, New York |isbn=978-0-7641-3885-0 |page=23}}</ref>

The cephalothorax is usually covered by a single, unsegmented carapace. The abdomen is segmented in the more primitive forms, but varying degrees of fusion between the segments occur in many groups. It is typically divided into a preabdomen and postabdomen, although this is only clearly visible in scorpions, and in some orders, such as the mites, the abdominal sections are completely fused.<ref>{{cite book |last1=Ruppert |first1=E. |last2=Fox |first2=R. |last3=Barnes |first3=R. |date=2007 |title=Invertebrate Zoology: A Functional Evolutionary Approach |edition=7th |publisher=[[Thomson Learning]] |isbn=978-0-03-025982-1 |name-list-style=amp |url-access=registration |url=https://archive.org/details/isbn_9780030259821 }}</ref> A [[telson]] is present in scorpions, where it has been modified to a stinger, and into a flagellum in the [[Palpigradi]], Schizomida (very short) and [[Uropygi|whip scorpions]].<ref>{{Cite book |last=Little |first=Colin |url=https://books.google.com/books?id=tfs8AAAAIAAJ&dq=palpigrades+schizomida+telson&pg=PA108 |title=The Colonisation of Land: Origins and Adaptations of Terrestrial Animals |date=1983-12-15 |publisher=Cambridge University Press |isbn=978-0-521-25218-8 |language=en}}</ref> At the base of the flagellum in the two latter groups there are glands which produce acetic acid as a chemical defense.<ref>{{Cite book |last1=Pinto-da-Rocha |first1=Ricardo |url=https://books.google.com/books?id=pbdpSKHkKDIC&dq=%2522Among+arachnids+%252C+chemical+defenses+have+been+recorded+for+Opiliones+%252C+Uropygi+%252C+and+Schizomida%2522&pg=PA382 |title=Harvestmen: The Biology of Opiliones |last2=Machado |first2=Glauco |last3=Giribet |first3=Gonzalo |date=2007-02-28 |publisher=Harvard University Press |isbn=978-0-674-02343-7 |language=en}}</ref> Except for a pair of [[pectines]] in scorpions,<ref>{{cite journal | pmc=5963125 | year=2018 | last1=Di | first1=Z. | last2=Edgecombe | first2=G. D. | last3=Sharma | first3=P. P. | title=Homeosis in a scorpion supports a telopodal origin of pectines and components of the book lungs | journal=BMC Evolutionary Biology | volume=18 | issue=1 | page=73 | doi=10.1186/s12862-018-1188-z | pmid=29783957 | bibcode=2018BMCEE..18...73D | doi-access=free }}</ref> and the [[spinneret]]s in spiders, the abdomen has no appendages.<ref>{{cite journal | url=https://www.nature.com/articles/npre.2008.2088.1 | doi=10.1038/npre.2008.2088.1 | title=Fossil evidence for the origin of spider spinnerets | year=2008 | last1=Selden | first1=Paul | last2=Shear | first2=William | journal=Nature Precedings | pages=1 | doi-access=free }}</ref>

Like all arthropods, arachnids have an [[exoskeleton]], and they also have an internal structure of [[cartilage]]-like tissue, called the [[wikt:endosternite|endosternite]], to which certain muscle groups are attached. The endosternite is even calcified in some [[Opiliones]].<ref>{{cite journal |first=J. |last=Kovoor |date=1978 |title=Natural calcification of the prosomatic endosternite in the Phalangiidae (Arachnida:Opiliones) |journal=[[Calcified Tissue Research]] |volume=26 |issue=3 |pages=267–269 |pmid=750069 |doi=10.1007/BF02013269|s2cid=23119386 }}</ref>

==Locomotion==
{{See also|Arachnid locomotion}}
Most arachnids lack [[extension (kinesiology)|extensor]] muscles in the [[Anatomical terms of location#Proximal and distal|distal]] joints of their appendages. Spiders and whip scorpions extend their limbs hydraulically using the pressure of their [[hemolymph]].<ref name=Sensenig2003>{{Cite journal |doi=10.1242/jeb.00182 |pmid=12517993 |issn=1477-9145 |volume=206 |issue=4 |pages=771–784 |last1=Sensenig |first1=Andrew T. |first2=Jeffrey W. |last2=Shultz |title=Mechanics of Cuticular Elastic Energy Storage in Leg Joints Lacking Extensor Muscles in Arachnids |journal=[[Journal of Experimental Biology]] |date=February 15, 2003 |s2cid=40503319 |name-list-style=amp|doi-access= |bibcode=2003JExpB.206..771S }}</ref> [[Solifuge]]s and some [[harvestmen]] extend their knees by the use of highly elastic thickenings in the joint cuticle.<ref name=Sensenig2003/> Scorpions, pseudoscorpions and some harvestmen have evolved muscles that extend two leg joints (the femur-patella and patella-tibia joints) at once.<ref name=Shultz2005>{{Cite journal |doi=10.1002/jmor.1052100103 |pmid=29865543 |issn=1097-4687 |volume=210 |issue=1 |pages=13–31 |last=Shultz |first=Jeffrey W. |title=Evolution of locomotion in arachnida: The hydraulic pressure pump of the giant whipscorpion, ''Mastigoproctus giganteus'' (Uropygi) |journal=[[Journal of Morphology]] |date=February 6, 2005 |s2cid=46935000 }}</ref><ref name=Shultz1992>{{Cite journal |issn=1477-9145 |volume=162 |issue=1 |pages=313–329 |last=Shultz |first=Jeffrey W. |title=Muscle Firing Patterns in Two Arachnids Using Different Methods of Propulsive Leg Extension |journal=Journal of Experimental Biology |access-date=2012-05-19 |date=January 1, 1992 |doi=10.1242/jeb.162.1.313 |url=http://jeb.biologists.org/content/162/1/313|doi-access=free |bibcode=1992JExpB.162..313S }}</ref> The equivalent joints of the pedipalps of scorpions though, are extended by elastic recoil.<ref name=Sensenig2004>{{Cite journal |doi=10.1636/S02-73 |issn=0161-8202 |volume=32 |issue=1 |pages=1–10 |last1=Sensenig |first1=Andrew T. |first2=Jeffrey W. |last2=Shultz |title=Elastic energy storage in the pedipedal joints of scorpions and sun-spiders (Arachnida, Scorpiones, Solifugae) |journal=[[Journal of Arachnology]] |date=2004 |s2cid=56461501 |name-list-style=amp|url=https://www.biodiversitylibrary.org/part/228777 }}</ref>
[[File:Haeckel Arachnida.jpg|thumb|"Arachnida" from [[Ernst Haeckel]]'s [[Kunstformen der Natur]], 1904]]

==Physiology==
{{See also|Hemolymph}}
There are characteristics that are particularly important for the terrestrial lifestyle of arachnids, such as internal respiratory surfaces in the form of [[Invertebrate trachea|trachea]]e, or modification of the [[book gill]] into a [[book lung]], an internal series of [[Blood vessel|vascular]] [[lamella (zoology)|lamella]]e used for [[gas exchange]] with the air.<ref name=Garwood>{{cite journal |last1=Garwood |first1=Russell J. |last2=Edgecombe |first2=Gregory D. |date=September 2011 |title=Early Terrestrial Animals, Evolution, and Uncertainty |journal=Evolution: Education and Outreach |volume=4 |issue=3 |pages=489–501 |doi=10.1007/s12052-011-0357-y |name-list-style=amp|doi-access=free }}</ref> While the tracheae are often individual systems of tubes, similar to those in insects, ricinuleids, pseudoscorpions, and some spiders possess sieve tracheae, in which several tubes arise in a bundle from a small chamber connected to the [[Spiracle (arthropods)|spiracle]]. This type of tracheal system has almost certainly evolved from the book lungs, and indicates that the tracheae of arachnids are not [[Homology (biology)|homologous]] with those of insects.<ref name=IZ/>

Further adaptations to terrestrial life are [[appendage]]s modified for more efficient locomotion on land, [[internal fertilisation]], special sensory organs, and water conservation enhanced by efficient [[excretory]] structures as well as a waxy layer covering the cuticle.{{citation needed|date=March 2025}}

The excretory glands of arachnids include up to four pairs of [[coxal gland]]s along the side of the prosoma, and one or two pairs of [[Malpighian tubule]]s, emptying into the gut. Many arachnids have only one or the other type of excretory gland, although several do have both. The primary nitrogenous waste product in arachnids is [[guanine]].<ref name=IZ/>

Arachnid blood is variable in composition, depending on the mode of respiration. Arachnids with an efficient tracheal system do not need to transport oxygen in the blood, and may have a reduced circulatory system. In scorpions and some spiders, however, the blood contains [[haemocyanin]], a copper-based pigment with a similar function to [[haemoglobin]] in vertebrates. The [[heart]] is located in the forward part of the abdomen, and may or may not be segmented. Some mites have no heart at all.<ref name=IZ/>

==Diet and digestive system==

Arachnids are mostly [[carnivore|carnivorous]], feeding on the pre-digested bodies of insects and other small animals. But ticks, and many mites, are parasites, some of which are carriers of disease. The diet of [[mite]]s also include tiny animals, fungi, plant juices and decomposing matter.<ref>{{cite journal | doi=10.1111/brv.12832 | title=Feeding habits and multifunctional classification of soil-associated consumers from protists to vertebrates | year=2022 | last1=Potapov | first1=Anton M. | last2=Beaulieu | first2=Frédéric | last3=Birkhofer | first3=Klaus | last4=Bluhm | first4=Sarah L. | last5=Degtyarev | first5=Maxim I. | last6=Devetter | first6=Miloslav | last7=Goncharov | first7=Anton A. | last8=Gongalsky | first8=Konstantin B. | last9=Klarner | first9=Bernhard | last10=Korobushkin | first10=Daniil I. | last11=Liebke | first11=Dana F. | last12=Maraun | first12=Mark | last13=Mc Donnell | first13=Rory J. | last14=Pollierer | first14=Melanie M. | last15=Schaefer | first15=Ina | last16=Shrubovych | first16=Julia | last17=Semenyuk | first17=Irina I. | last18=Sendra | first18=Alberto | last19=Tuma | first19=Jiri | last20=Tůmová | first20=Michala | last21=Vassilieva | first21=Anna B. | last22=Chen | first22=Ting-Wen | last23=Geisen | first23=Stefan | last24=Schmidt | first24=Olaf | last25=Tiunov | first25=Alexei V. | last26=Scheu | first26=Stefan | journal=Biological Reviews | volume=97 | issue=3 | pages=1057–1117 | pmid=35060265 | s2cid=246078291 | doi-access=free }}</ref> Almost as varied is the diet of [[Opiliones|harvestmen]], where we will find predators, decomposers and omnivores feeding on decaying plant and animal matter, droppings, animals and mushrooms.<ref>{{cite journal | doi=10.1636/JoA-S-20-002 | title=Diet, predators, and defensive behaviors of New Zealand harvestmen (Opiliones: Neopilionidae) | year=2021 | last1=Powell | first1=Erin C. | last2=Painting | first2=Christina J. | last3=Hickey | first3=Anthony J. | last4=Machado | first4=Glauco | last5=Holwell | first5=Gregory I. | journal=The Journal of Arachnology | volume=49 | s2cid=234364795 | doi-access=free }}</ref><ref>[https://www.wildlifetrusts.org/wildlife-explorer/invertebrates/spiders/common-harvestman Common harvestman | The Wildlife Trusts]</ref><ref>[https://www.discoverwildlife.com/animal-facts/insects-invertebrates/how-do-harvestmen-hunt/ How do harvestmen hunt? - BBC Wildlife Magazine]</ref> The [[harvestmen]] and some mites, such as the [[house dust mite]], are also the only arachnids able to ingest solid food, which exposes them to internal parasites,<ref name=pinto2007>{{cite book |last1=Machado |first1=Glauco |last2=Pinto-da-Rocha |first2=Ricardo |last3=Giribet |first3=Gonzalo |editor1-last=Pinto-da-Rocha |editor1-first=Ricardo |editor2-last=Machado |editor2-first=Glauco |editor3-last=Giribet |editor3-first=Gonzalo |date=2007 |title=Harvestmen: the Biology of Opiliones |publisher=[[Harvard University Press]] |isbn=978-0-674-02343-7 |name-list-style=amp}}</ref> although it is not unusual for spiders to eat their own silk. And [[Bagheera kiplingi|one species]] of spider is mostly herbivorous.<ref>[https://www.livescience.com/5759-rare-vegetarian-spider-discovered.html Rare Vegetarian Spider Discovered]</ref> Scorpions, spiders and pseudoscorpions secrete [[venom]] from specialized [[gland]]s to kill prey or defend themselves.<ref>{{cite journal | pmc=5983263 | year=2018 | last1=Santibáñez-López | first1=C. E. | last2=Ontano | first2=A. Z. | last3=Harvey | first3=M. S. | last4=Sharma | first4=P. P. | title=Transcriptomic Analysis of Pseudoscorpion Venom Reveals a Unique Cocktail Dominated by Enzymes and Protease Inhibitors | journal=Toxins | volume=10 | issue=5 | page=207 | doi=10.3390/toxins10050207 | pmid=29783636 | doi-access=free }}</ref> Their venom also contains pre-digestive enzymes that helps breaking down the prey.<ref>{{cite journal | pmc=3337657 | year=2012 | last1=Zeh | first1=J. A. | last2=Bonilla | first2=M. M. | last3=Adrian | first3=A. J. | last4=Mesfin | first4=S. | last5=Zeh | first5=D. W. | title=From father to son: Transgenerational effect of tetracycline on sperm viability | journal=Scientific Reports | volume=2 | page=375 | doi=10.1038/srep00375 | pmid=22540028 | bibcode=2012NatSR...2..375Z }}</ref><ref>{{cite journal | pmc=9030722 | year=2022 | last1=Delgado-Prudencio | first1=G. | last2=Cid-Uribe | first2=J. I. | last3=Morales | first3=J. A. | last4=Possani | first4=L. D. | last5=Ortiz | first5=E. | last6=Romero-Gutiérrez | first6=T. | title=The Enzymatic Core of Scorpion Venoms | journal=Toxins | volume=14 | issue=4 | page=248 | doi=10.3390/toxins14040248 | pmid=35448857 | doi-access=free }}</ref><ref>{{cite journal | pmc=5553785 | year=2017 | last1=Walter | first1=A. | last2=Bechsgaard | first2=J. | last3=Scavenius | first3=C. | last4=Dyrlund | first4=T. S. | last5=Sanggaard | first5=K. W. | last6=Enghild | first6=J. J. | last7=Bilde | first7=T. | title=Characterisation of protein families in spider digestive fluids and their role in extra-oral digestion | journal=BMC Genomics | volume=18 | issue=1 | page=600 | doi=10.1186/s12864-017-3987-9 | pmid=28797246 | doi-access=free }}</ref> The saliva of ticks contains anticoagulants and anticomplements, and several species produce a [[neurotoxin]].<ref>{{cite journal | pmc=8294578 | year=2021 | last1=Denisov | first1=S. S. | last2=Ippel | first2=J. H. | last3=Castoldi | first3=E. | last4=Mans | first4=B. J. | last5=Hackeng | first5=T. M. | last6=Dijkgraaf | first6=I. | title=Molecular basis of anticoagulant and anticomplement activity of the tick salivary protein Salp14 and its homologs | journal=The Journal of Biological Chemistry | volume=297 | issue=1 | page=100865 | doi=10.1016/j.jbc.2021.100865 | pmid=34118237 | doi-access=free }}</ref><ref>[https://www.ncbi.nlm.nih.gov/books/NBK470478/ Tick Paralysis - StatPearls - NCBI Bookshelf]</ref>

Arachnids produce digestive enzymes in their stomachs, and use their pedipalps and chelicerae to pour them over their dead prey. The digestive juices rapidly turn the prey into a broth of nutrients, which the arachnid sucks into a pre-buccal cavity located immediately in front of the mouth. Behind the mouth is a muscular, sclerotised [[pharynx]], which acts as a pump, sucking the food through the mouth and on into the [[oesophagus]] and [[stomach]]. In some arachnids, the oesophagus also acts as an additional pump.{{citation needed|date=March 2025}}

The stomach is tubular in shape, with multiple [[diverticula]] extending throughout the body. The stomach and its diverticula both produce digestive enzymes and absorb nutrients from the food. It extends through most of the body, and connects to a short sclerotised [[intestine]] and [[anus]] in the hind part of the abdomen.<ref name=IZ>{{cite book |first=Robert D. |last=Barnes |date=1982 |title=Invertebrate Zoology |publisher=Holt-Saunders International |location=Philadelphia, PA |pages=596–604 |isbn=978-0-03-056747-6}}</ref>

==Senses==

Arachnids have two kinds of eyes: the lateral and median [[ocelli]]. The lateral ocelli evolved from [[compound eye]]s and may have a [[tapetum lucidum|tapetum]], which enhances the ability to collect light. With the exception of scorpions, which can have up to five pairs of lateral ocelli, there are never more than three pairs present. The median ocelli develop from a transverse fold of the [[ectoderm]]. The ancestors of modern arachnids probably had both types, but modern ones often lack one type or the other.<ref name=pinto2007/> The [[cornea]] of the eye also acts as a lens, and is continuous with the cuticle of the body. Beneath this is a transparent vitreous body, and then the [[retina]] and, if present, the tapetum. In most arachnids, the retina probably does not have enough light sensitive cells to allow the eyes to form a proper image.<ref name=IZ/>

In addition to the eyes, almost all arachnids have two other types of sensory organs. The most important to most arachnids are the fine sensory hairs that cover the body and give the animal its sense of touch. These can be relatively simple, but many arachnids also possess more complex structures, called [[trichobothria]].{{citation needed|date=March 2025}}

Finally, slit sense organs are slit-like pits covered with a thin membrane. Inside the pit, a small hair touches the underside of the membrane, and detects its motion. Slit sense organs are believed to be involved in [[proprioception]], and possibly also hearing.<ref name=IZ/>

==Reproduction==
{{see also|Spider#Reproduction and life cycle|Scorpion#Reproduction}}
[[File:Vinegaroons in courtship (Thelyphonus sp) (8390306848).jpg|thumb|Courtship behavior of ''[[Thelyphonus]]'' sp.|280px]]
Arachnids may have one or two [[gonad]]s, which are located in the abdomen. The genital opening is usually located on the underside of the second abdominal segment. In most species, the male transfers sperm to the female in a package, or [[spermatophore]]. The males in harvestmen and some mites have a penis.<ref>{{cite journal | pmc=6225839 | year=2018 | last1=McLean | first1=C. J. | last2=Garwood | first2=R. J. | last3=Brassey | first3=C. A. | title=Sexual dimorphism in the Arachnid orders | journal=PeerJ | volume=6 | pages=e5751 | doi=10.7717/peerj.5751 | pmid=30416880 | doi-access=free }}</ref> Complex courtship rituals have evolved in many arachnids to ensure the safe delivery of the sperm to the female.<ref name=IZ/> Members of many orders exhibit sexual dimorphism.<ref name="McLeanGarwood2018">{{cite journal|last1=McLean|first1=Callum J.|last2=Garwood|first2=Russell J.|last3=Brassey|first3=Charlotte A.|title=Sexual dimorphism in the Arachnid orders|journal=PeerJ|volume=6|year=2018|pages=e5751|issn=2167-8359|doi=10.7717/peerj.5751|pmid=30416880|pmc=6225839|doi-access=free}}</ref>

Arachnids usually lay yolky [[egg (biology)|eggs]], which hatch into immatures that resemble adults. Scorpions, however, are either [[ovoviviparous]] or [[viviparous]], depending on species, and bear live young. Also some mites are ovoviviparous and viviparous, even if most lay eggs.<ref>{{Cite book |last=Auerbach |first=Paul S. |url=https://books.google.com/books?id=tdTInPqMCjMC&dq=%2522some+are+ovoviviparous%252C+and+a+few+are+viviparous%2522&pg=PA950 |title=Wilderness Medicine E-Book: Expert Consult Premium Edition - Enhanced Online Features |date=2011-10-31 |publisher=Elsevier Health Sciences |isbn=978-1-4557-3356-9 |language=en}}</ref> In most arachnids only the females provide parental care, with harvestmen being one of the few exceptions.<ref>{{cite journal |last1=Quesada-Hidalgo |first1=Rosannette |last2=Solano-Brenes |first2=Diego |last3=Requena |first3=Gustavo S. |last4=Machado |first4=Glauco |title=The good fathers: efficiency of male care and the protective role of foster parents in a Neotropical arachnid |journal=Animal Behaviour |date=April 2019 |volume=150 |pages=147–155 |doi=10.1016/j.anbehav.2019.02.007|s2cid=73728615 }}</ref><ref>{{cite journal |last1=Nazareth |first1=Tais M. |last2=Machado |first2=Glauco |title=Mating system and exclusive postzygotic paternal care in a Neotropical harvestman (Arachnida: Opiliones) |journal=Animal Behaviour |date=March 2010 |volume=79 |issue=3 |pages=547–554 |doi=10.1016/j.anbehav.2009.11.026|s2cid=53166528 }}</ref>

==Taxonomy and evolution==

===Phylogeny===
The [[Phylogeny|phylogenetic]] relationships among the main subdivisions of arthropods have been the subject of considerable research and dispute for many years. A consensus emerged from about 2010 onwards, based on both morphological and molecular evidence; extant (living) arthropods are a [[Monophyly|monophyletic]] group and are divided into three main clades: chelicerates (including arachnids), pancrustaceans (the [[paraphyly|paraphyletic]] crustaceans plus insects and their allies), and myriapods (centipedes, millipedes and allies).<ref name=MeusReumSimoRoed10/><ref name=RegiShulZwicHuss10/><ref name=RotaCampBrinEdge10/><ref name=CampRotaEdgeMarc11/><ref name=SharKaluPereGonz14/> The three groups are related as shown in the [[cladogram]] below.<ref name=RotaCampBrinEdge10/> Including fossil taxa does not fundamentally alter this view, although it introduces some additional basal groups.<ref name=LeggSuttEdge13/>
{{clade
|style=line-height:100%
|label1=  [[Arthropoda]] 
|1={{clade
   |label1=
   |1=[[Chelicerata]] (sea spiders, horseshoe crabs and '''arachnids''') [[File:Nymphon signatum 194389384 (white background).jpg|90 px]] [[File:Limulus polyphemus (aquarium) (white background).jpg|70 px]] [[File:Aptostichus simus Monterey County.jpg|60 px]]
   |label2=  [[Mandibulata]] 
   |2={{clade
      |label1=
      |1=[[Myriapoda]] (centipedes, millipedes, and allies) [[File:Scolopendra japonica アオズムカデ 大阪府 生駒山産.jpg|70 px]] <span style="{{MirrorH}}">[[File:Andrognathus corticarius A.jpg|100 px]]</span>
      |2=[[Pancrustacea]] (crustaceans and hexapods) <span style="{{MirrorH}}">[[File:Lobster png by absurdwordpreferred d2xqhvd.png|70 px]]</span> [[File:Platycheirus angustatus (Syrphidae) - (male imago), Elst (Gld), the Netherlands - 2.jpg|60 px]]
      }}
   }}
}}

The extant chelicerates comprise two marine groups: Sea spiders and horseshoe crabs, and the terrestrial arachnids. These have been thought to be related as shown below.<ref name=RegiShulZwicHuss10/><ref name=SharKaluPereGonz14/> (Pycnogonida (sea spiders) may be excluded from the chelicerates, which are then identified as the group labelled "Euchelicerata".<ref name=GiriEdgeWhee01/>) A 2019 analysis nests Xiphosura deeply within Arachnida.<ref name=Sharma2019/>

{{clade
|style=line-height:100%
|label1=  [[Chelicerata]] 
|1={{clade
   |1={{clade
      |label1=
      |1=[[Pycnogonida]] (sea spiders) [[File:Nymphon signatum 194389384 (white background).jpg|90 px]]
      }}
   |label2=  [[Euchelicerata]] 
   |2={{clade
      |label1=
      |1=[[Xiphosura]] (horseshoe crabs) [[File:Limulus polyphemus (aquarium) (white background).jpg|70 px]]
      |2='''Arachnida''' [[File:Aptostichus simus Monterey County.jpg|60 px]]
      }}
   }}
}}

Discovering relationships within the arachnids has proven difficult {{as of|2016|March|lc=true}}, with successive studies producing different results. A study in 2014, based on the largest set of molecular data to date, concluded that there were systematic conflicts in the phylogenetic information, particularly affecting the orders [[Acariformes]], [[Parasitiformes]] and [[Pseudoscorpiones]], which have had much faster evolutionary rates. Analyses of the data using sets of genes with different evolutionary rates produced mutually incompatible [[phylogenetic tree]]s. The authors favoured relationships shown by more slowly evolving genes, which demonstrated the monophyly of Chelicerata, Euchelicerata and Arachnida, as well as of some clades within the arachnids. The diagram below summarizes their conclusions, based largely on the 200&nbsp;most slowly evolving genes; dashed lines represent uncertain placements.<ref name=SharKaluPereGonz14/>
{{barlabel
|size=10
|at1=13
|label1=  Arachnopulmonata 
|cladogram=
{{clade
|label1=  '''Arachnida''' 
|1={{clade
   |label1=
   |state1=dashed
   |1=[[Acariformes]] [[File:Rote Samtmilbe Namibia.png|60px]]
   |state2=dashed
   |2={{clade
      |label1=
      |1={{clade
         |label1=
         |1=[[Opiliones]] [[File:Phalangium opilio 2 (Nemo5576) (white background).jpg|60px]]
         }}
      |2={{clade
         |1=[[Ricinulei]] [[File:Ricinulei from Fernandez & Giribet (2015).png|60px]]
         |2=[[Solifugae]] [[File:Ammotrecha itzaana 4414721993.png|60px]]
         }}
      }}
   |state3=dashed
   |3=[[Parasitiformes]] [[File:Ixodes scapularis P1170301a (white background).png|60px]]
   |state4=dashed
   |4={{clade
      |label1=
      |state1=dashed
      |1=[[Pseudoscorpiones]] <span style="{{MirrorH}}">[[File:Neobisium sylvaticum 03.png|60px]]</span>
      |2=[[Scorpiones]] [[File:Buthus mariefranceae (10.3897-zookeys.686.12206) Figure 1.jpg|60px]] |barbegin2=purple
      |label3=  [[Tetrapulmonata]] 
      |3={{clade
         |label1=
         |1=[[Araneae]] [[File:Aptostichus simus Monterey County.jpg|60 px]] |bar1=purple
         |2={{clade
            |1=[[Amblypygi]] [[File:Flickr - ggallice - Tailless whip-scorpion, La Muerta.png|60px]] |bar1=purple
            |2=[[Uropygi]] (Thelyphonida ''s.s.'') [[File:Whip Scorpion body (9672115742) (white background).png|60px]] |barend2=purple
            }}
         }}
      }}
   }}
}}
}}

[[File:Hubbardia pentapeltis female.jpg|thumb|''[[Hubbardia pentapeltis]]'' (Schizomida)]]
[[Tetrapulmonata]], here consisting of [[Spider|Araneae]], [[Amblypygi]] and [[Uropygi]] (Thelyphonida ''s.s.'') ([[Schizomida]] was not included in the study), received strong support. Somewhat unexpectedly, there was support for a clade comprising [[Opiliones]], [[Ricinulei]] and [[Solifugae]], a combination not found in most other studies.<ref name=SharKaluPereGonz14/> In early 2019, a molecular phylogenetic analysis placed the horseshoe crabs, [[Xiphosura]], as the sister group to Ricinulei. It also grouped pseudoscorpions with mites and ticks, which the authors considered may be due to [[long branch attraction]].<ref name = Sharma2019>{{cite journal |last2=Sharma |first2=P.P. |last1= Ballesteros |first1=J.A. |year= 2019 |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 |pages=896–917 |doi= 10.1093/sysbio/syz011 |doi-access=free |pmid=30917194 }}</ref> The addition of [[Scorpiones]] to produce a clade called Arachnopulmonata was also well supported. Pseudoscorpiones may also belong here, as all six orders share the same ancient whole [[genome duplication]],<ref>{{cite journal |doi=10.1098/rspb.2021.1168 | biorxiv=10.1101/2021.01.11.426205 | title=The genome of a daddy-long-legs (Opiliones) illuminates the evolution of arachnid appendages | date=2021 | last1=Gainett | first1=Guilherme | last2=González | first2=Vanessa L. | last3=Ballesteros | first3=Jesús A. | last4=Setton | first4=Emily V. W. | last5=Baker | first5=Caitlin M. | last6=Barolo Gargiulo | first6=Leonardo | last7=Santibáñez-López | first7=Carlos E. | last8=Coddington | first8=Jonathan A. | last9=Sharma | first9=Prashant P. | journal=Proceedings of the Royal Society B: Biological Sciences | volume=288 | issue=1956 | pmid=34344178 | pmc=8334856 }}</ref><ref>
{{cite journal
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 |doi-access = free |pmid = 28756775 |pmc = 5535294}}</ref> and analyses support pseudoscorpions as the sister group of scorpions, with this clade forming the sister group to Tetrapulmonoata within Arachnopulmonata.,<ref>{{cite journal | last1=Garbiec | first1=Arnold | last2=Christophoryová | first2=Jana | last3=Jędrzejowska | first3=Izabela | year=2022 | 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 | pmid=35440674 | pmc=9018881 | doi=10.1038/s41598-022-10283-z | bibcode=2022NatSR..12.6447G }}</ref><ref>
{{cite journal
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</ref><ref name=":1">{{Cite journal |last=Gainett |first=Guilherme |last2=Klementz |first2=Benjamin C. |last3=Setton |first3=Emily V. W. |last4=Simian |first4=Catalina |last5=Iuri |first5=Hernán A. |last6=Edgecombe |first6=Gregory D. |last7=Peretti |first7=Alfredo V. |last8=Sharma |first8=Prashant P. |date=July 2024 |title=A plurality of morphological characters need not equate with phylogenetic accuracy: A rare genomic change refutes the placement of Solifugae and Pseudoscorpiones in Haplocnemata |url=https://onlinelibrary.wiley.com/doi/10.1111/ede.12467 |journal=Evolution & Development |language=en |volume=26 |issue=4 |doi=10.1111/ede.12467 |issn=1520-541X|doi-access=free |hdl=11086/552527 |hdl-access=free }}</ref> with analysis of Solifugae genomes indicating that they do not have a whole genome duplication, making a previously suggested close relationship with pseudoscorpions unlikely.<ref name=":1" /> Genetic analysis has not yet been done for Ricinulei, or Palpigradi, but horseshoe crabs have gone through two whole genome duplications, which gives them five Hox clusters with 34 [[Hox gene]]s, the highest number found in any invertebrate, yet it is not clear if the oldest genome duplication is related to the one in Arachnopulmonata.<ref>{{Cite journal |last1=Shingate |first1=Prashant |last2=Ravi |first2=Vydianathan |last3=Prasad |first3=Aravind |last4=Tay |first4=Boon-Hui |last5=Garg |first5=Kritika M. |last6=Chattopadhyay |first6=Balaji |last7=Yap |first7=Laura-Marie |last8=Rheindt |first8=Frank E. |last9=Venkatesh |first9=Byrappa |date=2020 |title=Chromosome-level assembly of the horseshoe crab genome provides insights into its genome evolution |journal=Nature Communications |language=en |volume=11 |issue=1 |pages=2322 |doi=10.1038/s41467-020-16180-1 |pmid=32385269 |pmc=7210998 |bibcode=2020NatCo..11.2322S |issn=2041-1723}}</ref>

More recent phylogenomic analyses that have densely sampled both genomic datasets and morphology have supported horseshoe crabs as nested inside Arachnida, suggesting a complex history of terrestrialization.<ref>{{Cite journal |last1=Ballesteros |first1=Jesús A. |last2=Santibáñez López |first2=Carlos E. |last3=Kováč |first3=Ľubomír |last4=Gavish-Regev |first4=Efrat |last5=Sharma |first5=Prashant P. |date=2019-12-18 |title=Ordered phylogenomic subsampling enables diagnosis of systematic errors in the placement of the enigmatic arachnid order Palpigradi |journal=Proceedings of the Royal Society B: Biological Sciences |language=en |volume=286 |issue=1917 |pages=20192426 |doi=10.1098/rspb.2019.2426 |issn=0962-8452 |pmc=6939912 |pmid=31847768}}</ref><ref>{{Cite journal |last1=Ballesteros |first1=Jesús A. |last2=Santibáñez-López |first2=Carlos E. |last3=Baker |first3=Caitlin M. |last4=Benavides |first4=Ligia R. |last5=Cunha |first5=Tauana J. |last6=Gainett |first6=Guilherme |last7=Ontano |first7=Andrew Z. |last8=Setton |first8=Emily V.W. |last9=Arango |first9=Claudia P. |last10=Gavish-Regev |first10=Efrat |last11=Harvey |first11=Mark S. |last12=Wheeler |first12=Ward C. |last13=Hormiga |first13=Gustavo |last14=Giribet |first14=Gonzalo |last15=Sharma |first15=Prashant P. |display-authors=6 |date=2022-02-03 <!-- |editor-last=Teeling |editor-first=Emma --> |title=Comprehensive species sampling and sophisticated algorithmic approaches refute the monophyly of Arachnida |journal=Molecular Biology and Evolution |language=en |volume=39 |issue=2 |page=msac021 |doi=10.1093/molbev/msac021 |issn=0737-4038 |pmc=8845124 |pmid=35137183 |url=https://academic.oup.com/mbe/article/doi/10.1093/molbev/msac021/6522129 }}</ref> Morphological analyses including fossils tend to recover the Tetrapulmonata, including the extinct group the [[Haptopoda]],<ref name="WangDunlop2018">{{cite journal |last1= Wang |first1=B. |last2= Dunlop |first2=J.A. |last3= Selden |first3=P.A. |last4= Garwood |first4=R.J. |last5= Shear |first5=W.A. |last6= Müller |first6=P. |last7= Lei |first7=X. |year= 2018 |title= Cretaceous arachnid ''Chimerarachne yingi'' gen. et sp. nov. illuminates spider origins |journal=Nature Ecology & Evolution |volume= 2 |issue=4 |pages= 614–622 |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><ref name="GarwoodDunlop2017">{{cite journal |last1= Garwood |first1=R.J. |last2= Dunlop |first2=J.A.|last3= Knecht|first3=B.J.|last4= Hegna |first4=T.A. |year=2017 |title=The phylogeny of fossil whip spiders |journal=BMC Evolutionary Biology |volume=17 |issue=1 |page=105 |doi=10.1186/s12862-017-0931-1 |doi-access=free |pmid=28431496 |pmc=5399839 |bibcode=2017BMCEE..17..105G }}</ref><ref name=GarwoodDunlop2016>{{cite journal |last1= Garwood |first1=R.J. |last2= Dunlop |first2=J.A. |last3= Selden |first3=P.A. |last4= Spencer |first4=A.R.T. |last5= Atwood |first5=R.C. |last6= Vo |first6=N.T. |last7= Drakopoulos |first7=M. |year= 2016 |title=Almost a spider: A 305&nbsp;million-year-old fossil arachnid and spider origins |journal=Proceedings of the Royal Society B: Biological Sciences |volume= 283 |issue=1827 |page=20160125 |doi=10.1098/rspb.2016.0125 |pmid=27030415 |pmc=4822468}}</ref><ref name=GarwoodDunlop2014>{{cite journal |last1= Garwood |first1=R.J. |last2= Dunlop |first2=J. |year= 2014 |title=Three-dimensional reconstruction and the phylogeny of extinct chelicerate orders |journal= PeerJ |volume= 2 |page=e641 |doi= 10.7717/peerj.641 |doi-access=free |pmid= 25405073 |pmc= 4232842 }}</ref><ref name=Shultz2007>{{cite journal|last1= Shultz |first1=J.W. |year= 2007 |title=A phylogenetic analysis of the arachnid orders based on morphological characters |journal=Zoological Journal of the Linnean Society |volume= 150 |issue= 2 |pages= 221–265 |doi=10.1111/j.1096-3642.2007.00284.x |doi-access= free }}</ref> but recover other ordinal relationships with low support.

Cladogram of current understanding of chelicerate relationships, after Sharma and Gavish-Regev (2025):<ref name=":2">{{Cite journal |last=Sharma |first=Prashant P. |last2=Gavish-Regev |first2=Efrat |date=2025-01-28 |title=The Evolutionary Biology of Chelicerata |url=https://www.annualreviews.org/content/journals/10.1146/annurev-ento-022024-011250 |journal=Annual Review of Entomology |language=en |volume=70 |issue=1 |pages=143–163 |doi=10.1146/annurev-ento-022024-011250 |issn=0066-4170|url-access=subscription }}</ref>{{clade|{{clade
|1=[[Pycnogonida]] (sea spiders) [[File:Pseudopallene_pachycheira.jpeg|100px]]
|label2=[[Prosomapoda]]
|2={{clade
|1=[[Opiliones]] (harvestmen) [[File:Nemastomella dubia 2.jpg|100px]]
|state2=dashed
|2={{clade
|state1=dashed
|state2=dashed
|1=[[Palpigradi]] (microwhip scorpions) [[File:Live Eukoenenia spelaea in its cave habitat.png|100px]]
|2={{clade
|state1=dashed
|state2=dashed
|1=[[Solifugae]] (camel spiders) [[File:USMC-050510-M-7846V-002.jpg|100px]]
|2=[[Acariformes]] (mites, in part) [[File:Galumnidae sp.png|100px]]
}}}}
|3=[[Parasitiformes]] (mites, in part and ticks) [[File:Promegistus armstrongi 335075066.jpg|100px]]
|4={{clade
|state1=dashed
|state2=dashed
|1=[[Xiphosura]] (horseshoe crabs) [[File:Horseshoe crab (62577).jpg|100px]]
|2=[[Ricinulei]] [[File:Ricinulei_from_Fernandez_&_Giribet,_male_Ricinoides_karschii_from_Campo_Reserve,_Cameroon_(2015)_(cropped).jpg|100px]]
|label3=Arachnopulmonata
|3={{clade
|label1=Panscorpiones
|1={{clade
|1=[[Pseudoscorpiones]] (pseudoscorpions) [[File:Neobisium_sylvaticum_03.jpg|100px]]
|2=[[Scorpiones]] (scorpions) [[File:Buthus_mariefranceae_(10.3897-zookeys.686.12206)_Figure_1.jpg|100px]]
}}
|label2=[[Tetrapulmonata]]
|2={{clade
|1=[[Araneae]] (spiders) [[File:Theraphosa_blondi_MHNT.jpg|100px]]
|2={{clade
|1=[[Amblypygi]] (whip-spiders) [[File:Damon_johnstoni_–_Lydekker,_1879.png|100px]]
|2={{clade
|1= [[Schizomida]] (shorttailed whipscorpions) [[File:Hubbardia_pentapeltis_female.jpg|100px]]
|2=[[Uropygi]] (whip scorpions/vinegaroons) [[File:Whip_Scorpion_body_(9672115742)_(white_background).png|100px]]
}}}}}}}}}}}}}}|label1=[[Chelicerata]]}}

===Fossil history===
{{further|Evolution of spiders|Evolution of scorpions}}
[[File:Goniotarbus angulatus holotype fossil dorsal ventral.jpg|thumb|Fossil ''Goniotarbus angulatus'' ([[Phalangiotarbida]])]]
[[File:Kreischeria Vienna.jpg|thumb|Fossil of ''[[Kreischeria]]'' (Trigonotarbida)]]
The [[Uraraneida]] are an extinct order of spider-like arachnids from the [[Devonian]] and [[Permian]].<ref name=SeldSheaSutt08>{{Citation |last1=Selden |first1=P.A. |last2=Shear |first2=W.A. |last3=Sutton |first3=M.D. |date=2008 |title=Fossil evidence for the origin of spider spinnerets, and a proposed arachnid order |journal=Proceedings of the National Academy of Sciences |volume=105 |issue=52 |pages=20781–20785 |doi=10.1073/pnas.0809174106 |name-list-style=amp |pmid=19104044 |pmc=2634869|bibcode=2008PNAS..10520781S |doi-access=free }}</ref>

A fossil arachnid in 100 million year old (mya) [[amber]] from Myanmar, ''[[Chimerarachne yingi]]'', has spinnerets (to produce silk); it also has a tail, like the [[Palaeozoic]] Uraraneida, some 200 million years after other known fossils with tails. The fossil resembles the most primitive living spiders, the [[Mesothelae|mesotheles]].<ref>{{cite news |last1=Briggs |first1=Helen |title='Extraordinary' fossil sheds light on origins of spiders |url=https://www.bbc.co.uk/news/science-environment-42945813 |access-date=9 June 2018 |agency=BBC |date=5 February 2018}}</ref><ref name="WangDunlop2018" />

===Taxonomy===
[[File:Live Eukoenenia spelaea in its cave habitat.png|thumb|''Eukoenenia spelaea'' ([[Palpigradi]])]]

The subdivisions of the arachnids are usually treated as [[Order (biology)|orders]]. Historically, [[mite]]s and [[tick]]s were treated as a single order, Acari. However, molecular phylogenetic studies suggest that the two groups do not form a single clade, with morphological similarities being due to convergence. They are now usually treated as two separate taxa – Acariformes, mites, and Parasitiformes, ticks – which may be ranked as orders or superorders. The arachnid subdivisions are listed below alphabetically; numbers of species are approximate.{{citation needed|date=March 2025}}
;Extant forms
* [[Acariformes]] – mites (32,000 species)
* [[Amblypygi]] – "blunt rump" tail-less whip scorpions with front legs modified into [[whip]]-like sensory structures as long as 25&nbsp;cm or more (250 species)
* [[Spider|Araneae]] – spiders (51,000 species)
* [[Opiliones]] – phalangids, harvestmen or daddy-long-legs (6,700 species)
* [[Palpigradi]] – microwhip scorpions (130 species)
* [[Parasitiformes]] – ticks (12,000 species)
* [[Pseudoscorpion]]ida – pseudoscorpions (4,000 species)
* [[Ricinulei]] – ricinuleids, hooded tickspiders (100 species)
* [[Schizomida]] – "split middle" whip scorpions with divided exoskeletons (350 species)
* [[Scorpion]]es – scorpions (2,700 species)
* [[Solifugae]] – solpugids, windscorpions, sun spiders or camel spiders (1,200 species)
* [[Uropygi]] (also called Thelyphonida) – whip scorpions or vinegaroons, forelegs modified into sensory appendages and a long tail on abdomen tip (120 species)
;Extinct forms
* †[[Haptopoda]] – extinct arachnids apparently part of the [[Tetrapulmonata]], the group including spiders and whip scorpions (1 species)
* †[[Phalangiotarbida]] – extinct arachnids of uncertain affinity (30 species)
* †[[Trigonotarbida]] – extinct (late [[Silurian]] [[Cisuralian|Early Permian]])
* †[[Uraraneida]] – extinct spider-like arachnids, but with a "tail" and no [[Spinneret (spider)|spinnerets]] (2 species)

It is estimated that 110,000 arachnid species have been described, and that there may be over a million in total.<ref name=":0" />

==See also==
{{Portal|Arthropods}}
* [[Arachnophobia]]
* [[Endangered spiders]]
* [[Glossary of spider terms]]
* [[List of extinct arachnids]]
<!--*[[List of spiders of the United States and Canada]]-->

==References==
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}}

==External links==
{{Wikispecies |Arachnida}}
{{Commons category|Arachnida}}
* [http://www.nhm.ac.uk/our-science/collections/zoology-collections/arachnid-collections.html Arachnid], [[Natural History Museum, London]]

{{Arthropods}}
{{Arachnida}}
{{Taxonbar|from=Q1358}}
{{Authority control}}

[[Category:Arachnids| ]]
[[Category:Extant Silurian first appearances]]
[[Category:Llandovery first appearances]]