Editing Arachnid (section)

==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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 |date = July 2017
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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
 |first1  = A.Z.   |last1  = Ontano       |first2  = G.   |last2  = Gainett
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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" />