Editing Arachnid (section)

== 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>