Editing Arachnid (section)

===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" />