Arachnid

Arachnid; Temporal range: Early Silurian – present
	Representatives of the 12 extant orders of arachnids
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Arthropoda
Clade:	Arachnomorpha
Subphylum:	Chelicerata
Class:	Arachnida; Lamarck, 1801
Orders
	Ricinulei; Opiliones – harvestmen; Solifugae – camel spiders; Acariformes mites; Parasitiformes mites and ticks; †Phalangiotarbida (extinct); Palpigradi – micro-whipscorpions; Arachnopulmonata Panscorpiones Pseudoscorpiones – pseudoscorpions; Scorpiones – scorpions; ; Pantetrapulmonata †Trigonotarbida (extinct); Tetrapulmonata Schizotarsata †Haptopoda (extinct); Pedipalpi Amblypygi – whip spiders; Schizomida – short-tailed whipscorpions; Uropygi – vinegaroons; ; ; Serikodiastida †Uraraneida (extinct); Araneae – spiders; ; ; ; ; ?Merostomata ?Xiphosura – horseshoe crabs; ?Eurypterida – marine scorpions; ;

Arachnida (/əˈræknɪdə/) is a class of joint-legged arthropods, in the subphylum Chelicerata. Arachnida includes, among others, spiders, scorpions, ticks, mites, pseudoscorpions, harvestmen, camel spiders, whip spiders and vinegaroons.

Adult arachnids have eight legs attached to the cephalothorax, although the frontmost pair of legs in some species has converted to a sensory function, while in other species, different appendages can grow large enough to take on the appearance of extra pairs of legs. The term is derived from the Greek word ἀράχνη (aráchnē, 'spider'), from the myth of the hubristic human weaver Arachne, who was turned into a spider.

Almost all extant arachnids are 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.

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