Arachnid

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Template:Otheruses4 {{Taxobox | color = pink | name = Arachnid | image = Haeckel Arachnida.jpg | image_width = 240px | image_caption = "Arachnida" from Ernst Haeckel's Artforms of Nature, 1904 | regnum = Animalia | phylum = Arthropoda | subphylum = Chelicerata | classis = Arachnida | classis_authority = Cuvier, 1812 | subdivision_ranks = Orders | subdivision = See text. }} The arachnids, Arachnida, are a class of joint-legged invertebrate animals in the subphylum Chelicerata. Arachnids are named for the mythological figure Arachne. They are chiefly terrestrial arthropods, some 65,000 to 73,000 named species including the spiders, scorpions, harvestmen, ticks, and mites. The arachnids are easily distinguished from the insects by the fact that they have eight legs instead of six. The pedipalpi (leg-like mouthparts) of some species has instead been adapted for sensory, prey capture or reproductive functions. In solifugae the palpi are quite leg-like and make solifugae appear to have ten legs. Larval mites have only six legs: they grow their fourth pair when they molt into nymphs.

Arachnids are further distinguished by the fact they have no antennae and no wings. They have a two-segmented body, made up of the cephalothorax and abdomen. The cephalothorax is derived from the fusion of the cephalon or head and the thorax.

Arachnids are mostly carnivorous, feeding on the pre-digested bodies of insects and other small animals. Many are venomous - they secrete poison from specialized glands to kill prey or enemies. Others are parasites, some of which are carriers of disease. Arachnids usually lay eggs, which hatch into immature adults.

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Classification

Image:Victorian funnelweb.jpg Image:Adult deer tick.jpg

Trigonotarbida - Extinct
Amblypygi - "Blunt rump" tailless whip scorpions with front legs modified into whip-like sensory structures as long as 25 cm or more
Araneae - Spiders (40,000 species)
- Mesothelae - very rare, primitive spiders
- Opisthothelae - all spiders except mesothelae
  - Araneomorphae - most common spiders
  - Mygalomorphae - tarantulas and tarantula-like spiders
Phalangiotarbida - Extinct
Opiliones - Phalangids, harvestmen or daddy longlegs (6,300 species)
Palpigradi - Microwhip scorpions
Pseudoscorpionida - Pseudoscorpions
Ricinulei - Ricinuleids, Hooded tickspiders
Schizomida - "Split middle" whip scorpions with divided exoskeletons
Scorpiones - Scorpions (2,000 species)
Solifugae - Solipugids; windscorpions, sun spiders or camel spiders (900 species)
Haptopoda - Extinct
Uropygi - whip scorpions, with first legs modified as whip-like sensory organs and with a long thin tail at end of abdomen (100 species)
Acarina - Mites and ticks (30,000 species)
- Acariformes
  - Sarcoptiformes
  - Trombidiformes
- Opilioacariformes
- Parasitiformes - Holothyrans, ticks and mesostigmatic mites