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The arachnids, Arachnida, are a class of 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 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 solpugids the palpi are quite leg-like and make solpugids appear to have ten legs. Larval mites have
only six legs: they grow their fourth pair when they molt into nymphs.
Arachnids are mostly carnivorous, feeding on the pre-digested body 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.
Orders:
- 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)
- Opiliones - phalangids; harvestmen, daddy longlegs (4,500 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; wind scorpions, sun spiders or camel spiders (900
species)
- Uropygi - whip scorpions, with first legs modified as whip-like sensory organs
and with a long thin tail at end of abdomen (60 species)
- Acari - mites and ticks (30,000 species)
- Acariformes
- Sarcoptiformes
- Trombidiformes
- Opilioacariformes
- Parasitiformes -
Holothyrans, ticks and mesostigmatic mites
Also see the following articles for links to species by common name:
External links:
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