The arachnids, Arachnida, are a class of invertebrateanimals 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.
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Amblypygi - "Blunt rump" tailless whip scorpions with front legs modified into whip-like sensory structures as long as 25 cm or more
The Arachnida include the terrestrial chelicerates that everyone is familiar with, and that nearly everyone would rather not be too familiar with: spiders (Araneae), ticks and mites (Acari), and scorpions (Scorpiones).
For more information on arachnids on the WWW, try the Arachnida pages at the Tree of Life, or visit the very comprehensive arachnology home page at Antwerp University in Belgium and this fine Arachnida Sources Listing.
Shultz, J.W. Evolutionary morphology and phylogeny of the Arachnida.
The Arachnida also includes a diverse array of smaller groups, including scorpions (1200 species), whip scorpions (100 species), palpigrades (60 species), pseudoscorpions (2000 species), solpugids (900 species), and harvestmen (5000 species).
Arachnids have a pair of tagmata called a prosoma and opisthosoma.
Disclaimer: The Animal Diversity Web is an educational resource written largely by and for college students.
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