Arachnids (Encyclopedia of Science)
Arachnids (pronounced uh-RACK-nidz; class Arachnida) form the second-largest group of land arthropods (phylum Arthropoda) after the class Insecta. There are over 70,000 species of arachnids that include such familiar creatures as scorpions, spiders, harvestmen or daddy longlegs, and ticks and mites, as well as the less common whip scorpions, pseudoscorpions, and sun spiders. The marine horseshoe crabs and sea spiders are near relatives.
Arachnids have paired, jointed appendages (parts that are attached to the main body), a hardened exoskeleton (exo means "outer"), a segmented (divided into parts) body, and a well-developed head. Their body consists of two main parts: a fused head and thorax, and an abdomen. There are six pairs of appendages on the body: the first pair are clawlike fangs near the mouth used for grasping and cutting; the second pair serve as general-purpose mouth parts that may be modified for special functions; and the last four pairs of appendages are the walking legs.
Most arachnids live on land and breathe by means of book lungs (socalled because their thin membranes are arranged like the pages of a book) or by tracheae (small tubes that distribute air from the outside throughout the body), or both. Most are flesh-eating predators. They feed by piercing the body of their prey and directly...
(The entire section is 990 words.)
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