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Any member of the class Arachnida, primarily carnivorous arthropods having a well-developed head, a hard external skeleton, and four pairs of walking legs. Spiders and scorpions have a segmented body, but daddy longlegs, ticks, and mites do not. Arachnids range in size from tiny mites (0.003 in. [0.08 mm] long) to the 8-in. (21-cm) black scorpion of Africa. As arachnids grow, they molt several times (seemolting). Most are unable to digest food internally; instead, they inject their prey with digestive fluids and suck the liquefied remains. Arachnids are found worldwide in nearly every habitat. Most groups are free-living, but some mites and ticks are parasitic and can carry serious diseases of animals and humans. Venomous spiders and scorpions also may pose a danger to humans. However, most arachnids are harmless and prey on insect pests.
This entry comes from Encyclopædia Britannica Concise. For the full entry on arachnid, visit Britannica.com.