An arachnid is a member of the class Arachnida (phylum Arthropoda, see arthropod), which contains scorpions, spiders, harvestmen, ticks, mites, and solifugae (spider-like desert dwellers). Arachnids have an exoskeleton, four pairs of jointed, walking legs, two distinct body sections (cephalothorax and abdomen), chelicerate jaws (consisting of clawed pincers), and no wings. All are air-breathing. They lack antennae and wings. Most arachnids are terrestrial and live in soil, leaf litter, and low vegetation; some are blood-sucking parasites. About 60,000 species are known.


Insects, by contrast, have a three-part body and three pairs of legs; and most have wings. The legs of arachnids are formed on a plan similar to that of the insects, but the last segment, the tarsus ends in one, two, or three claws.