What is a tick and why is it not an insect?

Ticks are small parasitic arachnids that attach to vertebrates and feed on blood. Unlike insects, adult ticks have eight legs, no antennae, and a body plan typical of arachnids. They are closely related to mites and spiders and develop through egg, larva, nymph, and adult stages. Ticks are medically important because some species can transmit diseases such as Lyme disease and spotted fever. Because they spend time waiting in vegetation for a host, they are often encountered in wooded or grassy areas.