The order Acarina of the class Arachnida contains mites and ticks. Spiders are of course related. Members of Acarina differ from other arachnids in that the body is not segmented. The cephalothorax and abdomen are combined into one body region. Larval mites and ticks have 3 pairs of legs, whereas nymphs and adults have 4 pairs. Mites inhabit most environments from deserts to rain forests, mountain tops to tundra, and from ocean floors to lakes. The relatively few species parasitic on humans, sometimes causing dermatitis followed by allergic reactions.