Mites are arachnids and, as such, evolved from a segmented body with the segments organised into two tagmata: a prosoma (cephalothorax) and an opisthosoma (abdomen). However, only the faintest traces of primary segmentation remain in mites; the prosoma and opisthosoma are fused, and a region of flexible cuticle (the circumcapitular furrow) separates the chelicerae and pedipalps from the rest of the body. This anterior body region is called the capitulum or gnathosoma and, according to some works, is also found in Ricinulei. The remainder of the body is called the idiosoma and is unique to mites.
Most adult mites have four pairs of legs, like other arachnids, but some have fewer. For example, gall mites like
Phyllocoptes variabilis (family Eriophyidae) have a worm-like body with only two pairs of legs; some parasitic mites have only one or three pairs of legs in the adult stage. Larval and prelarval stages have a maximum of three pairs of legs; adult mites with only three pairs of legs may be called 'larviform'. Also members of the Nematalycidae within Endeostigmata, which live between sand grains, have often wormlike and elongated bodies with reduced legs.
The mouth parts of mites may be adapted for biting, stinging, sawing or sucking. They breathe through tracheae, stigmata (small openings of the skin), intestines and the skin itself. Species hunting for other mites have very acute senses, but many mites are eyeless. The central eyes of arachnids are always missing, or they are fused into a single eye. Thus, any eye number from none to five may occur.