Mites are small arthropods belonging to the class Arachnida and the subclass Acari (also known as Acarina). The term "mite" refers to the members of several groups in Acari but it is not a clade as it spans two different groups of arachnids; it also excludes the ticks, order Ixodida. Mites and ticks are characterised by the body being divided into two regions, the cephalothorax or prosoma (there is no separate head), and an opisthosoma. The scientific discipline devoted to the study of ticks and mites is called acarology.
Most mites are tiny, less than 1 mm (0.04 in) in length, and have a simple, unsegmented body plan. Their small size makes them easily overlooked; some species live in water, many live in soil as decomposers, others live on plants, sometimes creating galls, while others again are predators or parasites. This last group includes the commercially important Varroa parasite of honey bees, as well as the scabies mite of humans. Most species are harmless to humans but a few are associated with allergies or may transmit diseases.