acarina or acari are a taxon of arachnids that contains mites and ticks. The diversity of the Acari is extraordinary and its fossil history goes back to the Devonian era. As a result, acarologists (the people who study mites and ticks) have proposed a complex set of taxonomic ranks to classify mites. In most modern treatments, the Acari is considered a subclass of Arachnida and is composed of 2-3 superorders or orders: Acariformes, Parasitiformes, and Opilioacariformes; the latter is often considered a subgroup within the Parasitiformes. The monophyly of the Acari is open to debate and the relationships of the acarines to other arachnids is not at all clear. In older treatments the subgroups of the Acarina were placed at order rank, but as their own subdivisions have become better-understood it is more usual to treat them at superorder rank
Mites are members of Arachnida and, as such, should have 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 insensibly fused, and a region of flexible cuticle (the cirumcapitular furrow) separates the chelicerae and pedipalps from the rest of the body. This anterior body region is called the capitulum or gnathosoma and is also found in the 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 (superfamily Eriophyioidea) have a wormlike 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'.
The mouth parts of mites may be adapted for biting, stinging, sawing or sucking. They breath 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.
Ontogeny
typically consists of an egg, a prelarval stage (often absent), a larval stage (hexapod except in Eriophyoidea which have only two pairs of legs), and a series of nymphal stages. Any or all of these stages except the adult may be suppressed or occur only within the body of a previous stage. Larvae (and prelarvae) have a maximum of three pairs of legs (legs are often reduced to stubs or absent in prelarvae); legs IV are added at the first nymphal stage. Usually a maximum of three nymphal stages are present and they are referred to in sequence as protonymph, deutonymph, and tritonymph; however, some soft ticks have supernumary nymphal stages. The females of some Tarsonemidae bear sexually mature young. If one or more nymphal stages are absent, then authors may disagree on which stage(s) is(are) present. Only the Oribatida pass through all developmental stages.