Acari (or
Acarina) are a
taxon of
arachnids that contains
mites and
ticks. The diversity of the Acari is extraordinary and its
fossil history goes back to at least the early
Devonian period. 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 two or three
superorders or
orders:
Acariformes (or Actinotrichida),
Parasitiformes (or Anactinotrichida), 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.
