Acarina or Acari are a taxon of arachnis 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