Mite
Mites are not a defined taxon, but the name is used for members of several groups of the class Arachnida. Mites span two large orders of arachnids, the Acariformes and the Parasitiformes, which were historically grouped together in the subclass Acari, but genetic evidence suggests are not closely related. The Parasitiformes include ticks, which are sometimes semantically distinguished from mites.
Most mites are tiny, less than 1 mm (0.04 in) in length, and have a simple, unsegmented body plan. The small size of most species 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 type includes the commercially important Varroa parasite of honey bees, as well as scabies mites of humans. Most species are harmless to humans but a few are associated with allergies or may transmit diseases.
The mites are not a defined taxon, but is used for two distinct groups of arachnids, the Acariformes and the Parasitiformes. The phylogeny of the Acari has been relatively little studied, but molecular information from ribosomal DNA is being extensively used to understand relationships between groups. The 18 S rRNA gene provides information on relationships among phyla and superphyla, while the ITS2, and the 18S ribosomal RNA and 28S ribosomal RNA genes, provide clues at deeper levels.[1]
The third edition (2009) of the standard textbook A Manual of Acarology uses a system of six orders, grouped into three superorders:[2]
It has been subsequently argued that Opilioacariformes should be considered a subgroup of Parasitiformes, based on genetic evidence.[3][4]
The mite fossil record is sparse, due to their small size and low preservation potential.[5] The oldest fossils of acariform mites are from the Rhynie Chert, Scotland, which dates to the early Devonian, around 410 million years ago[6][5] while the earliest fossils of Parasitiformes are known from amber specimens dating to the mid-Cretaceous, around 100 million years ago.[5][7] Most fossil acarids are no older than the Tertiary (up to 65 mya).[8]
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