Mites

Definition

Mites are the most diverse of all arachnids, and among the oldest of all terrestrial animals. Fossils date mites from the early Devonian, nearly 400 million years ago. Belonging to the class Archnida, the term "mite" refers to the several groups in the subclass Acari, but it is not a clade, and excludes the ticks. Mites and ticks are similarly characterized by their bodies being divided into two parts, named the cephalothorax and opisthosoma. The three major lineages of mites are recognized as Opilioacariformes, Acariformes and Parasitiformes. [1] 45,000 species of mites have been described, which is believed to be only small 5% of the number of species estimated to be alive today.

Characteristics

The Acari have been defined by A Manual of Acarology to have the following characteristics [1]:

• Hexapod prelarva (lost in Parasitiformes and many derived Acariformes)

• Hexapod larval stage

• Three octopod nymphal stages (variously abbreviated in derived taxa)

• Gnathosoma delimited by a circumcapitular suture

• Palpcoxal endites fused medially forming a hypostome

• Hypostome with rutella or corniculi (lost in many derived Acariformes)

• Loss of external evidence of opisthosomal segmentation, i.e. without tergites or sternites

• Ingestion of particulate food (lost in many derived taxa)

Taxonomy

Acari have been variously ranked as a group made up of one to seven or more distinct orders. The orders comprise approximately half of the described arachnid diversity. Molecular information from ribosomal DNA is used to expose 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, show relationships at deeper levels. [2]

A Manual of Acarology uses a system of six orders, grouped into three superorders to classify Acari [2]:

• Superorder Opilioacariformes – a small order of large mites with about 20 known species
• Superorder Parasitiformes – ticks and a variety of mites
  • Holothyrida - predatory mites from the southern hemisphere
  • Ixodida – hard ticks and soft ticks
  • Mesostigmata – excess of 65 families and 10,000 described species, including bird mites, phytoseiid mites
    • Trigynaspida - large, diverse order
    • Monogynaspida - diverse order of parasitic and predatory mites
• Superorder Acariformes – contains over 300 families and over 30, 000 described species
  • Trombidiformes – plant parasitic mites (spider mites, peacock mites, gall mites, red-legged earth mites, etc.), snout mites, chiggers, hair follicle mites, velvet mites, water mites, etc.
    • Sphaerolichida - small order of mites containing two families
    • Prostigmata - large order of sucking mites
  • Sarcoptiformes
    • Oribatida – oribatid mites, beetle mites, armored mites (also cryptostigmata)
    • Astigmatina – stored product, fur, feather, dust, and human itch mites, etc.

References

[1] Walter, David Evans, Gerald Krantz, and Evert Lindquist. 1996. Acari. The Mites. Version 13 December 1996. http://tolweb.org/Acari/2554/1996.12.13 inThe Tree of Life Web Project, http://tolweb.org/

[2] Pepato, Almir R, et al. “Phylogenetic Position of the Acariform Mites: Sensitivity to Homology Assessment under Total Evidence.” BMC Evolutionary Biology, BioMed Central, 2 Aug. 2010, bmcevolbiol.biomedcentral.com/articles/10.1186/1471-2148-10-235.