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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

Most acarines are minute to small (e.g. 0.08–1.0 mm), but the largest Acari (some ticksand red velvet mites) may reach lengths of 10–20 mm. It is estimated that over 50,000 species have been described (as of 1999) and that a million or more species are currently living. The study of mites and ticks is called acarology (from the Greek: ?????, akari, a type of mite; and ?????, logos, "knowledge")^[1], and the leading scientific journals for acarology include Acarologia, Experimental and Applied Acarology and International Journal of Acarology.

Morphology

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 othermites 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^[2].

Ontogeny

Acarine 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^[2].

Diversity and Lifestyles

Acarines are extremely diverse. They live in practically every habitat, and include aquatic (freshwater and sea water) and terrestrial species. They outnumber other arthropods in the soil organic matter and detritus. Many are parasitic, and they affect both vertebrates and invertebrates. Most parasitic forms are external parasites, while the free living forms are generally predatory and may evenb e used to control undesirable arthropods. Others are detritivores that help to break down forest litter and dead organic matter such as skin cells. Others still are plant feeders and may damage crops.

Economic Importance

Damage to crops is perhaps the most costly economic effect of mites, especially by the spider mites and their relatives (Tetranychoidea), earth mites (Penthaleidae), thread-footed mites (Tarsonemidae) and the gall and rust mites (Eriophyidae). Some parasitic forms affect humans and other mammals, causing damage by their feeding, and can even be vectors of diseases such as scrub typhus and rickettsialpox. A well known effect of mites on humans is their role as an allergen and the stimulation of asthma in people affected by respiratory disease. The use of predatory mites (e.g. Phytoseiidae) in pest control and herbivorous mites that infest weeds are also of importance. An unquantified, but major positive contribution of the Acari is their normal functioning in ecosystems, especially their roles in the decomposer subsystem ^[1].

Taxonomy

Rust mite (size: 30 micrometres)

Male tick (size: 2 mm)

The phylogeny of the Acari is still disputed and several taxonomic schemes have been proposed for their classification. Most researchers agree that the Acari contain two separate lineages: the Acariformes (also Actinotrichida) and the Parasitiformes (also Anactinotrichida). However, some researchers treat these groups as superorders and others as orders.

• Acariformes are the most diverse group of mites.
  • 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.
  • Sarcoptiformes - oribatid, endeostigmatan, and astigmatan mites
    • Endeostigmata - basal sarcoptiform lineages
    • Oribatida - oribatid mites, beetle mites, armored mites (also cryptostigmata)
    • Astigmata - stored product, fur, feather, dust, and human itch mites, etc.
• Parasitiformes are the group including ticks and a variety of mites.
  • Mesostigmata - bird mites, phytoseiid mites, raubmilben
  • Ixodida - hard and soft ticks
  • Holothyrida - holothyrans
• Opilioacariformes are a small order or suborder of parasitiform mites that are superficially similar to harvestmen (opiliones, hence their name).

• Taxa incertae sedis:
  • Family Phytoseiidae
  • Genus Acarophenox
  • Genus Eniochthonius
  • Genus Entrombidium
  • Genus Nothrus
  • Genus Petauralges
  • Genus Rhinoseius

The Subclass Acari is a member of the Class Acarida. Here is the complete "parentage" of Acari:

• Domain: Eukaryota Whittaker & Margulis,1978 - eukaryotes
  • Kingdom: Animalia Linnaeus, 1758 - Linnaeus, 1758 - animals
    • Subkingdom: Bilateria (Hatschek, 1888) Cavalier-Smith, 1983 - (Hatschek, 1888) Cavalier-Smith, 1983 - bilaterians
      • Branch: Protostomia Grobben, 1908 - Grobben, 1908 - protostomes
        • Infrakingdom: Ecdysozoa Aguinaldo et al., 1997 ex Cavalier-Smith, 1998 - Aguinaldo et al., 1997 ex Cavalier-Smith, 1998 - ecdysozoans
          • Superphylum: Panarthropoda Cuvier
            • Phylum: Arthropoda Latreille, 1829
              • Class: Acarida
                • Subclass: Acari Leach, 1817

The Subclass Acari is further organized into finer groupings including:

• Order (7): Actinedida · Astigmata · Holothyrida · Ixodida · Mesostigmata · Opilioacarida · Oribatida