Common name: Mites and ticks.
Morphology: The Acari (or mites) can be recognized by having an entire, fused body, with no obvious segmentation. In this respect they resemble, within the Arachnida, only the spiders (Araneae), but the body of the latter is subdivided into a cephalothorax (bearing the legs) and abdomen, whereas the body of the Acari is entire, and the legs may be borne on the anterior and posterior parts of the body. In addition, spiders usually have four pairs of simple eyes (ocelli), caudal spinnerets and do not undergo a six-legged larval stage. Mites have at most two pairs of eyes, rarely (and only in spider mites) anterior spinnerets, and their development includes a larval stage.
Mites are usually 0.5 mm or less in length (the ticks, or Metastigmata, can be twice or more that size), with an oval body, which may have a trough (the sejugal furrow between the second and third pair of legs. The mouthparts consists of the inner chelicerae, and the outer palpi. Both, which occur in diverse shapes in the various suborders, serve as sensory organs, for grasping, wounding and piercing food, as well as for spermatophore transfer by some males. Most mite nymphs and adults bear four pairs of legs, which also show much variation in their structure and function. Gas exchange is through a pair of stigmata, which may be placed near the mouthparts, anteriorly on the body, or between and behind the legs, or lacking; their positions provide the names of the various suborders. A pair of ducts, the peritremes, lead from the stigmata into the acarine body. The genital and anal apertures are usually situated between and behind the fourth pair of legs. Exceptions to the above general description are the Eriophyoidea, the gall or rust mites, characterized by a small (0.1-0.3 mm), annulate, worm-like body and only two pairs of legs. The body of certain animal parasites, like Demodex, is also worm-like, but they have four pairs of legs.
Economic importance: Most acarine plant pests in the Middle East belong to the suborder Prostigmata, including the Eriophyoidea, the spider mites (Tetranychidae), the false spider mites (Tenuipalpidae), and the Tarsonemidae. Also in the same suborder are the Trombiculidae, vectors of scrub typhus. Ticks, which suck the blood of their vertebrate hosts and transmit many diseases, are the most important acarine medical and veterinary pests. Rhizoglyphus robini is one of the rare plant pests in the suborder Astigmata, whereas others, like Acarus siro, are major pests in stored foods. Also in the same suborder are Psoroptes, Sarcoptes and Dermatophagoides , which cause diseases and allergy in animals and humans. Some Mesostigmata parasitize chickens (Dermanyssus) and bees (Varroa). The Phytoseiidae, also in this suborder, are major natural enemies of acarine plant pests and of thrips (Thysanoptera).
Life history: The great diversity within the subclass is reflected in the very different life histories and habitats of its members. The common cycle consists of egg, larva, 2-3 nymphal stages and adults. However, one or more of the juvenile stages may be omitted; the females in some families giving birth to larvae, nymphs or even to adults. In other cases one and even two nymphs are repressed, and some of the stages may be heteromorphic to the others, differing in habitat, life style and diet. Acari are usually terrestrial and feed on diverse diets, such as dead organic matter, on fungi (some being pests of mushrooms), on green plants, on living prey or they are external or internal parasite of invertebrates or vertebrates. The Hydrachnidia, a group of superfamilies in the suborder Prostigmata, are aquatic (living in fresh or in salt water), parasitizing the larvae of mosquitoes and midges Diptera as well as water bugs (Hemiptera ), beetles (Coleoptera)and other invertebrates.
An acarine generation can be quite rapid, requiring less than one week, although soil mites may develop only a single (or less) annual generation. Several plant-feeding and predatory species undergo a winter or (seldom) summer diapause. Mite reproduction may be entirely sexual, by arrhenotoky or totally asexual (thelytokous). Some parasitic mites (like Varroa, which attacks bees) produce only 5-7 progeny, whereas ticks can lay thousands of eggs. The fecundity of other species lies between these extremes, often being around 50 offspring per female. Mites were found in all habitats where at least some humidity is available. Besides cultivated and uncultivated land, and diverse invertebrate and vertebrate hosts and their nests, they also occur near the north and south poles, on mountain peaks where no other arthropods live, in deserts, in the littoral, in warm springs (50°C) and down in the deep sea (to a depth of 5,000 m). Most Acari require high relative humidities for survival and reproduction, but some spider mites Tetranychidae thrive under relatively arid conditions. Acarine dispersal is by active walking or swimming (water mites), by phoresy on vertebrates and invertebrates, by winds (webbing spider mites, many rust mites) and by being on transported commercial plants. Dispersal is usually by one stage, whether a juvenile (the hypopus of the Astigmata) or an adult (spider mites). The Acari are divided into two orders, the Parasitiformes and the Acariformes. The former contains the ticks (Metastigmata) and the Mesostigmata. The latter order consists of the Astigmata, Cryptostigmata (also known as Oribatei or Oribatida) and Prostigmata. Approximately 55,000 species of mites have been named, but this is probably only 10% of the world acarine fauna.
Alberti, G. and Coons, L.B. 1999. Acari: mites. In: Microscopic Anatomy of Invertebrates (Ed. by F.W. Harrison & R.F. Foelix), Wiley-Liss, New York, pp. 515-1265.
Halliday, R.B., OConnor, B.M. and Baker, A.S. 1999. Global diversity of mites. In: Nature and Human Society: the Quest for a Sustainable World (Ed. by P.H. Raven), National Academy Press, Washington, pp. 192-203.
Hughes, A.M. 1976. The Mites of Stored Food and Houses. Her Majesty’s Stationery Office, London.
Jeppson, L.R., Keifer, H.H. and Baker, E.W. 1975. Mites Injurious to Economic Plants. University California Press, Berkeley, California.
Krantz, G.W. 1978. A Manual of Acarology, 2nd Edition. Oregon State University Book Store, Corvallis, Oregon.
Walter, D.E. and Proctor, H.C. 2013. Mites: Ecology, Evolution and Behaviour, Life at a Microscale. 2nd Edition. Springer.
A website advertising books about the Acari: http://www.intercept.co.uk/gb/not.asp?id=OX2Y3OLAXASOFJ