Acari
Taxonomic placing: Arthropoda, Chelicerata, Arachnida.
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](entry/Stigmata.html
), 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](entry/Peritremes.html
), 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 (Claparede) is one of the rare plant pests in the suborder Astigmata, whereas others, like Acarus siro L., 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 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) 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 50,000 species of mites have been named, but this
is probably only 10% of the world acarine fauna.
References
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