Parasitic Insects, Mites and Ticks: Genera of Medical and Veterinary Importance/Acarines general

Acarines (Acarina)
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Characters of mites and ticks of veterinary importance
The Order Acarina contains many sub-Orders of mites, each of which has numerous Families. There is one sub-Order of ticks, with two Families. Most species of mites are free-living but all the ticks are obligate blood feeding ectoparasites. Otherwise the mites and ticks have closely similar body form and physiology, despite the ticks being larger and more conspicuous than mites. The acarines separated from the insects very early during evolution and for technical and clinical purposes they need to be considered separately, despite mites and ticks sometimes being called insects during informal speech.
 * Photograph shows a group of house-dust mites, genus Dermatophagoides; these free-living mites are a source of allergens causing respiratory disease in humans and domestic animals. Note their size against textile fibers typical of their natural habitat. These mites are not parasitic but are included in the fields of medical and veterinary entomology. (Photograph by Gilles San Martin).

The body wall (integument) of acarines forms an exoskeleton typical of arthropods, but this tends to be flexible rather than stiff as in adult insects. The body is not clearly divided into segments. Segmentation has become greatly reduced during evolution so that the body consists of a small anterior portion (gnathosoma) bearing the mouthparts, and a large posterior portion (idiosoma) bearing the legs. The integument is often textured with fine striations, tubercles, or scales. Setae commonly appear on much of the body and legs. In addition there may be stout spines.

Piercing mouthparts usually consist of a pair of chelicerae with cutting teeth which can be protruded from paired cheliceral sheaths. These sheaths are dorsal. A flat blade-like hypostome is ventral to the cheliceral sheaths. These sheaths and the hypostome form between them one tube for both blood-sucking and salivation. Sensory palps accompany the feeding mouthparts.
 * Photograph shows an adult female Hard-tick, genus Hyalomma, an ectoparasite of livestock animals, dogs and others. Skin of host is penetrated by the long forward projecting mouthparts. This is an adult tick, showing division into a small anterior gnathosoma with the mouthparts, and a large posterior idiosoma bearing four pairs of strong legs.

Adult acarines have four pairs of legs, larvae have three pairs. Antennae are absent from acarines but many species have sensory structures on their palps, and sometimes on their front legs, as Haller's organ. Eyes are present in some genera of ticks, and these are always borne on the idiosoma. The smaller mites are semi-transparent pale white or yellow. All ticks are grey or brown, also some have bright colored patterns on their dorsal surface. Hard plates of sclerotized integument are often present, especially in the family Ixodidae, called Hard-ticks after this character.

All acarines have a life-cycle with an incomplete metamorphosis, that for Soft-ticks is typical, as below (see also life-cycle of an ixodid tick: Hard ticks). There may be several nymphal stages. The life-cycle may be completed in about one week, as with the small mites permanently parasitic on their hosts, or it may take several years with the large ticks adapted for survival for long periods whilst waiting for hosts to arrive.
 * Diagram represents the life-cycle of an Ornithodoros Soft-tick feeding on pigs. This cycle is an incomplete metamorphosis. These Soft-ticks take multiple small meals at nymphal and adult stages of their life cycle.

Infestations with mites and ticks are often at low levels, not causing clinically significant disease, but liable to flare up into a state of disease in circumstances favourable to the parasite, such as season, or poor immune state of their host.

The information here on mites is ordered to emphasize their clinical signs and location at skin or other organs. Taxonomy of mites is complex and currently is not settled, thus it is clearer to present the mites in this book in relation to their medical and veterinary relationships.

Glossary

 * Acarine = Collective term for all the mites and all the ticks, in the Order Acarina.
 * Antenna = A paired sensory organ on head of insects and other arthropods but absent from all acarines.
 * Chelicerae = Paired organs that form part of the piercing structures of acarines, often long and sharp and may have moveable claws at their tips.
 * Exoskeleton = Acarines, as arthropods, have an external skeleton with muscles acting on joints from inside.
 * Gnathosoma = Anterior section of body of acarines, the part bearing the mouthparts and sensory palps.
 * Haller's organ = Sensory pits on the fore-legs of some acarines, used for finding hosts.
 * Hypostome = A paired organ of the mouthparts, acts together with the cheliceral sheaths to form a tube for blood sucking and salivation.
 * Idiosoma = Posterior section of body of acarines, contains gut, reproductive organs and so on. Acarines have reduced segmentation and do not have a head, thorax and abdomen as found in insects.
 * Integument = The outer body wall of acarines and insects, which also acts as their external skeleton.
 * Palp = Paired and segmented sensory organs associated with the mouthparts of acarines and insects.
 * Scales = Extensions of the integument of some mites, giving a distinctive surface texture.
 * Setae = Thin, long, moveable extensions of the integument of acarines and insects; often very long on mites.
 * Striations = Patterns of fine lines, like finger-prints, on the integument of many acarines.

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