Members of the Acari are minute or small arachnids and, although sharing many features of insects, usually lack obvious body segmentation. Unlike insects, mites have no antennae, compound eyes or wings; also, the body of a mite is composed of just two main sections: a gnathosoma (which bears a pair of segmented pedipalps and the mouthparts - including, ventrally, a beak-like hypostome) and a sac-like idiosoma (which bears the legs).
The pedipalps have up to five main segments: trochanter, femur, genu, tibia, tarsus, the tarsal segment often bearing at its basal inner angle an articulating, two-, three- or four-pronged sixth segment called the apotele (Fig. 164). The mouthparts of mites are adapted for biting or piercing and usually include a pair of, often chelate (for grasping) or needle-like (for piercing), chelicerae; the chelicerae often terminate in one fixed and one moveable, thumb-like digit, and often coalesce to form a needle-like stylophore.
Adult mites, unlike insects, are usually 8-legged, and each leg has up to six segments: coxa, trochanter, femur, genu, tibia and tarsus (Fig. 165); also, the legs often end in a soft ambulacrum (which may comprise a pretarsus and a pad-like pulvillus), at the tip of which may
Fig. 165 Segmentation of the leg of a mite.
be inserted one or more claws; the ambulacrum on leg I may be small or absent. In some groups of mites, the idiosoma is subdivided by a sejugal furrow into an anterior propodosoma and a posterior hysterosoma (Fig. 166), each bearing two pairs of legs. Mites within the superfamily Eriophyoidea (order Prostigmata) possess just two pairs of legs, both of which arise from the propodosoma. Ocelli, when present, are located on the propodosoma.
The body of a mite often bears more or less sclerotized plates (shields) and these are useful for distinguishing between various groups. Identification of mites, however, usually requires high-powered microscopical examination;
detailed examination of setae on the body and appendages (chaetotaxy) is of particular importance but is a specialist task and beyond the scope of the present work. Features of the shield which overlies the propodosoma of eriophyid mites (in the present work termed the prodorsal shield - also widely known as the cephalo-thoracic shield, the dorsal shield and the propodosomal shield), and the number and orientation of the setae arising from it, are of considerable taxonomic significance; although brief mention of the more gross features is made in the specific descriptions (see Part II, p. 255 et seq.), their full appreciation requires the use of a scanning electron microscope.
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