Life cycle

Adult mites, like ticks, have eight legs and therefore are not insects. They can be distinguished from ticks by the absence of teeth on the hypostome of the mouthparts and in having setae (bristles) on the body as well as the legs. But the principal medically important species (scabies mite, scrub typhus mite, house-dust mite and follicle mite) can most readily be recognized by their characteristic shapes.

Sarcoptes scabiei, the scabies or itch mite, occurs on people worldwide. Morphologically they are indistinguishable from S. scabiei infesting wild and domesticated animals, including dogs, horses and pigs. Mites on such animals are considered to be the same species as those infecting people but physiologically adapted for life on non-human hosts. In animals they cause the condition known as mange. Mites living on animals very rarely infect humans, but if they do the infection can persist for several weeks.

Scabies mites are not vectors of any disease but cause conditions known as scabies, acariasis, and crusted or Norwegian scabies.

External morphology

The female mite (0.30–0.45mm) is just visible without the aid of a hand lens. It is pale and disc-shaped. Dorsally the mite has numerous tiny peg-like spines and a few bristles (setae), and both dorsally and ventrally there are wavy lines across the body, giving the mite a striated appearance (Fig. 18.1, Plate 31). Adults have four pairs of short and cylindrical legs divided into five ring-like segments. The first two pairs of legs end in short stalks called pedicels which terminate in thin-walled roundish structures often termed ‘suckers’. In females the posterior two pairs of legs do not have ‘suckers’ but end in long and very conspicuous bristles. There is no distinct head, but the short and fat palps and pincer-like chelicerae of the mouthparts protrude anteriorly from the body.

Adult male scabies mites are only 0.20–0.25mm long, and apart from their small size may also be distinguished from females by the presence of ‘suckers’ on the last pair of legs (Fig. 18.2).