Life cycle

Adult mites, like ticks, have eight legs and therefore are not insects. They canbe 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 medicallyimportant species (scabies mite, scrub typhus mite, house-dust mite and folliclemite) can most readily be recognized by their characteristic shapes.

Sarcoptes scabiei, the scabies or itch mite, occurs on peopleworldwide. 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 asthose infecting people but physiologically adapted for life on non-human hosts.In animals they cause the condition known as mange. Mites living on animals veryrarely infect humans, but if they do the infection can persist for severalweeks.

Scabies mites are not vectors of any disease but cause conditions known asscabies, 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. Itis pale and disc-shaped. Dorsally the mite has numerous tinypeg-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 fourpairs of short and cylindrical legs divided into five ring-like segments. Thefirst two pairs of legs end in short stalks calledpedicels which terminate in thin-walledroundish structures often termed‘suckers ’. Infemales the posterior two pairs of legs donot have ‘suckers’ but end in long and very conspicuous bristles.There is no distinct head, but the short and fat palps and pincer-likechelicerae of the mouthparts protrudeanteriorly from the body.

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