The male measures 200 µ to 235 µ in length, by 145 µ to 190 µ in breadth. By preference, he lives under the scales which the presence of the parasite produce on the human host. The female is markedly larger than the male, measuring 330 µ to 450 µ in length by 250 µ to 350 µ in breadth. Her two anterior legs end in stalked suckers, whilst the two posterior end in hairs. The legs, like Malvolio’s, are curiously ‘cross-gartered’ with chitinous bars and rings.
At first she promenades about with the male on the surface of the human skin, but when they have paired the female begins to tunnel in the epidermis. The poor male, having been used, dies. As the mother-mite tunnels she begins to lay eggs, leaving them one by one behind her as she burrows deeper and deeper into the epidermis. Hence those that are nearer the entrance of the tunnel are always more advanced in age and development than those farther in. She always works head forward, and as her tunnel is but slightly bigger than the breadth of her body, she cannot turn round, and she is prevented from retreating by the backward hairs or spines of her body. Hence she burrows always forward, until she has dug her own grave at the far end of her excavation.
She is said to live two or three months and to lay one or two eggs a day. Thus one female is, in time, enough to infect seriously a single host. The egg is, relatively to the size of the mother, enormous: its length being 150 µ and its width 100 µ. The egg is hatched out after three to six days, and the young larva is hexapodous—that is, as is so usual in Acarines, six-legged. It escapes from the burrow on to the skin and soon tunnels into the epidermis of its host, where it moults and transforms, about the ninth day, into a four-legged nymph. At the end of another six days the mites moult again, and at this period one can distinguish nymphs of two sizes: the larger female, and the smaller male.
Fig. 38.—A diagrammatic view of the tunnel made by the female of Sarcoptes scabiei, with the eggs she has laid behind her as she burrows deeper and deeper. The black dots represent the excrement. (After Guiart and Grimbert.)
Within a month after hatching the Sarcoptes has become adult, and the sexes are occupied in seeking each other on the surface of the skin, and it is in this stage that they are easily passed by personal contact from one human being to another.
Fig. 39.—A female Sarcoptes scabiei, with four eggs in different stages of development; × about 180. (After Bourguignon.)
Many animals suffer from Sarcoptes; and the fact that this genus can be transferred to man from the horse, the ox, the sheep, the goat, the dog, the cat, the camel, the lion, &c., is a slight argument in favour of their being one species. There is another undoubtedly distinct species which causes serious epidemics, especially in Norway; but that is hardly likely to enter into the scope of this book.
Sarcoptes scabiei, the itch-mite, is, however, a cause of serious trouble in an army ‘in being.’ The tunnel or gallery in which the female mite burrows is the only lesion produced directly by the parasite. To the naked eye it presents a little whitish or greyish line, varying in length from some millimetres to one or even three centimetres, the longer ones occurring most frequently on the hands or wrists. It is of course open at one end, and ends in a cul-de-sac, which is slightly swollen, and here it is the female has taken up her abode. She is visible as a small white, brilliant spot. Besides the wrist, and the inner faces of the fingers—the interdigital areas—the palms of the hands are most commonly affected.