Fig. 367.—Sarcoptes scabiei: female, dorsal aspect. 200/1. (After Fürstenberg.)
The body is oval or nearly circular and whitish in colour, with transverse rows of striæ partly interrupted on the back. There are transverse rows of small bristles on the dorsal surface, and groups of trichomæ on the front, sides and back. There are chitinous hairs at the base of the legs; the two first pairs are provided with pedunculated ambulacra in both sexes, the two posterior pairs terminate each with a long bristle in the female; in the male the third pair of legs terminate in a bristle, the fourth pair with a pedunculated ambulacrum. The anus is situated at the posterior border of the dorsal surface.
At one time numerous species were differentiated, according to the form of the Acarus, the number, position and size of the hairs and spines, even according to the hosts, etc. All these characteristics, however, fluctuate so considerably that absolute differentiation is impossible; the supposed species may be regarded in the same light as Mégnin did, as varieties. It is also hardly possible to distinguish the mite of human scabies (S. hominis) from that of a number of domestic animals (S. squamiferus). It is best, therefore, to accept one single species (S. scabiei), which may give rise to different races or castes by living in the skin of man and mammals, but can pass from one host to the other.
[Canestrini and Kramer, in their monograph of the Sarcoptidæ, enumerate eighteen distinct species of this genus, from the dog, goat, camel, horse, ferret, lion, wolf, sheep, pig, etc., and two species parasites of man (scabiei and scabiei-crustosæ). There is no doubt that they are distinct species.—F. V. T.]
Fig. 368.—Sarcoptes scabiei: male, ventral aspect. 200/1. (After Fürstenberg.)
The S. scabiei of man (S. scabiei var. hominis) (length of male 0·2 to 0·3 mm., and breadth 0·145 to 0·190 mm.; length of female 0·33 to 0·45 mm., and breadth 0·25 to 0·35 mm.) lives in the tunnels that it excavates in the epidermis, and attacks by preference places with thin skin, such as between the fingers, in the bend of the elbows and knees, in the inguinal region, on the penis, on the mammæ, but may also affect other parts. The tunnels, which vary from a few millimetres to a centimetre and more long, do not run straight, but are somewhat tortuous; the female is found at the terminal end. The tunnels contain the excrement and oval eggs (0·14 mm. in length) of the parasite; the males are rarely met with, as they die off after copulation; the females die after depositing their eggs. The six-legged larvæ hatch out after four to eight days, and after about a fortnight, during which time they change their skins three times and undergo metamorphosis, they begin themselves to burrow. Transmission from person to person rarely is effected through linen, but by direct contact (as in coitus); transmission can be artificially effected on horses, dogs and monkeys, but not on cats.
The smaller S. scabiei-crustosæ, Fürstenberg, is the cause of the itch that occurs chiefly in Norway; it is not certain whether this is a distinct species of itch mite.
[This is quite a distinct species, which is recorded from Germany and France. Mégnin (Parasitology, 1880, p. 165) described this as S. scabiei var. lupi. The female is 140 µ long, 340 µ broad; the male is 170 µ long by 150 µ broad. In Science (March 3, 1893, p. 125) is recorded that at the Indiana Academy of Science Dr. Robert Hessler referred to “a case of that extremely rare and almost extinct form of itch known as ‘Norway itch,’ the scabies norvegica of Hebra, 1852.” The afflicted man was covered with thick, creamy white, leathery scales; some of these scales measured over an inch in diameter and 1/10 in. thick. A constant shedding of scales went on, a handful being gathered daily. They were found full of mites and eggs and riddled with passages. Under treatment the mites were killed and the skin became normal. Dr. Hessler made a calculation of the number of eggs and mites, amounting to ova and shells 7,004,000, mites in all stages 2,009,000.—F. V. T.]