_Notoedres cati_ (usually known as _Sarcoptes minor_) is a species of itch mites which produce an often fatal disease of cats. The body is rounded and it is considerably smaller than _Sarcoptes scabiei_, the female (fig. 61) measuring 215-230µ long and 165-175µ wide; the males 145-150µ by 120-125µ. The most important character separating _Notoedres_ from _Sarcoptes_ is the position of the anus, which is dorsal instead of terminal. The mite readily transfers to man but does not persist, the infestation usually disappearing spontaneously in about two weeks. Infested cats are very difficult to cure, unless treatment is begun at the very inception of the outbreak, and under ordinary circumstances it is better to kill them promptly, to avoid spread of the disease to children and others who may be exposed. [Illustration: 61. Notoedres cati, male and female. After Railliet.] Demodecidæ The DEMODECIDÆ are small, elongate, vermiform mites which live in the hair follicles of mammals. The family characteristics will be brought out in the discussion of the species infesting man, _Demodex folliculorum_. [Illustration: 62. Demodex folliculorum. (×200) After Blanchard.] _Demodex folliculorum_ (fig. 62) is to be found very commonly in the hair follicles and sebaceous glands of man. It is vermiform in appearance, and with the elongate abdomen transversely striated so as to give it the appearance of segmentation. The female is 380-400µ long by 45µ; the male 300µ by 40µ. The three-jointed legs, eight in number, are reduced to mere stubs in the adult. The larval form is hexopod. These mites thus show in their form a striking adaptation to their environment. In the sebaceous glands and hair follicles they lie with their heads down (fig. 63). Usually there are only a few in a gland, but Gruby has counted as many as two hundred. [Illustration: 63. Demodex folliculorum. Section through skin showing the mites in situ. Magnification of Nos. 1, 2, 6 and 7, ×150; Nos, 3, 4, 5, ×450. After Megnin.] The frequency with which they occur in man is surprising. According to European statistics they are found in 50 per cent to 60 per cent or even more. Gruby found them in forty out of sixty persons examined. These figures are very commonly quoted, but reliable data for the United States seem to be lacking. Our studies indicate that it is very much less common in this country than is generally assumed. The Demodex in man does not, as a rule, cause the slightest inconvenience to its host. It is often stated that they give rise to comedons or "black-heads" but there is no clear evidence that they are ever implicated. Certain it is that they are not the usual cause. A variety of the same, or a very closely related species of _Demodex_, on the dog gives rise to the very resistant and often fatal follicular mange. HEXAPODA OR TRUE INSECTS