Say what the use, were finer optics giv’n,
T’ inspect a mite, not comprehend the heav’n.
(Pope, Essay on Man.)
Demodex
We have seen that harvest mites are wont to insert their heads—or rather their mouth-parts—into the skin of human beings, but other mites show less restraint, and insert their whole bodies. One of these, the well-known Demodex folliculorum, is, according to Guiart and Grimbert, ‘Le plus commun des parasites de l’homme et nous en sommes presque tous porteurs.’ Without taking quite so gloomy a view, Demodex is undoubtedly widely distributed in the skin of mankind and of other mammals.
Fig. 34.—(a) Demodex in hair-follicle of dog; magnified. (After Neumaun.) (b) Demodex folliculorum; highly magnified. (After Railliet.)
There are differences of opinion as to whether this form should be split up into numerous species, or subspecies, according to the genus of the mammals upon which it lives. We, at any rate, will confine our attention to the human kind and so avoid losing ourselves in the tortuous maze of synonymy and the arid discussion of a meticulous classification so dear to the analytical German mind. To us a Demodex shall be a Demodex, and we will leave it at that.
Unlike the majority of mites, Demodex is a good deal longer than it is broad. But even for a mite it is very small, and shows signs of bodily degradation associated with its parasitic habit of life. Its shape is adapted to its habitat, which is the sebaceous glands of the skin. The long abdomen appears to be segmented, but the annulations are not true segments. The legs are reduced to conical stumps. The male is 300 µ[12] long and 40 µ broad across the cephalothorax. The female is, as usual, larger, measuring 380 µ in length by 45 µ in breadth. The minute larvae have, as is so often the case with mites, but three pairs of legs, and are 60 µ to 100 µ in length.
This parasite, which lives on all parts of the skin of the human body, is perhaps most commonly seen on the nose and in the passages leading into the ear. It can be expressed by firmly pressing over the black spot which indicates its presence in the skin of the nose or elsewhere any small cylindrical tube, such as a watch-key. When expressed it is not always easy to see, as coming away with it is a mass of sebaceous matter which can best be dissolved off with oil on the microscopic slide. Whether this particular parasite causes much disease is not known. But in some cases it is certainly associated with acne and other skin disorders; and as it is also found in hair-follicles, it may possibly destroy the hair. It is apparently spread by personal contact.