Fig. 370.—Demodex folliculorum of the dog. (After Mégnin.)
As in Sarcoptes scabiei, numerous varieties of this species are known; the form parasitic on man lives in the hair-follicles, the meibomian and sebaceous glands, and hardly ever causes inconvenience; the male measures 0·3 mm. in length and the female about 0·4 mm. in length. The eggs 0·06 to 0·08 mm. in length, 0·04 to 0·05 mm. in breadth, and are thin-shelled. The creatures are always attached with the head end downwards in the parts mentioned; they are most frequent in the sebaceous glands of the face, by the nose, lips and forehead, but they may be present on the abdomen and on other parts of the body. They may occasionally obstruct the excretory gland ducts, thus causing inflammation of the gland (comedones); their agglomeration in the meibomian glands sets up inflammation of the margins of the eyelids. There are generally only a few specimens in a gland. According to some statements Demodex occurs in 50 per cent. of mankind and even in children; they survive the death of their hosts by several days.
The variety living in the dog (D. folliculorum var. canis) is smaller than the variety living in man, and produces a skin disease resembling scabies in these animals. According to Zürn they may also live on man; nevertheless, no other investigator has recorded a similar observation, and attempts at artificial infection have proved negative.[354]
[Ten distinct species of Demodex are given by Canestrini and Kramer (“Demodicidae und Sarcoptidae,” Das Tierreich, 1899, vii). The species are certainly distinct.
[The species living on the dog (D. canis, Leydig, 1844) is cosmopolitan. According to the British Medical Journal (February 22, 1913, p. 407), dog mange may be caught by humans. Whitfield and Hobday describe in the Veterinary Journal seventeen cases which have come under their observation.—F. V. T.]
Order. Pentastomida.
Family. Linguatulidæ.
Arachnida greatly altered in consequence of their parasitic manner of life; for a long time they were regarded as helminthes. The body is elongated, vermiform, flattened or cylindrical, and more or less distinctly annulated. The head, thorax, and abdomen are not defined from each other (fig. 371). The elliptical mouth, surrounded by a chitinous ring, is situated at the anterior end, on the ventral surface, and the intestine leading straight through the body opens at the posterior end. Two retractile hooks are at the sides of the mouth (fig. 372); these are usually considered to be the terminal joints of two pairs of legs, but it appears to be more correct to regard them as the remains of the antennæ and palpi (Stiles). According to this opinion, the legs in the adult state are completely degenerated.