688 



DISEASES OF THE SKIN. 



penetrating a little farther each time, leaving its deposited eggs 

 to occupy the space previously inhabited by itself. The direc- 

 tion of the canal is oblique, the portion firet formed being, of 

 course, nearest the surface. As the old epidermis is thrown off, 

 new layers of cuticle being formed from the deeper strata, the 

 first-laid eggs are gradually thrust upwards to the surface, where 

 they are finally extruded ; while the recently deposited ova 

 remain in the canal, close to the parent female, whose instincts 

 lead her to make the canal in such a way that her eggs reach 

 the surface about the time the young ones are ready to come 

 out of the shell. The newly hatched acari, male and female, 

 crawl about the skin, and are said to moult their skins three 

 times before they reach maturity; this occupies nearly five 

 weeks. They then enter into sexual intercourse; the female 

 burrows the skin, as before described, and dies at the blind end 

 of the canal or caniculus, after depositing her ova ; whilst the 

 male rambles about on the surface, entering into sexual inter- 

 course wdtli many females, a proof of the necessity for fewer 

 males than females. 



Authors differ as to the 

 time required for incuba- 

 tion ; some say that egg- 

 hatching occurs in eight or 

 ten days ; whilst Gerlach 

 brings forward experiments 

 to prove that three days are 

 sufficient for this purpose. 



SARCOPTES EQUI. 



The males of this pretty 

 large species, which is visible 

 to the naked eye, are about 

 yV" in length and breadth, 

 resembling a square notched 

 at its four angles; arched 

 on its back and belly, and Fig. 120.— Sarcoptesequi.— (Gerlach.) 

 tolerably thick; the body covered with alternate furrows and 

 varied lines, running transversely on the back and abdomen 

 in a semicircular form. On the back there are small tubercles. 



