Mites Injitrioiis to Domestic Animals. 79 



Demodex, are internal parasites, but are very difEerent in shape, 

 being almost circular in outline. Demodex agrees with Psorergates and 

 certain other Trombidiid mites {Myobia, etc.) in having the sexual 

 aperture of the male on the dorsal surface, and also in the structure 

 of the appendages of the larvae and nymphs which are quite rudi- 

 mentary, merely consisting of an epimeron supporting a circular 

 disc crowned with minute denticles (fig. 52). It is probable, there- 

 fore, that the Demodicidae are descended from pilicoloua mites like 

 Myohia, living at the base of the hairs of the host, and have subse- 

 quently become internal parasites, the very small size and elongation 

 of the body being special adaptations to. this mode of life. Besides 

 the forms of Demodex mentioned below as causing skin trouble in 

 domestic animals, Demodex occurs in man, bats, hedgehogs, moles, 

 badgers, foxes, deer, rabbits, field mice, rats, house mice, etc. 



Fig. 67. 



(a) Sternal plate of Lipo7iyssus bursa, 9 (p. 89). (b) Sternal plate of 

 L. sylviaruui, 9 . (Original.) 



Genus Demodex, Owen, 1843. 



Demodex canis, Leydig, 1859. 

 (Fig. 48.) 



An elongated species, usually more than six times as long as 

 wide. Spinule on capitulum very short and pointed. 



D. canis is present in the skin disease called demodectic or 

 follicular mange, a very serious complaint which is most difficult 

 to cure. In typical cases the mites are found in enormous numbers, 

 and are associated with a bacterium (Staphylococcus jnjogenes albus 

 or some allied form). The mite plays an important part in the 

 disease by preparing the ground for the invasion of the bacterium 

 by dilating the follicles and sweat glands, and possibly also carries 

 and introduces the germ. The clinical symptoms of follicular 

 mange vary very greatly ; sometimes it assumes a pustular form 

 characterised by numerous pustules accompanied by various crusts, 

 etc. ; in other cases it is a squamous affection with numerous 

 epidermal pellicles, and sometimes the irritation is not very great 



