SARCOPTES SCABIEI 



covered by transverse rows of folds partly interrupted on the 

 back. There are transverse rows of small bristles on the dorsal 

 surface, and groups of trichomae on the front, sides and back. 

 There are chitinous hairs at the base of the legs ; the two first 

 pairs (of legs) are provided with pedunculated ambulacra in both 

 sexes, the two posterior pairs terminate each with a long bristle 

 in the female ; in the male the third pair of legs terminate in a 

 bristle, the fourth pair with a pedunculated ambulacrum. The 

 anus is situated at the posterior border of the dorsal surface. 



Formerly numerous kinds 

 were differentiated, according 

 to the form of the acarus, the 

 number, position and size of 

 the prickles and spines, even 

 according to the hosts, &c. 

 All these characteristics, how- 

 ever, fluctuate so consider- 

 ably that absolute differ- 

 entiation is impossible ; the 

 conjectural species might 

 almost be regarded in the 

 same light as Megnin did, 

 as varieties. It is also 

 hardly possible to distin- 

 guish the mite of human 

 scabies {Sarcoptes hominis} 

 from that of a , number of 

 domestic animals (S. squami- 

 ferus'}. The simplest plan, 

 therefore, is to accept one 

 single species - - Sarcoptes 

 scabiei which may give rise 

 to different races or varieties 

 by living in the skin of man 

 and a few mammals, but can 

 pass from one host to the other. 



The Sarcoptes scabiei of man (Sarc. scabiei, var. hominis) 

 (length of male 0-2 0-3 mm. and breadth 0-145 0*190 mm. ; 

 length of female 033 0*45 mm. and breadth 0*25 035 mm.) lives in 

 the tunnels that it excavates in the epidermis, and attacks by prefer- 

 ence places with thin skin, such as between the fingers, in the bend 

 of the elbows and knees, in the inguinal region, on the penis, on 

 the mammae, but may also affect other parts. The tunnels, from a 

 few millimetres to a centimetre and more long, do not run straight, 

 but are somewhat tortuous ; the female is found at the terminal 

 end, The tunnels contain the excrements and oval eggs (0*14 mm. 



FIG. 243. Sarcoptes scabiei, female, dorsal 

 aspect. 200 i. (After Furstenberg.) 



