694 MEDICAL ENTOMOLOGY 



in the form of a triangle on each side behind the origin of the second 

 pair of legs ; in addition there are two rows of three larger spines in the 

 posterior part of the body. On each side of the anus there are two long 

 bristles projecting backwards. 



This species is the well-known sarcopt of scabies ; it burrows into 

 the skin, forming a tunnel at the bottom of which the female deposits her 

 eggs ; her presence causes irritation which results in time in the forma- 

 tion of an eczematous condition of the skin. 



The eggs hatch in about five days and the larva which emerges sheds 



its skin three times, the last moult resulting in the formation of the 



nymph. These are of different sizes, and when the 



Life history of Sar- next mou i t ta ^ es p i ace t h e smaller become males and 



coptes scabei 



the larger females ; the former do not moult again, but 



the latter pass through another change, when they become mature. The 

 males often remain at the surface of the skin,, and die as soon as the act 

 of copulation is completed. 



Sarcoptes scabei has many varieties which have been described as 

 species by different observers ; 'they are mainly characterized by differences 

 in size, and this is said to depend on the thickness of the skin of the 

 host. Those parasitic on the Pachydermata are the largest, next in size 

 are those found on the Carnivora, next those on the Ruminants, and so 

 on. There is a variety of Sarcoptes scabei 'for 'almost every common 

 domestic animal ; .it should be noted that they are not restricted to one 

 species of animal but may pass from one to the other. 



GENUS PSOROPTES, GERVAIS 



Body oval ; capitulum conical, considerably elongated and without 

 cheeks (cp. Sarcoptes). The legs are short and thick, particularly the an- 

 terior ones, and, unlike those of Sarcoptes, they are visible outside the 

 lateral margins of the body ; the suckers are carried on a long three- 

 jointed pedicel. The male has copulatory suckers and is armed with 

 two knob-like prolongations. 



This genus also contains one species, Psoroptes communis, with a 

 number of varieties related to the species of the host; they can only be 

 distinguished from each other by their size. These sarcopts do not 

 burrow into the skin, but live among the scabs produced by their 

 irritating bites. 



GENUS CHORIOPTES, GERVAIS 



Body oval ; capitulum conical and as broad as it is long, without 

 cheeks. Legs are long and thick and visible beyond the sides of the 



