158 Veterinary Medicine. 



Sarcoptes (sarx flesh, coptein to cut) which burrow, forming 

 galleries under the epidermis ; the psoroptes (psora itch, ptessein 

 to conceal) which live on the surface in the shelter of hair, scurf 

 and scab, and tend to spread over the whole body ; and the 

 symbiotes (sym with, bios life) which also live on the surface 

 sheltering under hair, scales and scabs and attack especially the 

 hind feet and legs. 



Sarcoptes are distinguished by round or slightly oval body ; 

 short rostrum and legs, the last pair being concealed beneath the 

 abdomen or nearly so ; the tarsus has often a sucker ; the male is 

 devoid of abdominal bosses and copulatory suckers. 



Each gnaws its way into the epidermis and constructs its own 

 gallery, the galleries of the larger females being much more 

 spacious than those of the males. The union of the sexes takes 

 place in these chambers, the males leaving their own burrows to 

 seek the females and being therefore sometimes caught on the 

 surface. The site of the gallery is marked in delicate white skins 

 (man, sheep, etc.) by a fine red line, but this is not observable in 

 the horse or ox. It may be straight, curved or crooked, broken 

 at intervals by a minute vesicle and its outlet is marked by a 

 papule or vesicle, which is especially marked when the larvae 

 burrow in the delicate, soft, young epidermis at this point. In 

 the gallery of a female are u.sually found eggs, empty shells and 

 larvse. The burrowing habit of the sarcoptes makes them the 

 most inveterate of the psoric acari as they cannot be readily 

 reached by local dressings to the skin. 



Psoroptes {Dermatodedes : Dermatocoptes) . These have an 

 ovoid body with long conical rostrum, long thick legs, all visible 

 around the margin of the body, with ambulatory suckers, and in 

 the male copulatory suckers and abdominal prolongations. 



These live in colonies on the surface of the epidermis, seeking 

 the shelter of the hair and later of the scabs which form from 

 their bites and the consequent itching, rubbing, gnawing and 

 scratching. They migrate in bodies when disturbed by comb or 

 brush or by rubbing, and on this account as well as because of 

 the crowding due to rapid encrease they tend to extend over the 

 whole body. In warm weather they leave the shelter of hair and 

 scabs to sun themselves on the hairs, and retreat again to their 

 shelter on the advent of cold. lyiving on the surface it is com- 



