SOME HUMAN PARASITES 291 



The screw- worm fly, Chrysomyia macellaria (fig. 91), is 

 confined to America and the West Indies. The adult flies 

 are about ten millimetres long and of a metallic green or 

 blue colour, marked with three longitudinal black lines on 

 the thorax; they have hairy abdomens, black legs, and 

 transparent wings. The larva (fig. 92) is dirty white in 

 colour and is divided into twelve segments, each one of 

 which is furnished with one or more rings of spines. The 

 larval form takes its sustenance from men or animals ; eggs 

 are laid by the adult fly wherever there is a wound, indeed 

 blood has a great attraction for these flies. Sometimes ovi- 

 position takes place in the human ear or nostril, especially 

 if there is any discharge from either organ. The pupa is 

 dark brown and spiny. 



HUMAN BODY MITES 



Of the mites inhabiting the human body, the itch mite 

 is becoming a rare pest in civilised communities ; the follicle 

 mite rarely attains serious proportions ; and the mites 

 causing grain itch, copra itch, and the like only attack 

 people engaged in certain trades. 



The itch mite, Sarcoptes scabei (fig. 93), occurs not only on 

 man, but on various domestic and wild animals, among them 

 being the horse, sheep, goat, camel, hog, dog, ferret, llama, 

 hare, rabbit, lion, wolf, fox, and wombat. The adults are 

 flattened, circular, and very minute ; their general structure 

 is shown in the figure. All stages in the life-cycle are 

 passed on the host, in fact many successive generations may 

 occur on the same host. In man, an attack of the mite is 

 characterised by intense itching, usually at the base of the 

 fingers and between the knuckles, though in bad cases the 

 whole hand may be inflamed and covered with pustules. 

 The itching is caused by the mites burrowing in the skin ; 

 as they extend their channels deeper and deeper they de- 

 posit their eggs, and the young which develop from them 

 feed on the surrounding tissue. After four moults maturity 



