136 Accidental or Facultative Parasites 



determined. Fortunately, several workers have undertaken com- 

 parative studies along this line. The most comprehensive publica- 

 tion is that of Banks (1912), entitled "The structure of certain dip- 

 terous larvae, with particular reference to those in human food." 



Without attempting an exhaustive list, we shall discuss here the 

 more important species of Diptera whose larvae are known to cause 

 myasis, either external or internal. The following key will serve 

 to determine those most likely to be encountered. The writers 

 would be glad to examine specimens not readily identifiable, if 

 accompanied by exact data relative to occurrence. 



a. Body more or less flattened, depressed; broadest in the middle, each segment 



with dorsal, lateral, and ventral fleshy processes, of which the laterals, 

 at least, are more or less spiniferous (fig. 101). Fannia ( = Homalomyia). 

 In F. canicularis the dorsal processes are nearly as long as the laterals; 



in F. scalaris the dorsal processes are short spinose tubercles. 

 aa. Body cylindrical, or slender conical tapering toward the head; without 

 fleshy lateral processes (fig. 105). 



b. With the posterior stigmata at the end of shorter or longer tubercles, or if not 



.placed upon tubercles, then not in pit; usually without a "marginal button" 

 and without a chitinous ring surrounding the three slits; the slits narrowly 

 or broadly oval, not bent (fig. 171 i). A caly pirate muscidce and some species 

 of Anthomyiida. To this group belong the cheese skipper (Piophila casei, 

 figs. 98, 99), the pomace-fly (Drosophila ampelophila) , the apple maggot 

 (Rhagoletis pomonella), the cherry fruit fly (Rhagoletis cingulata) , the small 

 dung fly (Sepsis violacea, fig. 170), the beet leaf-miner (Pegomyia vicina, 

 fig. 171 i), the cabbage, bean and onion maggots (Phorbia spp.) et. al. 

 bb. Posterior stigmata of various forms, if the slits are narrowly oval (fig. 171) 



then they are surrounded by a chitin ring which may be open ventro- 



mesally. 



c. Integument leathery and usually strongly spinulose; larvae hypodermatic or 



endoparasitic Bot flies (fig. 171, f, g, k). Oestridce 



cc. Integument not leathery and (except in Protocalliphora) spinulae restricted 

 to transverse patches near the incisures of the segments. 



d. The stigmal plates in a pit; the lip-like margin of the pit with a number of 



fleshy tubercles; chitin of the stigma not complete; open ventro-mesally, 



button absent (fig. 171 e) Flesh flies. Sarcophaga 



dd. Stigmata not in a pit. 



e. The chitin ring open ventra-mesally ; button absent (fig. 171 c). Screw- 



worm fly Chrysomyia 



ee. The chitin ring closed. 



/. Slits of the posterior stigmata straight; marginal "button" present (fig. 171 b); 

 two distinct mouth hooks, fleshy tubercles around the anal area. Phormia 

 (fig. 171 f), Lucilia and Calliphora (fig. 172, a, b), Protocalliphora (fig. 171, j), 

 Cynomyia (fig. 171, a). Blow flies, bluebottle flies Calliphorina 



