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). Acalyptrate muscidce and some species 
of Anthomyiidce. 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 ( Pliorbia 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 ) spinulse 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. 
/. vSlits 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 . Calliphorince 
