538 MISC. PUBLICATION 657, U. S. DEPT. OF AGRICULTURE 
majority of the species are scavengers, others are parasitic. One 
species, Wohlfahrtia vigil (Wlkr.), is said to be the frequent cause 
of cutaneous myiasis In man and mammals. 
Species of the genus Sar cophaga are both saprophytic and parasitic. 
A large number of them breed in mammal excrement, others in dead 
fish. Dead and dying caterpillars or pupae are also attractive breed- 
ing material. There are many records of parasitism, some of which 
appear doubtful. Several species, the most common of which is the 
grasshopper maggot (S. /ellyi Ald.), are parasitic on grasshoppers 
in the Western States. The same species is also said to have been bred 
from the cerambycid borer Plectrodera scalator. In the South, S. poli- 
stensis Hall has been reared from Polistes tewanus Cress. 
One of the most interesting recent observations concerning sar- 
cophagid parasitism is that of Sarcophaga aldrichi Park. Hodson 
(226) reports that the species will not only attack normal larvae and 
cocooned pupae of the forest tent caterpillar (Jalacosoma disstria), 
but it will also breed in carrion. Moreover, when the species devel- 
ops parasitically there is only one generation a year, whereas two 
generations develop when it breeds in carrion. Unpublished observa- 
tions by H. J. MacAloney are to the effect that S. aldrichi was the 
most important insect-control factor in an outbreak of the forest tent 
caterpillar in northern Minnesota in 1938. 
Famity CALLIPHORIDAE 
The Calliphoridae differ from the Sarcophagidae in that the hind- 
most posthumeral bristle is almost always lower (more ventrad) than 
the presutural bristle. The body color is frequently metallic green, 
blue, or yellow. The aristae are plumose. 
The larvae are of the usual muscid form readily differentiated from 
the sarcophagids in that the posterior spiracles are flush. 
The habits of the species are variable. One of the best-known species 
is the screwworm fly (Cochliomyia americana C. and P.) which is 
a major pest of cattle and other animals in the Southeast and West. 
This species deposits its eggs in wounds, the larva invading the tissues 
and eventually causing death if the animal is not found and treated. 
The greenbottle fly (Lucilia sericata Meig.) and the black blowfly 
(Phormia regina Meig.) are common scavengers and will attack sheep, 
cattle, and other animals if these have sores or wounds in which the 
flies can deposit their eggs. One of the most important control meas- 
ures in connection with these species is the prompt destruction of 
carcasses in which the flies breed prolifically. 
Species of the genus Protocalliphora are parasitic on nestling birds. 
Johnson (257) reported that in a Massachusetts locality in 1929 prob- 
ably 80 percent of the first brood of bluebirds died from the effect of 
the blood-sucking larvae of Protocalliphora spp. Other host species 
are the mourning dove, tree swallow, warblers, and crested flycatcher. 
Famity DROSOPHILIDAE 
Pomace Flies 
The Drosophilidae are small, plump-appearing flies with con- 
spicuous frontal bristles. 
