SUBFAMILY CALLIPHORINAE 265 
If any conelusions can be drawn from this preliminary survey, 
the figures seem to indicate that females develop supernumerary 
bristles in the acrostichal series more frequently than do males, 
that specimens of sericata and pallescens rarely, if ever, occur 
with only two postacrostichal bristles on each side, and that these 
two species can be separated from caerulewiridis, mexicana, and 
probably most of the species which have only two postacrostichal 
bristles on each side by this sole difference. 
Distribution. It is often stated that sericata is cosmopolitan 
in distribution but this is doubtful. No specimens were collected 
in Central and South America in 1942 and 1948, nor on any 
islands of the Central or Southwest Pacific in 1944 and 1945, al- 
though over 100,000 such flies were collected in those areas. It is 
the most abundant species of Phaenicia in North America, occur- 
ring most frequently in collections in northern United States and 
southern Canada. 
Biology, habits, and immature stages. Egg. Macrotype, large, 
pale yellow with thick chorion which is striate and faintly reticu- 
late. 
Wardle (1930) and Melvin (1934) studied the effect of tem- 
perature and humidity upon the incubation period of sericata 
eggs. The latter author found that 42.37 hours were required at 
59° F., but only 8.09 hours at 99° F.; no eggs hatched at tem- 
peratures above 99°. The optimum temperature for development 
of eggs of this species appeared to be about 94° F., at which tem- 
perature only 8.10 hours were required to complete the stage. 
Larva. The larvae of sericata are usually found feeding upon 
decomposing flesh. In one instance larvae of sericata were found 
in small numbers in hen manure (Illingworth, 1923), and Thom- 
sen and Hammer (1936) found occasional larvae in pig manure. 
In the vast majority of cases, however, the larvae of sertcata will 
be found in decomposing meat or animal matter. 
First instar: Wardle (1930) stated that this stage is a short, 
nonfeeding period, terminated by a molt within 2 or 3 hours 
after hatching. 
Spines lightly pigmented, segments 2-7 each completely en- 
circled with spines at the anterior border; on segment 8 spines 
absent on dorsal surface except on 12, which is provided with 
long, almost colorless spines on border of posterior cavity. 
Second instar: According to Wardle, this is the true feeding 
period, lasting from 114 to 914 days, depending upon tempera- 
ture; it is also terminated by a molt. 
Segments 2-7 each bearing complete bands of spines at the 
anterior border, segment 8 usually with a narrow dorsal spine- 
