- PARASITES AND ENEMIES 75 
nests of B. lapidarius. When the infested nest is 
under the ground the full-fed caterpillars often spin 
a web to the mouth of the tunnel and, climbing up 
by this, spin their cocoons on the surface under a 
stone or other protection. 
Another insect that devours the brood is the 
larva of Brachycoma devia, a two-winged fly. This 
fly much resembles the common house-fly, but it 
belongs to a different family, the Zachznznz, many 
of the species of which, in their larval stage, are 
parasitic in the bodies of various insects. The 
larva of Brachycoma is of the shape usual in fly 
maggots, namely, tapering to a point at the mouth 
and truncated at the tail. It is white and trans- 
lucent and bears no spines. Large specimens 
attain a length of 2 inch. Only about a dozen of 
these maggots are usually found in a nest at a time, 
but occasionally there are more, and I have seen the 
brood in a nest of B. pratorum completely eaten up 
by them. In the hot summer of 1911 the maggots 
were seen in most of my outdoor nests. Ina strong 
nest of terrestris that I dug up, several of the queen 
cocoons had a soft watery appearance and were 
found when opened to contain the maggots ; there 
were two, sometimes three, nearly full-sized maggots 
in each cocoon with the shrivelled remains of the 
Bombus larva, but there were no holes in the cocoons 
so that the maggots must have allowed themselves 
to be imprisoned with the Bomdbus larva when it was 
spinning. I have seen the cocoons of B. meuscorum 
similarly occupied by the maggots, and find that it 
