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 Brae hy coma devia, a two-winged fly. This 

 fly much resembles the common house-fly, but it 

 belongs to a different family, the Tachinince, many 

 of the species of which, in their larval stage, are 

 parasitic in the bodies of various insects. The 

 larva of Brae hy coma 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 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 191 1 the maggots 

 were seen in most of my outdoor nests. In a 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 Bombus larva when it was 

 spinning. I have seen the cocoons of B. muscorum 

 similarly occupied by the maggots, and find that it 



