282 A BOOK OF INSECTS 



woven fibres of grass or moss. Such nests are often 

 approached through an arched passage of the same 

 material, and are usually cunningly hidden amongst the 

 herbage. But carder bees readily adapt themselves to 

 circumstances, and have been known to make their homes 

 in such places as straw-filled packing-cases or deserted 

 birds' nests. In one instance a carder bee (Bombus 

 agrorum) actually invaded a wren's nest, and heaped up 

 its brood cells amongst the eggs, which were eventually 

 deserted by the parent bird. Another little humble-bee 

 family flourished for a time in the mud-walled habitation 

 of a house-martin, from which the rightful owners had 

 apparently been evicted ; but the bees, in their turn, were 

 subsequently victimised by the social caterpillars of a 

 small moth (Aphomia sociella) which feed upon wax and 

 similar substances. These laid waste the bees' nest, 

 ultimately spinning their cocoons amongst the wreckage. 

 Parasites and guest insects, or inquilines, are very 

 numerous in the nests of humble-bees. Some seem to 

 play the part of scavengers, and to be tolerated for this 

 reason. For example, the grubs of the two- winged flies 

 known as Volucella appear to eat what the bee larvae 

 reject ; whereas the grubs of other flies and certain beetles 

 are pestilential scourges which destroy both larvae and 

 pupae, and effect the downfall of the community. The 

 cuckoo humble-bees of the genus Psithyrus (Apathus of 

 some authors) resemble their hosts so closely that it 

 is difficult to tell them apart without close scrutiny. 

 But they have no workers, and the females lack food- 

 gathering appliances — notably the pollen-baskets of the 

 hind tibiae. Unfitted to perform their own domestic 

 duties, they take up their abode in the nests of social 

 humble-bees ; and strangely enough each species of 

 Psithyrus usually resembles in colouring the Bombus 



