Ill 



PSITHYRUS, THE USURPER-BEE 



The most interesting and, in my experience, the 

 deadliest enemies to which several of the commonest 

 species of humble-bees are liable to fall a prey are 

 bees so closely resembling the true humble-bees 

 themselves that only a student can tell the differ- 

 ence between them. These bees were first recog- 

 nised to be distinct from the ordinary humble-bees 

 in 1802 by Kirby, who noticed among other differ- 

 ences that the females lacked the pollen-collecting 

 apparatus on the hind legs, 1 and thirty years later 

 Lepeletier gave them the name of Psithyrtis. 



Each species of Psithyrtis breeds only in the 

 nests of its own particular species of Bombus. The 

 Psithyri produce no workers, and some of the early 

 observers wh'o saw them in the nests of the Bombi, 

 noticing that they appeared to live in perfect 

 harmony with their hosts, suggested that they 

 might render them some important service. But 

 it was soon discovered that their association with 

 the Bombi was to the disadvantage of the latter, and 

 they came to be regarded as commensals living 



1 Monographia Apu/u Anglia, vol. i. pp. 209, 210. 



59 



