vii] THE SOCIAL-BEES 93 



In point of numbers a Bombus community falls 

 far below other species of social Hymenoptera. A 

 strong subterranean nest may have a population of 

 about 300 or 400 individuals ; while that of a surface- 

 builder seldom contains more than half that number, 

 and frequently much fewer. Smith gives statistics of 

 a nest of B. mnscorum, which yielded 25 females, 

 36 males and 59 workers. 



The number present is largely determined by the 

 amount of attention received from one or other of the 

 five species of Psithyrus which play "cuckoo" upon 

 the humble-bees, especially upon the subterranean 

 species. As a rule the Psithyrus both in appearance 

 and in structure closely resembles the Bombus which it 

 patronises, but is of larger dimensions ; for example, 

 Ps. rupestris invades the nests of B. lapidarhis, and 

 like its host is a large black bee with a red tail ; the 

 "cuckoos' are, however, entirely destitute of all 

 apparatus for the collecting of pollen, and only occur 

 in the form of males and large females : there are no 

 " workers." The Psithyrus is admitted to the nest 

 of the Bombus without any demur and actually goes 

 so far as to build cells, which are recognisable by 

 their greater size, for the reception of her eggs. 

 Hence we may presume that the "cuckoo" habit has 

 been but recently acquired, and that the call of duty 

 has not quite ceased to make itself heard. Beyond 

 this, however, Psithynts does not go. She leaves the 



