m PSITHYRUS, THE USURPER-BEE 69 



the cocoons of all the species that I have observed 

 (rupestris, vestalis, distinctus, and barbutellus) differ 

 from those of their Bombus hosts in that they 

 quickly lose their fresh lemon tint, and become of a 

 dull ochreous colour, and later turn semi-transparent, 

 crackling when they are dented qualities that are 

 possessed in only a very slight degree by the 

 Bombus cocoons. The cocoons of Ps. rupestris 

 form compact clusters like those of its host B. 

 lapidarius, while those of Ps. vestalis, like those 

 of B. terrestris, are only loosely attached to one 

 another. 



The Psithyrus kills the Bombus queen before 

 she has laid the full number of worker eggs, con- 

 sequently nests containing Psithyri are not very 

 populous, the number of workers seldom exceeding 

 eighty. In nests of B. lapidarius containing Ps. 

 rupestris I have never known a lapidarius queen 

 to be reared ; similarly in nests of B. terrestris 

 attacked by Ps. vestalis I have never seen a young 

 terrestris queen. 



In a Psithyrus- ridden nest a large number of the 

 workers become fertile. In digging out a nest of 

 B. terrestris, I can generally tell before reaching 

 the nest that it has been victimised by Ps. vestalis if 

 many workers containing eggs, known by their 

 shining black abdomens with the yellow band on 

 the second segment more or less obliterated, rush 

 out of the hole to greet me. But it is a remarkable 

 fact that as long as the Psithymis queen reigns in 

 the nest I have not known a Bombus male to be 



