NOTES ON PSITHYRUS. 25 



of the Bremus colony. She has a powerful sting, and her integu- 

 ment is so thick that her opponents are unable to penetrate it with 

 their stings. But, like Siegfried and Achilles, the Psithyrus is 

 vulnerable in certain places e.g., the neck and it is chiefly for 

 this reason that she is not always successful. 



However, as already indicated, a Psithyrus queen sometimes does 

 gain admittance to a B remits colony without losing her life and 

 may remain with the colony until she dies. Here the question 

 arises : What is the attitude of such a Psithyrus queen and the 

 rightful owners of the nest to each other? The two foremost 

 European authorities on the subject do not agree on this point. 

 Hoffer ('Si, '88), who studied the habits of the European Psithyri 

 more extensively than any other investigator, repeatedly found the 

 Psithyrus queen living peacefully with the members of the Bremus 

 colony, including the host queen. Sladen ('99, '12, '15), on the 

 other hand, claims that the Psithyrus queen, at least those of 

 Psithyrus rupestris Fabricius and Psithyrus vestalis Foucrier, al- 

 ways kills the Bremus queen, 1 either at the time the Psithyrus 

 queen enters the nest or a few days later, when the Psithyrus is 

 about to begin egg-laying. After the Psithyrus has committed this 

 murder, the Bremus workers, according to Sladen ('12), con- 

 stantly watch for an opportunity to avenge their mother by killing 

 the usurper. He (p. 60 ff.) very vividly describes this phase in 

 the life history of the Psithyrus queen and the afflicted bumble-bee 

 colony as follows : "It is the practice of the Psithyrus female to 

 enter the nest of the Bomb us, to sting the queen to death, and then 

 to get the poor workers to rear her young instead of their own 

 brothers and sisters. 



' The way in which the Psithyrus queen proceeds in order to 

 ensure the success of her atrocious work has all the appearance of 

 a cunning plan, cleverly conceived and carried out by one who not 

 only is a mistress of the crime of murder, but also knows how to 

 commit it at the most advantageous time for herself and her future 

 children, compelling the poor orphans she creates to become her 



i On July 14, 1911, Sladen ('12, p. 70) took a nest of Bremus hortorum 

 Linnceus, containing 49 workers of B. hortorum, and 16 young queens and 

 2 males of Psithyrus barbutellus Kirby. From this rather meager evidence he 

 concludes that Ps. barbutellus is probably " parasitic in the same deadly way 

 as Ps. rupestris and vestalis." 



