24 O. E. PLATH. 



Smith ('55), Hoffer ('81, '88), Sladen ('99, '12, '15), and Prison 



C'16,'21). 



STRUCTURE. 



Both sexes of the genus Psithyrus so closely resemble those of 

 the genus Bremus that anyone, other than a specialist, would see 

 no difference between them ; and even the specialist sometimes has 

 difficulty in determining whether a certain male is a Bremus or a 

 Psithynts, so that it has occasionally happened that Bremus males 

 have been described as Psitliyri, and vice versa. It is quite easy, 

 however, to tell a Bremus female from a Psithyrus, because the 

 latter lacks corbiculce, or pollen baskets, a fact which was first 

 noticed by Kirby (1802, I., pp. 209, 210). Illiger (1806, p. 173), 

 suspecting a corresponding difference in habits, separated them 

 from the true bumble-bees, and Newman ('34, p. 404) later gave 

 the group the generic name of Apathus. This term held sway for 

 over 40 years, when it was discovered that the name Psithyrus had 

 been given to the group by Lepeletier in 1832 (p. 273). 



LIFE HISTORY AND HABITS. 



According to Hoffer ('88) and Sladen ('12), the young Psi- 

 thyrus queen, like the young Bremus queen, hibernates in the 

 ground, but reappears somewhat later in spring than does the latter. 

 Having no apparatus for collecting pollen, she is unable to found a 

 colony of her own, as does the Bremus queen, but, like the Euro- 

 pean cuckoo and some of our American cowbirds, perpetuates her 

 kind by entering, and laying her eggs in, the nests of her more 

 industrious cousins of the genus Bremus. The latter, like the 

 dupes of the European cuckoo, rear the larvae of these lazy guests 

 instead of their own. As will be seen later, these latter, at least 

 those in the earlier stages of development, are probably systemati- 

 cally destroyed by the Psithyrus. 



That the Psithyrus queen does not always gain admittance to a 

 Bremus colony without a struggle is indicated by the frequent dis- 

 covery in bumble-bee nests of dead or disabled Psithyri or Bremi, 

 or both, and is confirmed by direct observation when a Psithyrus 

 first enters, or is placed in, a Bremus nest. In these encounters 

 the Psithyrus has a great advantage over the individual members 



