-22 O. E. PLATH. 



As in the case of the affinis-ashtoni colony, the members of the 

 vagans-laboriosus colony became more and more inactive as the 

 weather grew colder. On September 12 the old vagans queen dis- 

 appeared from the nest, and four days later the old Psithyrus van- 

 ished. The two had lived together for about five weeks. 



From what has been said it is evident that Ps. ashtoni and Ps. 

 laboriosus do not always, if ever, kill the host queen, and in this 

 respect they behave like the two other American Psithyri whose 

 hosts are known. One of these, Ps. insular is, was found breeding 

 in a colony of B. flavifrons by Sladen ('15), and the other, Ps. 

 variabilis, was found repeatedly in the nests of B. pennsylvanicus 

 by Frison ('16). In the six infested nests reported by these two 

 authors, each one contained the old Bremus queen, and this despite 

 the fact that in both cases young Psithyri had emerged, or were 

 about to emerge. 



All of these observations on the habits of our American PsitJiyri 

 agree with Hoffer's ('Si, '88) account 1 and make Sladen's claim 

 (that Psithyrus rupestris and vestalis always kill the host queen) 

 extremely doubtful. Sladen ('12) based his conclusions on the 

 fact that he never found a living Bremus queen in a Psithyrus- 

 ridden nest. 2 Against this we have the positive evidence of Hoffer 

 ('88, pp. 126, 148), who, as in the case of Ps. carnpcstris, quadri- 

 color, and barbntdlus, also found the queens of Ps. rupestris and 

 vestalis living in peace with the host queen, in one case after some 

 of the offspring of the Psithyrus had emerged. Further inquiry 

 into the queen-killing habit of Ps. rupestris and vestalis will un- 

 doubtedly show that Sladen ('99, '12, '15) based his conclusions on 

 insufficient evidence. 



This same criticism applies to another of Sladen's ('12) con : 

 elusions. He (p. 68) believes that Psithyrus queens do not kill 



1 Recent observations by Wheeler and Taylor ('21) indicate that Vespa 

 arctica Rohwer, which has Psithyrus-like habits, being a social parasite on 



Vespa diabolica De Saussure, may also sometimes, if not always, live in peace 

 with the host queen. 



2 Such negative evidence, as the work of Wheeler and Taylor ('21) indi- 

 cates, is very unsatisfactory. Of nine Vespa diabolica nests taken by these 

 two authors comparatively early in the season (before August 4), only one 

 contained the old diabolica queen, and yet, with a single exception, these nests 

 were not parasitized by Vespa arctica. 



