36 O. E. PLATH. 



three queens of B. lapidarius, which usually builds underground, 

 started their nests in different parts of his house, and that a few 

 weeks later numerous Ps. rupcstris queens were hunting for these 

 nests. These observations of Hoffer, and the fact that Ps. labori- 

 osus queens repeatedly appeared in the writer's B remits colonies 

 kept in a third-story window (about 30 feet above ground), make 

 it evident that PsitJiyms queens are able to find bumble-bee nests, 

 even if the latter are located at a considerable altitude. Natural 

 selection in this direction is therefore of little benefit to a B rein us 

 species as long as the members of the genus Psitlivrus retain their 

 acute sense of smell. 



How exceedingly keen the olfactory sense of these social para- 

 sites actually is 1 can be inferred from the following observation : 

 On July 3, an exceptionally pleasant day, colony No. 2 (B. bimacu- 

 latus) was dug up. The comb had hardly been exposed when a 

 Ps. laboriosus queen swooped down upon it. She was captured 

 and a half a minute later another laboriosus queen was buzzing 

 about the comb. This one was also caught; but within another 

 minute a third laboriosus queen alighted on the comb, and this 

 despite the fact that not a single Psithyrus had been noticed in the 

 vicinity previously. 



This sudden appearance of the three Psitliynis queens also sug- 

 gests an error in one of Sladen's ('12) experiments. As already 

 stated, this author believes that the Brcinus workers of a Psithyrus- 

 ridden colony are constantly watching for an opportunity to kill the 

 PsitJiynts, even if the latter has been living with the colony for a 

 considerable period. The observations on which Sladen ('12) 

 chiefly based his conclusion were briefly as follows : On July 9, 

 1911, this author (cf. p. 251 ff.) dug up a nest of B. lapidarius 

 containing 71 workers and a large amount of Ps. rupestris brood, 

 but the rupcstris queen was nowhere to be seen. Suspecting she 

 had hidden herself in a side hole, he left a lump of cocoons in the 

 nest cavity to attract her, and returning a quarter of an hour later 

 actually found a rupcstris queen on the comb, and concluded that 

 he was dealing with the same Psithyrus queen which had been liv- 

 ing in the nest. However, judging from Sladen's ('12, p. 253) 



: Observations of Latter ('06) and the Raus ('iS) indicate that this is also 

 true of some solitary parasitic bees and wasps. 



