6o 



HYMENOPTERA 



Fig. 23. — Psilhyrus restalis. Britain. A, Female, 

 B, outer side of hind leg. 



no harm beyond appropriating a portion of their food supphes. 

 Sehmiedeknecht says they are commensals, not parasites ; but it 

 must be admitted that singularly few descriptions of the habits 

 and life-histories of these interesting Insects have been recorded. 



Hoffer has, however, 

 made a few direct 

 olDservations which 

 confirm, and at the 

 same time make 

 more definite, the 

 vague ideas that 

 have been generally 

 prevalent among 

 entomologists. He 

 found and took 

 home a nest of Bom- 

 Inis variabilis, which 

 contained also a 

 female of Psithyrus 

 campestris, so that he was able to make observations on the two. 

 The Fsithyrus was much less industrious than the Bombus, and 

 only left the nest somewhat before noon, returning home again 

 towards evening ; after about a month this specimen became still 

 more inactive, and passed entire days in the nest, occupying itself 

 in consuming the stores of honey of its hosts, of which very large 

 ([uantities were absorbed, the Psithyrus being much larger than the 

 host-bee. The cells in which the young of the Psithyrus are hatched 

 are very much larger than those of the Bomius, and, it may therefore 

 be presumed, are formed by the Psithyrus itself, for it can scarcely 

 be supposed that the Bombus carries its complaisance so far as 

 to construct a cell specially adapted to the superior stature of its 

 uninvited boarder. When a Psithyrus has been for some time a 

 regular inhabitant of a nest, the Bombus take its return home from 

 time to time as a matter of course, displaying no emotion what- 

 ever at its entry. Occasionally Hoffer tried the introduction of 

 a Psithyrtis to a nest that had not previously had one as an in- 

 mate. The new arrival caused a great hubbub among the Bombus, 

 which rushed to it as if to attack it, but did not do so, and the 

 alarm soon subsided, the Psithyrus taking up the position in 

 the nest usually affected by the individuals of the species. On 



