HYMEXOPTEEA-ACULEATA. 419 



necessary, for instead of apparently living in 

 amity with their hosts, as is usnal with most of 

 the inquiline bees, the female Psithyrus, like 

 Melecta, enters the nest of her selected Bombus 

 as a murderous enemy, and, as actually observed 

 by Mr. F. W. L. Sladen (v. " Ent. Mon. Mag.," 

 October, 1899), fights with and slays the owner 

 " queen " before taking possession, or sometimes, 

 as noted by Mons. E. Hoffer, gets killed herself. 

 A victorious Psithyrus not only appropriates the 

 nest of the conquered species, but forces the 

 workers therein to collect food for and to rear her 

 own progeny. The various species of Psithyri 

 are usually inquiline with the following Bombi, 

 viz. : — 



P. rupestris in nests of B. lapiclarius. 



P. vestalis ,, ,, B. terrestris. 



P. barbutellus ,, ,, B. hortorum and prato- 



rum. 



P. quadricolor „ ,, B. pratorum and jonel- 



lus. 



P. campestris ,, ,, B. agrorum, venustus, 



and latriellellus. 



The first three resemble their hosts in colour 

 as well as in form, while the last two only do so 

 in the latter particular for the most part. 



P. rupestris, Fab. — Only noted so far from West Kirby 

 Thurstaston and Heswall ; also Chester, 

 E.C.T. ; possibly overlooked. 



P. vestalis, Fourc. — Widely distributed and often 

 abundant, e.g., West Kirby and Heswall ; South- 

 port, B.C., Chat Moss, J.E.H., Helsby, E.N., 

 Chester, E.N. and E.C.T., Delamere, E.C.T. and 

 R.N. 



