GENUS PSITHYRUS 205 



Males. The beginner does not find it easy to 

 distinguish the Psithyrus males from those of 

 Bombus, there being no very striking characters 

 to rely upon. At first glance they differ most 

 noticeably in their somewhat thinner and coarser 

 coats, and particularly in their more shining, because 

 less densely-clothed, abdomens. When the 1st segment 

 is clothed with yellow hairs the brightness of the 

 yellow is reduced by the black skin underneath. 

 The face is rounder and shorter than in the males 

 of most species of Bombus. The forceps of the 

 armature are pale and membranous, while in Bombus 

 they are brown and horny. Some of the best dis- 

 tinguishing characters are to be seen in the hind 

 tibiae : these, as in the females, are, on the outer 

 side, transversely convex and hairy all over, while in 

 the males of Bombus the tibiae are flattened, bare (or 

 almost so) in the centre, and more shining than in 

 Psithyrus, except in the males of the carder-bees, 

 which in respect of these tibial characters occupy 

 an intermediate position ; but as the males of the 

 carder-bees have the joints of the antennae swelled 

 underneath (only the last six joints in B. derha- 

 mellus and B. sylvaruni), a peculiarity not found in 

 the Psithyri and in the other Bombi, they can easily 

 be excluded from comparison. 



