226 AMERICAN HYMENOPTERA. 



The most striking differences in genitalia, between Bombus 

 and Psithyrtis^ are presented by the volsellae. The volsellse 

 of Bombus are hard and horny and have well developed api- 

 cal projections. Those of Psithyrus are semi-membraneous 

 and are destitute of apical projections. The weak volsellae 

 of Psithyrus seem to indicate that in that genus the legs are 

 used to grasp the females, during copulation, more than they 

 are in Bombus. The lateral elevations on the female hypopy- 

 gium of Psithyrus may have some use in this connection. 



Wings. — These organs are large and powerful. They vary 

 in color, between the various species and castes, from nearly 

 hyaline to very dark brown with violaceous reflections. Over 

 the base of each fore wing, lies a small, arched, chitinous plate, 

 known as the tegula (fig. 47). This plate is rather pointed 

 at one end, but is broadly rounded at the other. Toward 

 the pointed end, the outer surface is rather thickly punctate 

 and bears considerable long hair. Besides the tegulse, there 

 are several other, though smaller, chitinous pieces connected 

 with the bases of the four wings. These have not all been 

 studied in detail by the writer, but two of them, larger and 

 more conspicuous than the rest, may be mentioned here. 

 One of these pieces (fig. 52, SC) is sub-triangular in general 

 outline and lies, for the most part, on the front side of the 

 base of the wing. This piece is the enlarged basal portion of 

 the subcosta of Comstock's classification, the remainder of 

 that vein having entirely disappeared. The other piece (fig. 

 52, AP) runs from the middle of the wing base back beyond 

 the hind margin of the base. This piece is elongate, rounded 

 in front and pointed behind. It is known as the median plate 

 of the wing base or the median basal plate. There are also 

 several small pieces connected with the bases of the hind 

 wings, but these have not been studied by the writer. 



The nomenclature of the veins and cells, used by Cresson 

 and others, has been followed in this paper, but drawings 

 have been included which name the parts according to the 

 Comstock system (figs. 3, 4, 7, 8). 



Fore wings (figs. 3, 5, 7 and 9). — The radial cell is elon- 

 gated and very narrowly rounded at its outer end. There is 



