228 AMERICAN HYMENOPTERA. 



first discoidal cell, the median cell is the largest cell in the 

 wing. It is bounded distally by the slanting, but nearly 

 straight, basal vein, and the tnedian vein separates it behind 

 from the elongate and narrow submedian cell. The distal 

 end of the submedian cell is limited by the transverse ?ne- 

 dian vein. In Psithyrus and the subgenus Bombus, this vein 

 forms an obtuse inner angle with the median vein and coal- 

 esces for a short distance with the base of the discoidal vein. 

 In Bombias, the inner angle formed with the median vein is 

 acute and there is practically no coalescence with the dis- 

 coidal vein (fig. 14). The submedian and second discoidal 

 cells are limited behind by the anal vein. Posterior to the 

 anal vein is the long, narrow and open anal cell. The basal 

 portion of the wing is margined in front by the costal vein. 

 The outer part of the wing is free from closed cells. Along 

 the margin of the wing, behind the distal portion of the anal 

 cell, is a downward fold of the wing membrane. This is 

 known as the fre7ial /old and, with the frenal hooks on the front 

 of the hind wing, it serves to hold the two wings together in 

 action. 



Hind wings (figs. 4, 6, 8 and 10). — The hind wing has but 

 three closed cells. They are all elongate and basal. The 

 anterior one is very narrow and is called the costal cell. It 

 is bordered in front by the costal vein and separated behind 

 from the ynedian cell by the sub-costal vein. The median cell 

 is the largest cell of the wing. It is widest in the middle 

 and tapers toward both ends. It is separated from the sub- 

 median cell behind it by the median vein. The sub-median 

 cell is limited distally by the transverse-median vein and is 

 bordered behind by the anal vein. In Psithyrus and the sub- 

 genus Bombus, the transverse median vein is nearly straight 

 and forms a strongly acute inner angle with the median vein 

 and a strongly obtuse inner angle with the anal vein. In 

 Bombias, the transverse median vein is usually curved from 

 end to end and forms nearly a right angle with both the me- 

 dian and the anal veins (fig. 15). The radial cell is open 

 and is separated from the median by the radial vein. The 

 limits of the single cubital and first and second discoidal cells 



