234 AMERICAN HYMENOPTERA. 



toward its apex. In Psithyrtis females, this fringe is rather 

 long throughout the length of the metatarsus. The hind 

 metatarsi, in all groups, and castes, are proportionately much 

 shorter, in comparison with the tibiae, than are those of the 

 front and middle legs. In Bombus males and in Psithyrus, the 

 hind metatarsi vary considerably in the proportion of their 

 length to their breadth and this variation is of some value 

 in classification. The outer face of the hind metatarsus of 

 the male is usually longitudinally concaved, but in a very 

 few species (e. g. — fratermis) it is fiat. 



The segments of the hind tarsi, beyond the metatarsi, are 

 not different from those of the front and middle tarsi in 

 any important respect, except that they are proportionately 

 longer. 



Secondary Sexual Characters. 



Aside from the presence of a sting in the females and of 

 copulatory organs in the males, the following important 

 sexual distinctions, which are common to the two genera, 

 may be mentioned : 



1. The males have thirteen segments in the antennae while the fe- 

 males have only twelve. The segments of the flagellum are generally 

 longer in the males and its basal segments are often of different rela- 

 tive length from what they are in the female. 



2. The males have seven exposed abdominal segments while the fe- 

 males have only six. 



3. The mandibles of the male are narrow and always two toothed 

 while those of the female are broad and either without teeth or with 

 three or more teeth. 



4. The mandible of the male is heavily bearded on the outer side, 

 while that of the female is not. 



5. The labrum of the female has either tubercles or tubercular areas 

 with a transverse, shelf-like projection or carina between them. These 

 elevations are absent on the labrum of the male. 



6. The epipygium and hypopygium of the female are pointed api- 

 cally, while those of the male are rounded. 



7. The coloration of the pile is markedly different in the males of 

 some species from what it is in the females. 



8. The male clypeus, as a rule, is very much more covered with hair 

 than that of the female. 



Besides the above described secondary sexual distinctions, 

 Bombus and Psithyrus have certain such characters which are 



