HENRY J. FRANKLIN. 221 



without punctures or hair. They are, however, very deli- 

 cately reticulated, though, to the unaided eye, they appear 

 perfectly smooth and shining. The smooth and hairless 

 condition of these areas is evidently due to their friction 

 against the hind coxae. 



The last exposed segment in both sexes is different in 

 form from the ones that precede it. In this paper, the last 

 exposed tergum is called the epipygiiun and the last exposed 

 sternum the hypopygmm (figs. 28 and 43). The last ex- 

 posed segment, taken as a whole, is pyramidal and bluntly 

 pointed at the apex in the females, the sixth tergum and 

 sternum opposing each other as epipygium and hypopygium. 

 In the males, the seventh tergum is convex above and broadly 

 rounded at the apex. In this sex the seventh tergum is the 

 epipygium and has the sixth sternum opposed to it as the 

 hypopygium. The thorax and all the anterior segments of 

 the abdomen possess no particular characters of value in 

 species determination aside from the character of the pile. 

 The hypopygium, however, often presents distinctive struc- 

 tural characters. In the females of Psithyrus, it has a strongly 

 elevated carina on each side of the apical portion (fig. H). 

 These elevations are absent in Bombus, but some Bombtis 

 females have a slight median carina on the apical portion. 



Within the cavity of the last exposed segment are lodged 

 the sexual organs and their appendages. All the details of 

 these structures have not been fully worked out by the 

 author, but, as far as they have been studied, they will now 

 be described for the two sexes separately. 



The sting and its appendages (fig. 16). — In general structure 

 and relationship, these organs are much like those of the 

 honey bee already so well described by Mr. Snodgrass (same 

 paper as previously cited). The sting itself is a somewhat 

 curved, tapering shaft with its tip directed posteriorly and 

 its base swollen into a bulk-like enlargement. This shaft is 

 not solid, but is a hollow organ made up of three pieces 

 which surround a central canal. One of these pieces is dor- 

 sal and the other two are ventral. The ventral pieces slide 

 lengthwise on track-like ridges of the dorsal piece, the latter 



TRANS. AM. ENT. SOC. , XXXVIII. 



