204 american hymenoptera. 



External Anatomy. 

 Integument. 



The chitinous covering of these insects is very hard and 

 firm in all groups, species and castes. There are consider- 

 able differences in it's toughness, however. Species of the 

 genus Psithyrus have a noticeably tougher covering than 

 have those of Bombus in the same sex. In Bombus, the queen 

 always has a thicker integument than that of the workers 

 and males. 



The plates of the head and also of the thorax are more or 

 less solidly fused together so as to make these portions of 

 of the body very rigid. The abdomen is quite flexible be- 

 cause the intersegmental membranes allow for considerable 

 movement, but each separate segment is in itself quite 

 rigid, though there is enough spring in the dorsal and ventral 

 plates to allow for considerable lateral compression, under 

 pressure, of this region of the body. 



Most of the external surface of the integument is very 

 thickly punctate. These punctures are the places of origin 

 of the hairs which form the clothing. There are several 

 kinds of these hairs and there is a great difference in the 

 relative fineness and coarseness of these different kinds. 

 There is a corresponding variation in the size of the punc- 

 tures which give settings to the hairs. The principal integu- 

 mental appendages may be classified as follows : 



1. Minute unbranched hairs, found principally on and around the 

 eyes, on the antennae, on the wings, on the mandibles, on the region 

 in front of that bearing the long branched hairs on each abdominal 

 segment and on the spathae of the genitalia of the male. 



2. Longer unbranched hairs, found mainly on the tongue, on the 

 maxillae, on each abdominal segment in front of the longer, branched 

 hairs and on the sting palpi. 



3. Minute branched hairs, present mostly on the legs (especially the 

 tarsi), on the apex of theepipygium andhypopygium, around the eyes, 

 on the propodeum, on the labrum and on the genitalia of the male. 



4. Long, coarse hairs with short branches (fig. 18). These hairs 

 make up most of the pile on the dorsum of the abdomen. They are 

 also strongly intermixed on the thorax and on the head with the kind 

 next described. They make up most of the long pile on the legs, in- 



