14 THE ABDOMEN. 



cellule (as in JEhnphytus), one as in Poecilosoma, or two 

 as with Tenthredo, &c. 



Specific characters, too, are sometimes afforded by 

 the position of the nervures. In this respect the form 

 of the accessory nervure is often useful. Sometimes it 

 is received at a greater or less distance in front of the 

 transverse median nervure (called then appendiculated) 

 (PI. X, fig. 13), or it may be joined to the transverse 

 median (PL X, fig. 13 a), when it is said to be inter- 

 stitial. 



The posterior wing has, on the costa, a number of 

 hooks, which fit into the thickened brim of the lower 

 edge of the front wing, so that in this way the two 

 remain united in flight. 



It only remains to add that with individual speci- 

 mens of most species, one or other of the cross ner- 

 vures may be absent, while, less frequently, greater 

 aberrations are met with. The species of Dineura (and 

 the Nematina generally) are especially liable to vary in 

 this respect ; with D. stilata, for instance, the trans- 

 verse radial nervure is as often absent as present. 



In the radial, cubital, and transverse and recurrent 

 nervures, are usually found small, white, blistered 

 spaces, which have been called by Walsh "bullae." 

 These exist in other groups of Hymenoptera ; and in 

 the Ichneumonidce have been shown by Walsh* to have, 

 from their constancy in position, some value from a 

 systematic point of view. They do not, however, 

 appear to have an equal value in the Tenthredinidce, 

 although in some cases they would seem to differ in 

 position in closely allied species or genera, and conse- 

 quently their presence or absence is worth mentioning 

 in specific descriptions, or even in generic ones. 



The Abdomen. 



The abdomen is joined to the thorax by its entire 

 width. It is, as a rule, longer than the head and 



* Proc. Ent. Soc. Phil., v, p. 209, and vi, p. 242. 



