302 SYSTEMATIC ARRANGEMENT 



comprised between the sub-medial nervure and the 

 posterior margin of the wing, which are denomi- 

 nated the posterior cells. 



"Such is/' says Lacordaire, of whose accurate 

 summary we have occasionally availed ourselves in 

 the preceding description; "such is, with a few 

 modifications relating rather to the form and size 

 of the cells than their number, the reticulation of 

 the wings of the Hymenoptera, composing the 

 genera Tenthredo, Cimbex, Allantus, Urocerus, 

 Sirex, &c. On examining the series of genera to 

 the opposite limits of the order, we perceive changes 

 introduced more or less important in proportion as 

 we recede from the groups just named. Even among 

 the Evanii, which are very nearly allied to them, 

 we observe only four principal nervures, and the 

 cubital and discoidal cells are each of them reduced 

 to one. In almost all the genera which follow, the 

 nervures, instead of extending to the edge of the 

 wings, terminate in the middle of the surface. If 

 the cells are closed they retain their usual form; 

 but if, as often happens, they are not united by trans- 

 verse nervures, they remain open, and are then said 

 to be incomplete. It likewise happens occasionally, 

 that a cell at its base does not occupy the same space 

 as that which precedes it, and is united to the latter 

 by a kind of stalk more or less elongated ; the cell 

 so circumstanced is said to be petiolated. Following 

 these degradations we see the cells gradually dis- 

 appear by turns, till we come to Psilus, in which 



