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 (he 
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 docs 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 ^etiolated. Following 
these degradations we see the cells gradually dis- 
appear by turns, till we come to Psilus, in which 
