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302 comprised between the sub-medial nervure and the posterior margin of the wing, which are denominated 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 transverse 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 disappear by turns, till we come to Psilus, in which