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Rh the two sexes. In the genus just named, for example, the male antennæ are quite simple, and filiform, while the female exhibits the remarkable peculiarity just alluded to. The prothorax is usually small in hymenopterous insects, but in many cases it is quite distinct, while in the Chalcididæ, Uroceridæ, &c. it forms a considerable portion of the dorsal area. The latter, however, generally consists of the mesothorax, which is here greatly enlarged, in order to afford sufficient momentum to the fore wings, on which the power of flight, in this order, chiefly depends. It is therefore this section of the thorax, which, in descriptive language, is spoken of simply as the thorax. The metathorax is proportionably diminished, but still holds sufficient prominence to correspond with the importance of the hinder wings, which are also essential to flight; and it seems to become enlarged as the size of the hinder wings increases.

The structure of the wings, and the arrangement of the nervures, have been perhaps more carefully studied in this than in any of the other orders, partly in consequence of their having been made the basis of a systematic classification of the genera. Taken in connection with characters derived from other parts, they must be admitted to afford valuable marks of distinction, both among the Hymenoptera and Diptera. It is, therefore, important, that the designations of the nervures and the cells formed by their intersection should be explained. When the nervures of the upper wings exist in their