<*F INSECTS. 299 



the two sexes. In the genus just named, for ex- 

 ample, the male antennae 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 Chalcididse, Uroceridae, &c. it forms a con- 

 siderable portion of the dorsal area. The latter, 

 however, generally consists of the meso thorax, 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 there- 

 fore 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 es- 

 sential 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 maxi- 



