OP INSECTS. 
299 
the two sexes. In the genus just named, for ex- 
ample, the male antennee 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 Chalcididce, Uroceridce, &c. it forms a con- 
siderable 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 there- 
fore this section of the thorax, which, in descriptive 
language, is spoken of simply as the thorax. The 
metatliorax 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- 
