284 



1NSECTA. 



theca has a small accessory vesicle (i) connected with it, not found 

 in the former examples. 



(329.) In many insects, especially of the Hymenopterous order, 

 the generative apparatus is terminated externally by peculiar instru- 

 ments provided for the purpose of introducing the eggs into a 

 proper situation. This is particularly remarkable in the Ichneu- 

 mons, which deposit their ova in living caterpillars ; and in the 

 saw-flies (Tenthredo), whose eggs are insinuated into the sub- 

 stance of the leaves, or even of the branches of trees. To describe 

 all the contrivances employed for this purpose would lead us far 

 beyond our prescribed limits : one example of an organ of this 

 description must suffice. 



In the Sir ex gigas (Jig. 128) the ovipositor consists appa- 



Fig. 128. 



rently of three pieces of considerable length, seen in the figure to 

 project from the inferior margin of the abdomen. Of these pieces, 

 two form a sheath enclosing a third, called the terebra, or borer, which 

 in the Tenthredo contains two saws of extremely beautiful construc- 

 tion, as we learn from an account of them given by Professor Peck, 

 and quoted by Kirby and Spence :* the original description, which 

 it would be unpardonable to abbreviate, is as follows : " This in- 

 strument," says Professor Peck, " is a very curious object ; and, in 

 order to describe it, it will be proper to compare it with the tenon- 

 saw used by cabinet-makers, which, being made of a very thin plate of 

 steel, is fitted with a back to prevent its bending. The back is a piece 



* Introd. to Entom. vol. iv. p. 161 . 



