STRUCTURE OF INSECTS. 327 



a groove all round, or an incisian, as it is tech- 

 nically termed. In the Hyraenoptera, this inci- 

 sion is so deep as to leave only a narrow pedicle, 

 like a neck, connecting these two divisions of 

 the body. In some this pedicle is short, in 

 others long : in the former case, an exceedingly 

 refined mechanism is resorted to for effecting 

 the necessary movements in a part so bulky 

 compared with the narrowness of the surface of 

 attachment.* 



Insects in their perfect state have constantly 

 six legs, which are the developements of the six 

 proper legs of the same animal in its larva con- 

 "* dition : all the spurious legs having disappeared 

 during its metamorphosis. We have seen that 

 in the myriapoda, the result of developement is 

 an increase in the number both of segments and 

 of legs ; the reason of which is that, being terres- 

 trial animals, a lengthened form was more useful 

 and accordant with their destination ; but in 

 winged insects, where the object is to procure 

 the means of flight, the organs require to be con- 

 centrated, and all superfluous parts must be re- 

 trenched and discarded from the fjibric. The 

 multiplication of organs, which, in the former 

 case, indicated the progress of a higher develope- 

 ment, would in the latter have been the source 



* For the details of this structure I must refer to writers on 

 entomology, and in particular to Kirby and Spence's "Introduc- 

 tion to Entomology," vol. iii. p. 701. 



