434 



Insect Architecture. 



than a dozen sometimes occur arranged as closely together 

 as possible. 



It is very essential to the grub that the hole of the tumor 

 should remain constantly open ; for by this aperture a com- 

 munication with the air necessary for respiration is pre- 

 served ; and the grub is thence placed in the most favour- 

 able position for receiving air. Its spiracles for respiration, 

 like those of many other grubs, are situate'd immediately 

 upon the posterior extremity of the body. Now, being almost 

 always placed in such a situation as to have this part above, 



Bumps or wurbles produced on cattle by the Ox breeze-fly. 



or upon a level with the external aperture, it is enabled to 

 respire freely.* 



We have not so many examples of galls of this kind as 

 we have of vegetable galls; and when we described the 

 surprising varieties of the latter, we did not perceive that 

 it was essential to the insects inhabiting them to preserve 

 a communication with the external air : in the galls of trees, 

 openings expressly designed or kept free for the admission 

 of air are never observed. Must the grub, then, which 

 inhabits the latter have less need of respiring air than tho 



*' Keaumur, !v, 549. 



