ANIMAL-GALLS, 



359 



may be seen touching each other ; and more than a dozen 

 sometimes occur arranged as closel}' 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- 



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



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 situated immediately 

 upon the posterior extremity of the body. Now, being 

 almost always placed in such a situation as to have this 

 part above, 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 Ihe 

 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 ad- 

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

 which inhabits the latter have less need of respiring air 

 than the grub of the breeze-flies in a flesh-gall ? Without 



* Keaumur, iv. 549. 



