ANIMAL GALLS. 413 



one cow only three or four bumps may be observed, 

 while upon another there will appear from thirty to 

 forty. They are not always placed on the same parts, 

 nor arranged in the same manner: commonly, they 

 are near the spine, but sometimes upon or near the 

 thighs and shoulders. Sometimes they are at remote 

 distances from each other; at other times they are 

 so near that their circumferences meet. In certain 

 places, three or four tumours may be seen touching 

 each other; and more than a dozen sometimes occur 

 arranged as closely together as possible. 



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

 tumour should remain constantly open; for by this 

 aperture a communication with the air necessary for 

 respiration is preserved; and the grub is thence 

 placed in the most favourable position for receiving 

 air. Its spiracles for respiration like those of many 

 other grubs are situated immediately upon the pos- 

 terior 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 the- surprising varieties of the latter, we did 

 not perceive that it was essential to the insects inha- 

 biting 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 the 

 grub of the breeze-flies in a fresh gall ? Without 

 doubt, not; but the apertures by which the air is 

 admitted to the inhabitants of the woody gall, al- 

 though they may escape our notice, in consequence 



* Rf aumur, iv. 549. 



VOL IV 34* 



