386 INSECT ARCHITECTURE 



to the oak-apple, and are probably formed at a season 

 when the fly perceives, instinctively, that the buds of 

 the young branches are unfit for the purpose of nidifi- 

 cation. 



There is another oak-gall, differing little in size 

 and appearance from the oak-apple, but which is 

 very different in structure, as, instead of giving pro- 

 tection and nourishment to a number of grubs, it is 

 only inhabited by one. This sort of gall, besides, is 

 hard and woody on the outside, resembling a little 

 wooden ball of a yellowish colour, but internally of 

 a soft, spongy texture. The latter substance, how- 

 ever, incloses a small hard gall, which is the im- 

 mediate residence of the included insect. Galls of 

 this description are often found in clusters of from 

 two to seven, near the extremity of a branch, not in- 

 corporated, however, but distinctly separate. 



We have obtained a fly very similar to this from a 

 very common gall, which is formed on the branches 



Woody-Gall on a Willow branch, drawn from a specimen. 



of the willow. Like the one-celled galls just de- 

 scribed, this is of a hard, ligneous structure, and forms 

 an irregular protuberance, sometimes at the ex- 



