336 IXSECT ARCHITECTURE. 



opinion that these fibres are the diverted nervures of the 

 leaves, which would have sprung from the bud in which 

 the gall-fly had inserted her eggs, and actually do carry 

 sap-vessels throughout the substance of the gall. 



Eeaumur says the perfect insects (^Cynips quercus) issued 

 from his galls in June and the beginning of July, and were 

 of a reddish-amber colour. We have procured insects, 

 agreeing with Eeaumur's description, from galls formed on 

 the bark or wood of the oak, at the line of junction between 

 the root and the stem. These galls are precisely similar in 

 structure 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. 



Root Galls of the Oak, produced by ('unips quercus ivferus? drawn from a specimen. 



There is another oak-gall, differing little in size and 

 appearance from the oak-apple, but which is very difierent 

 in stnicture, as, instead of giving protectioil and nourish- 

 ment 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, 

 however, encloses a small hard gall, which is the immediate 

 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 incorporated, however, but dis- 

 tinctly separate. 



We have obtained a fly very similar to this from a very 

 common gall, which is formed on the branches of the 

 willow. Like the one-celled galls just described, this is of 

 a hard, ligneous structure, and forms an irregular protuber- 

 ance, sometimes at the extremity, and sometimes on the 

 body, of a branch. But instead of one, this has a consider- 

 able number of cells, irregularly distributed through its 



