GALL-FLIES. 385 



oak-apple is commonly as large as a walnut or small 

 apple, rounded, but not quite spherical, the surface 

 being irregularly depressed in various places. The 

 skin is smooth, and tinged with red and yellow, like a 

 ripe apple; and at the base there is, in the earlier part 

 of the summer, a calys or cup of five or six small 

 brown scaly leaves; but these fall off as the season ad- 

 vances. If an oak-apple be cut transversely, there is 

 brought into view a number of oval granules, each con- 

 taining a grub, and embedded in a fruit-looking fleshy 

 substance, having fibres running through it. As these 

 fibres, however, run in the direction of the stem, they 

 are best exhibited by a vertical section of the gall; 

 and this also shows the remarkable peculiarity of 

 each fibre terminating in one of the granules, like a 

 footstalk, or rather like a vessel for carrying nourish- 

 ment. Ri aumur, indeed, is of 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. 



Reaumur 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 Reaumur's descrip- 

 tion, from galls formed on the bark or wood of the 



Root-Galls of the Oak, produced by Cynips quercus ivferus? drawn 

 from a specimen. 



oak, at the line of junction between the root "and the 

 stem. These galls are precisely similar in structure 

 VOL. IV. 33 



