GALL-FLIES. 



335 



which are covered with down, hair, or wool, though by no 

 means so copiously as the one which we have just described. 

 Among the plants so affected are the germander speedwell, 

 wild thyme, ground-ivy, and others to which we shall after- 

 wards advert. 



Oak-apple Galls, one being cut open to show the vessels running to granules. 



The well-known oak-apple is a very pretty example of 

 the galls formed by insects ; and this, when compared with 

 other galls which form on the oak, shows the remarkable 

 difference produced on the same plant by the punctures of 

 insects of different species. The 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 calyx or cup of five or six 

 small brown scaly leaves ; but these fall off as the season 

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

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

 ing a grub ; and embedded in a fruit-looking fleshy sub- 

 stance, having fibres running through it. As these fibres, 

 however, run in the direction of the stem, they are best 

 exhibited b}^ a vertical section of the gall ; and this also 

 shows the remarkable peculiarity of each fibre terminating 

 in one of the granules, like a foot-stalk, or rath(ir like a 

 vessel for carrying nourishment. Reaumur, indeed, is of 



