108 SOCIAL HABITATIONS. 
The oak is a tree that seems to be especially loved 
by gall-insects, which deposit their eggs in its leaves, 
its twigs, its flowers, and even in its roots. One of the 
most familiar examples of oak-galls is that which is 
called the oak-apple, and which is produced by a species 
of insect called Cynips terminalis. Although the insect 
is not of very great size, the gall which it produces is 
sometimes enormous, being as large as a common golden 
pippin or nonpareil apple, and therefore very conspicuous 
upon the tree. It is coloured in the same manner as the 
cherry-gall, but seldom so brilliantly, and the exterior is 
not so smooth and polished. 
The resemblance to a veritable fruit is much closer at 
the beginning of the season than in the autumn, as a 
number of small leaf-like projections surround its base, 
just as if they were a half-withered calyx. These, how- 
ever, fall off as the summer advances, and are no more 
seen. 
If the oak-apple be cut with a knife, the first touch of 
the steel betrays a marked difference between its sub- 
stance and that of the cherry-gall. Its texture is neither 
so firm nor so juicy, but is of a softer, drier, and more 
woolly character. Moreover, the knife passes through 
several resisting substances, which, when the gall is quite 
severed, prove to be separate cells, each containing a 
erub. From each of these cells, which are extremely 
variable in number, a kind of fibre runs toward the base 
of the gall, and it is the opinion of some naturalists that 
these fibres are in fact the nervures of leaves which 
would have sprung from the bud in which the gall-fly 
has deposited her eggs, and which, in consequence of 
the irritating fluid injected into the tree, are obliged to 
develop themselves in a new manner. 
To procure the insects of this and many other galls is 
