THE GALL-FLIES. 247 



scaly gall, which has been compared to a miniature 

 artichoke. Several other species are nourished upon 

 different parts of the oak-tree, and one (the C. \Bio- 

 rhiza] aptera) even resides in galls upon its root. 



All these galls, even those of different species upon 

 the same part of the free, exhibit a difference of size, 

 form, and structure so considerable, that it seems sur- 

 prising how causes so nearly identical can produce 

 such various results, and many theories have been 

 put forward to account for the formation of these cu- 

 rious excrescences. None of these however, plausible 

 though they may be, can account for the phenomena, 

 and we can scarcely go beyond the bare statement of 

 facts given above. Most of the conjectures of authors 

 attribute the formation of galls to an extravasation 

 of sap, but this can never account for the production 

 of regularly organized structures, such as the majority 

 of these excrescences are. 



Amongst the galls found upon the common oak, 

 there is one which deserves a further notice, as its 

 nature has been the subject of some controversy, it 

 having 'been regarded by some authors as a small para- 

 sitic fungus. On the lower surface of oak-leaves in 

 the latter part of summer, we constantly meet with 

 small flattish circular discs, which when detached are 

 seen to have been fixed to the leaf by a very short 

 stalk. The remainder of the lower surface is smooth 

 and free, and the outer surface, or that exposed to 

 the air, is covered with minute hairs. These little 

 discs, which are commonly known as oak- spangles, 

 remain attached to the leaves, even when these fall 

 from the trees, and they may be found in great 

 abundance during the winter upon the fallen leaves 

 in oak-woods. They remain in this condition until 



