HYMENOPTERA 



35i 



The oak-apples. — There are various kinds of galls found on the 

 leaves and stems of oaks that are commonly known as oak-apples, a 

 name suggested by the spherical form and large size of some of them. 

 Several of these are quite similar in external appearance but are markedly 

 different in internal structure. In all there is a firm outer wall and a 



Fig. 594. — An oak-apple. 



small, central larval cell (Fig. 594). The part of the gall between the 

 larval cell and the outer wall differs in structure in the galls of different 

 species of gall-flies; in some it is filled with a spongy mass of tissue, in 

 others the larval cell is held in place by a small number of filaments that 

 radiate from it to the outer wall. 



The large oak-apple, Amphibolips confluens. — This is the largest 

 of our common oak-apples, measuring from 1 to 2 inches in diameter. It 

 occurs on several species of oak and is usually attached to a vein or the 

 petiole of a leaf. The space between the larval cell 

 and the outer wall of the gall is filled with a spongy 

 mass of tissue, in which in some of the galls there 

 are many radiating fibers, as shown in the figure 

 above, but in other galls these fibers are indistinct, 

 the space being filled with an amorphous mass of 

 tissue. 



In spite of the fact that these galls are com- 

 mon and conspicuous the life-cycle of the species 

 that produces them has not been fully worked out. 



The large empty oak-apple, Amphibolips 

 inanis. — In this gall the space between the 

 central larval cell and the outer shell contains 

 only a few, very slender, silky filaments, which 

 hold the larval cell in place (Fig. 595). The 

 gall measures from 1 to if inches in diameter, and is found on the 

 leaves of the scarlet oak and red oak. Externally this gall resembles 



Frc. 595. — The large empty 

 oak-apple. 



