HYMENOPTERA 



937 



leaf-veins ; from these eggs hatch the larv^ that cause the growth of 

 the oak hedgehog galls. In the sexual form the wings are well- 

 developed. The name bicolens was proposed for this form bv Kinsey 

 ('20). 



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. Sev- 

 eral of these are 

 quite similar in ex- 

 ternal appearance 

 but are markedly 

 different in internal 

 structure. In all 

 there is a firm outer 

 wall and a small, 

 central larval cell 

 (Fig. 1 162). The 

 part of the gall be- 

 tween the larval 

 cell and the outer 

 wall differs in struc- 

 ture in the galls of 

 different species of 

 gall-flies; in some 

 it is filled with a 

 spongy mass of tis- 

 sue, 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, Amphtholips conflilens. — This is the largest 

 of our common oak-apples, measuring from 18 to 50 mm. 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 common and conspicuous 

 the life-cycle of the species that produces them has not been fully 

 worked out. What is known of it is based chiefly on the observations 

 of Walsh (1864), little has been added during the long interval since 

 this publication. 



The oviposition has not been observed. The galls appear on the 

 leaves early in the spring; from some of them there emerges in June 

 a generation of gall-flies consisting of both males and females; and 

 from other galls there emerges in the autumn a generation of gall-flies 

 which consists only of females. 



An oak-apple. 



