WHAT IS AN "OAK APPLE"? 



697 



THE WINGLESS FEMALE 

 (FIRST GENERATION) 



Which oviposits in the ter- 

 minal buds of the oak in 

 winter. 



(Magnified.) 



OFFSPRING OF THE 

 ABOVE. 



One of the winged, 

 sexual generation of 

 flies, which comes 

 from the oak apple 

 i n summer - time. 

 They lay eggs in the 

 rootlets of oak trees, 

 and their eggs pro- 

 duce agamic, wing- 

 less females. 

 (Magnified.) 



the tender root- 

 lets of the oak. 

 As a result, galls 

 are produced 

 upon the root- 

 lets; and in them 

 the winter gen- 

 eration of wing- 

 less females 

 comes to matur- 

 ity. These root 

 galls, by the 

 way, are much 

 smaller and 

 harder than the 

 oak apples. In- 

 deed, they are 

 not unlike little 

 nuts, or st(;nes, 

 and those who 



both males and 

 females, whereas 

 the wingless 

 winter genera- 

 t i o n consists 

 e n t i r e 1 y o f f e- 

 males endowed 

 with the mys- 

 terious power of 

 parthenogenesis, 

 or virgin repro- 

 duction. Also, 

 the females of 

 the winged sum- 

 mer brood be- 

 have in quite a 

 different manner 

 from that 

 adopted by their 

 forerunners, the 

 larger wingless 

 females. Instead 

 of ovipositing in 

 oak buds, they 

 work their way 

 beneath the shal- 

 low soil and lay 

 their eggs within 



wish to find them must search witli 

 care. Each root gall, too, has only a 

 few inmates (perhaps only one or t\vo), 

 wliereas the oak apple generall}' con- 

 tains several scores of grubs. 



Let me now briefly summarise the life 

 cycle of the oak apple gall-fl\-. in order 

 that we may realise how markedly it 

 differs from the life cycles of insects in 

 general. In the first place we saw a tiny 

 insect abroad in the dead of winter. la\-ing 

 her eggs at the heart of an oak bud.' As 

 spring advanced, this bud developed into 

 an oak apple gall, which provided food 

 and shelter for a number of grubs. In 

 June these grubs completed their trans- 

 formation within the gall, aiul came forth 

 as perfect flies, differing in an extraordinary 

 manner from their parent. 



Now, in the case of an ordinary insect, 

 the story would end here. To continue 

 our observations would l)e merely to 

 verify facts which \\e had already noted. 

 Not so, however, with the gaU-flies. We 

 saw that the egg-laying of the second 

 generation of flies resulted in galls quite 

 different from the oak apple, and that 

 from these gaUs came wingless female 



SECTION THROUGH GALL FROM ROOTLET OF AN OAK 



(GREATLY ENLARGED). 



Showing chambers, one of which contains a grub. 



