696 



THE NATURE BOOK 



fed, each grub 

 changes to a 

 pupa , and 

 towards the 

 end of June 

 emerges from 

 the gall as a 

 perfect fly. 

 Now these 

 flies d i ff e r 

 markedly 

 from their 

 parent — the 

 ant-like insect 

 which laid her 

 eggs in the 

 oak bud. 

 They are in- 

 ferior to her 

 in size, while 

 each has four 

 wings. More- 

 over, this gen- 

 eration of the 

 oak apple gall- 

 fly comprises 



SECTION THROUGH AN OAK APPLE (ENLARGED). 



Showing chambers wherein the grubs undergo 

 transformation. 



gall-flies known to science is bred from 

 a perfectly distinct and characteristic 

 gall. These galls all originate in ex- 

 actly the same manner ; i.e. in the egg- 

 laying of an insect. The reason why 

 they differ so markedly has yet to be 

 discovered. Yet I believe that when 

 this difficult question is cleared up, we 

 shall find that whereas the plant makes 

 the gall in so far as the supply of con- 

 structive energy is concerned, the tiny 

 grub (or, in certain cases, the develop- 

 ment and swelling of the egg), by irritat- 

 ing the sensitive cells in a peculiar \va.y, 

 determines the form which the gall 

 shall take. In other words, each kind 

 of gall-grub probably gnaws the cells in 

 a manner quite different from that 

 adopted by all other kinds of gall- 

 grubs, just as the caterpillars of butter- 

 flies and moths have characteristic 

 methods of feeding upon leaves. 



Let us now return to our oak apple. 

 As the spring advances, the little colony 

 of grubs within, each in its scjiarato 

 cell, feeds and grows fat. When full 



I 



t 





OAK APPLE IN EARLY SUMMER. 



