INSECTS. 



219 



egg is deposited, may begin an extraordinary production of cellular tissue 

 around the egg, soon resulting in a marked altera! ion of the corresponding 

 part of the plant. On the other hand, the gall may not begin to be formed 

 until after the hatching of the egg. Stranger still is the fact that if two 

 different species sting the same plant, the resulting galls will be entirely 

 different in their character; while if the species stings Two different kinds 

 of plants, the galls will be much alike. 



Inside the gall the larva feeds and grows, and it may remain inside it 

 until it attains the perfect condition, or it may bore its way out. descend 



Fig. 218. — a, sumac-gall; b, willow-gall; c, cabbage-willow gall. 



to the ground, and there undergo the rest of its changes. In connection 

 with this growth some interesting features have recently been discovered 

 in Europe. From one form of oak-leaf gall a gall-fly hatches out. which 

 immediately proceeds to deposit its eggs, not in the leaf as did its parent, 

 but in the buds, and the resulting galls are very unlike those of the 

 preceding generation. Now, when the proper time comes, from this 

 second generation of galls a fly hatches which does not in the least resem- 

 ble its parent, but which, until its history was known, was classed in 

 an entirely distinct genus. This in its turn lays eggs, which develop 

 into the first form. As the reader will see, this is an alternation of gen- 

 erations, recalling those detailed in the earlier pages of this volui 

 There is stilL another fact to be noticed in this connection. The indi- 

 viduals of one generation will all be females, while in the next both 

 males and females will occur. In still other forms, like the rose-gall fly. 

 shown in Figure 217, several generations will occur without the appear- 

 ance of males. 



