268 ANIMAL LIFE 



\vlirro gall-formation has undoubtedly begun before 

 the time of hatching are probably to be explained as 

 due to the influence of the larva making itself felt 

 through the egg-membrane. 



The larva of the gall-fly having no cause to wander 

 in search of food, and being shut off from most of the 

 changeable conditions of free life, possesses a simpler 

 structure than that of the saw-fly. Its colour, like 

 that of most hidden animals, is white. No limbs 

 are present and the bent body merely turns about 

 from side to side, as it slowly drains the wall of sappy 

 or pulpy material that has grown around it. This 

 supply lasts it for several months, after which it pupates 

 and issues as a fly in the next spring. In many cases 

 it has been found that the gall formed by it in turn is 

 different from that which produced it, and whilst the 

 first generation is entirely composed of females, the 

 galls, stimulated by their larvae, produce in the next 

 season both sexes ; and this generation is so different 

 in appearance that it was for some time considered to be 

 a totally distinct form. To what cause we can attri- 

 bute this difference in the two successive generations 

 is not yet thoroughly clear, but the fact that the two 

 galls are unlike points to a slight difference in the 

 food. The influence of change of diet upon young 

 insects is known to be a far-reaching one, and as any 

 check in the flow of sap is sufficient to convert an 

 aphis of one kind into an aphis of a different category, 

 and as a stimulating food will produce a queen bee 

 from a worker egg, so it may be in this case, that a 



