LIFE-HISTORIES OF GALL-FLIES 267 



Gall-flies. — In contrast to the free-living larva; 

 of saw-flies, the gall-fly's young spend their whole 

 life within excrescences on oaks, willows, and roses. 

 How these galls are made has long been a moot problem. 

 That some great advantage is gained from these local 

 inflammations of plant-tissues is clear, both from 

 the great variety of gall-forming creatures and the 

 close association in one and the same gall of diverse 

 insects, some of which have entered it after the 

 inflammation had begun ; and it seems probable that 

 the attraction is due to a very concentrated form 

 of easily digestible, nutritious, and inflamed tissue. 

 Each plant responds in a characteristic fashion. The 

 bedeguar or rose gall, and witches-brooms on birches, 

 are modified shoots ; the oak-apple is an aborted leaf ; 

 the willow-spot is merely an inflamed part of a leaf ; 

 and it was long thought that the plant parasite, be it 

 fly, scale, or mite, inserted an irritant fluid with its 

 egg into the bud or leaf to which each plant responded 

 by these several wonderfully specific outgrowths, 

 thereby concentrating tender rich juicy tissue into a 

 compact space about the egg, and so furnishing a 

 supply of stimulating food to the larva. In the case 

 of the gall-flies, however, irritation does not usually 

 follow the insertion of the egg until the larva hatches 

 out, and there sometimes follows an interval of ten 

 months before the gall begins to grow. From such 

 observations it has been concluded that the gall is 

 due, not to an irritant fluid injected by the parent, 

 but to something secreted by the larva ; and the cases 



