io6 On the Formation of 



making use of the rudimentary Aphiloirix fecimdatrix 

 galls is also of interest, because it is undoubtedly by 

 a further development of this habit that the inquilines 

 or lodgers have branched off from the Cynipidae, to 

 which they are so nearly related. The countless 

 inquilines of the oak gall-flies, which regularly take 

 possession of the great majority of galls, are the worst 

 enemies we have to contend with in our observations. 

 In their whole organization they are so nearly related 

 to the true gall-flies that they can only be distinguished 

 from them by the most minute characteristics, and 

 undoubtedly they have been developed from them. 

 By the use of a gall already formed, the prosperity of 

 their progeny is much more certainly insured ; un- 

 fortunately, however, inquilines are thus much more 

 easily reared and collected than the true gall-maker. 



It is evident from the descriptions of individual galls 

 already given, that they can be produced on all parts 

 of the oak — on leaves, flowers, trunk, root, and buds. 

 In each of these organs the gall-fly finds the same 

 formative zone which only needs the insertion of an 

 Qggy and escape from it of a larva, to excite gall 

 formation. We know also that the gall-fly proceeds 

 with judgement in selecting sometimes the tender 

 leaves, sometimes the terminal buds, and sometimes 

 the flower buds for its particular purpose. In spite, 

 however, of every care numerous galls fail, as we have 

 already noted, even where there can be no doubt that 

 the eggs had been laid by the fly. 



What causes this frequent failure of the galls ? We 

 might at first imagine that there had been some inter- 



