Galls by Gall- Flies. 105 



a new species. At any rate this much is certain, that 

 the influence of the larva is necessary, not only for 

 the early stage of gall formation, but for the completion 

 of its later development. The first cells are arranged 

 in a circle around the larva, and mark it out as the 

 central point which is to dominate their growth in the 

 future. 



A circumstance which might lead to serious error 

 ought to be mentioned here. A gall-fly occasionally 

 selects a spot where the formation of an earlier gall 

 has already begun, and on this spot arises its future 

 gall. In one case I was able to observe this satis- 

 factorily. The gall of Aphilotrix fecundatrix, which is 

 produced by the little Andricus pilosus, is formed at 

 the end of June or beginning of July. At first it is 

 only recognizable by an enlargement or expansion of 

 the bud ; but about this time Andricus ciirvator 

 emerges, which also lays its eggs in buds. Owing 

 to the prevalence of the Aphilotrix fecundatrix galls it 

 not infrequently happens that Andricus curvator lays 

 an ^gg in the same bud, probably because the ovi- 

 positor can penetrate more easily into a swollen bud 

 of this kind. Later we find at the base of the mature 

 Aphilotrix fecundatrix gall, and between the bud scales, 

 the Aphilotrix collaris gall, which is produced by 

 Andficus curvator. I have often found two or three 

 Aphilotrix collaris galls in one Aphilotrix fecundatrix 

 gall. As the Aphilotrix collaris galls are easily over- 

 lot)ked by reason of their minute size, it is obvious 

 that in breeding a doubt might arise as to the origin 

 of the flies. This peculiarity of Andricus curvator 



