ON THE ORIGIN OF THE FORMS OF GALLS. 85 



when they are attached to the leaves on the tree ; 

 it is only when they reach the ground that the 

 larva and the gall become fully grown. The very 

 small size of many galls must aiford them some 

 protection, for there is little inducement for inqui- 

 line flies to lay their eggs in them, the gall only 

 affording nourishment to a single larva. On the 

 other hand there is not the slightest doubt but 

 that the large size of some of the galls must be of 

 the greatest use as a protection to the larvae. In 

 the case of the large monothalamous galls, the 

 larva merely requires the central layer for food, 

 and this layer, too, is of a very strong texture. 

 The larva is protected by a layer of from two to 

 three lines, or even half an inch or more according 

 to the sj)ecies, and this must be pierced by the 

 parasite before it can reach its victim — and hence 

 the ichneumons ^\itli short ovij)ositors are pre- 

 cluded from attacking them. In a small gall, if a 

 cuckoo-fly dei:)osited its eggs in it at the proper 

 time, the tenant was certain to be killed, but the large 

 galls may afl'ord food foi- both. I do not mean to say 

 that this is always the case, but I have more than 

 once reared both the gall and the inquiline flies from 

 the same gall. I have observed that the Synerfjii.s 

 — the cuckoo-fly — lays its eggs in the galls of Cijuij^s 

 Kollarl while the latter A\'as very young, and in 

 this case it would doubtless kill tlie CijnijJs, but if 

 the eggs had been deposited later on both species 

 might have reached maturity. It is, in fact, a most 

 interesting subject to trace how the i^arasitic and 

 inquiline flies have had their bodies and tlieir 

 habits modified so as to enable them to prey on 

 the gall-flies — the former so as to enable them to 

 reach into the centre of the large galls — the latter 

 so that they may ai3i:>ear at the only time when 

 they can attack with any advantage, i.e., when the 

 galls are soft. 



I have already stated that the pilosity on some 

 galls serves to protect them by assimilating them 



