ON THE ORIGIN OF THE FORMS OF GALLS. 33 



resemble each other very closely. It is true that 

 some of them have longer or shorter ovipositors 

 than others, but this has reference solely to the 

 situation in which the eggs are laid. There is no 

 appreciable difference either between the larvae, 

 which, as has been explained, are the most active 

 agents in causing the galls to grow. I am not 

 iiware of any variation in their manner of 

 feeding; they all feed on a particular layer of the 

 gall, that, namely, which surrounds them — a layer 

 of cells containing much starchy matter. I believe, 

 however, that we may explain the forms and struc- 

 tures of galls by reference to well-known principles. 



Consider what exists with external-feeding 

 larvae. These have many enemies ; they are 

 preyed upon by birds, reptiles, and carnivorous 

 insects of all sorts, while there is the enormous 

 army of ichneumons ever ready to lay their eggs in 

 their chosen victims. To guard against these insidious 

 foes, we find that some are uneaten by birds OAving 

 to their bodies containing noxious secretions, or by 

 being covered by stinging hairs, while many have 

 peculiar habits which enable them to escape 

 danger. Large numbers escape death by their 

 bodies imitating the surroundings, animate or 

 inanimate. This is not mere theory; the facts I 

 have stated can be proved beyond dis^^ute, and are 

 independent of whatever views we may hold as to 

 their origin. 



Now apply the same reasoning to the elucidation 

 of the forms of galls. The gall clearly stands in 

 the same relation to the gall-fly as does its skin to 

 tlie external-feeding larva. The gall-larva is per- 

 fectly passive ; it is hidden, and, therefore, there is 

 no use in itself bearing the protecting sj)ines and 

 hairs found on so many external-feeding cater- 

 pillars ; neither can their bodies assume i3articular 

 shapes so as to elude, in this Avay, the keen eye- 

 sight of the ichneumons. If the larvae are to escape 

 they can only do so by the galls themselves being 



