104 On the Formation of 



in what way that differentiation proceeds which gives 

 to each gall its characteristic form, and regulates its 

 position and season. 



It is the value of these different structures, as pro- 

 tective contrivances, which has secured their evolution 

 by the gall. 



Pubescence, in particular, assumes an endless variety 

 of forms ; it may appear as a delicate down or as a thick 

 felt : sometimes the hairs exude a sticky sap which 

 deters parasites from approaching the gall ; and even 

 smooth galls like Aphilotrix Sieholdi secrete a juice 

 which, as already mentioned, serves to attract ants. 

 These protect them like sentinels, driving other insects 

 off, and often constructing a protective mantle of earth 

 around the galls. 



The influence of the larva enters as a necessary 

 factor into the normal course of gall formation. If 

 the larv^a perishes before the gall is formed, it is well 

 known that a stunting of the gall takes place. It has 

 been already mentioned in the description oi Aphilotrix 

 fecundatrix that when a roundish, undeveloped, inner 

 gall is found, parasites are always present ; it happens 

 in the same way that when 2iY\' Aphilotrix collaris gall 

 is pricked by parasites, it grows in an anomalous 

 manner, and becomes consolidated with the base of 

 the bud. In the galls of Aphilotrix Sieholdi we often 

 find a similar alteration ; for if the gall is pricked by 

 a parasite in the course of its development, it remains 

 small, scarcely projects above the bark, is not regularly 

 striped in the normal manner, and has altogether an 

 appearance so different that it was formerly mistaken for 



