THEOEIES OF GALL ORIGIN. 21 



eluded that the origin of so many gall-structures by 

 so many differently constructed insects — by insects 

 differing in mode of oviposition, in habits and form of 

 larvse, &c. — is hardly explainable by any one theory. 

 Thus the instrument of oviposition in some gall-mak- 

 ing beetles is the mouth ; with most insects it is the 

 ovipositor; while the Aphidse cause gall- structures 

 apparently by sucking the plants to obtain food for 

 themselves and progeny. Even in the Hymenoptera 

 we find two radical distinctions in habits of the insects, 

 as has already been pointed out (vol. ii, p. 185), — that 

 is to say, in the Tenthredinidse the gall is already 

 formed before the larva quits the egg; while in the 

 Cynipidse the birth of the larva is synchronous with 

 the formation of the gall — until the larva is born and 

 commences feeding there is no gall-formation. 



Let us see what takes place when a Oynips ovi- 

 posits in the leaf or bud, as the case may be. What is 

 the first factor in gall -formation ? This ; the growth of 

 the larva, as has already been pointed out. The second 

 is that the egg must be laid in, or in contact with, 

 the cambium layer. The cambium layer is the seat of 

 active growth in the plant. It is formative, not formed 

 matter. It is most active naturally in the spring. The 

 catkin galls are quickest in coming to maturity, the 

 birth of the larva and gall and its development to the 

 fly state occupying only a few days. The galls formed 

 on the young leaves are not much behind the catkin 

 ones in rapidity of growth ; while, on the other hand, 

 those found on the bark or roots take many months 

 in coming to maturity. Whenever the larva ceases 

 feeding no further growth takes place on those galls 

 which remain attached, except with twig galls like 

 those of Andricus inflator, which continue growing 

 along with the twig long after the fly has escaped. 

 On the other hand, there are galls which drop off in 

 an immature state in the autumn, resuming growth in 

 the spring. Even some of the succulent galls can be 

 detached before maturity without their development 



