1912] Morphology and Biology of Insect Galls 373 



Biorhiza forticornis Walsh is produced on Neuroterus batatus Fitch the 

 stimuli from the different producers are exerted on nearly the same 

 region of the host at the same time, as both these species are stem galls 

 and commence to develop just as the buds are opening. In such a case 

 as this if we assume that the protoplasm of the host has acquired char- 

 acteristics necessary to the production of a certain form of gall, it seems 

 unlikely that it could also possess the characteristics that would enable 

 it to originate an entirely different type at the same time. 



With the exclusion of the likelihood that the genetic characteristics 

 of the protoplasm have been modified in any way, we must turn to the 

 environmental factors to account for the shape of the gall and the re- 

 lations of its various zones. Among these one feature that must have a 

 certain amount of controlling effect is the direction in which the stimulus 

 is applied. The various types of dimple and pouch galls, in which a curv- 

 ing of the affected organ is a very marked feature, are originated by 

 stimuli disseminated in one direction only, while a Cynipid gall with its 

 characteristic, spherical inner gall arises when the influence is about 

 equally distributed in all directions. In some species it is also clear that 

 the location of the egg has produced an effect on the external form of the 

 gall. If the egg is deposited on the epidermis of the host and the tissues 

 grow up around it, a gall of the type produced by Cecidomyia ocellaris O.S. 

 results (Fig. 33). Even in the Cynipidse this factor has been in operation. 

 In species of Andricus the openings of the canals give a characteristic 

 appearance to the galls, and in A . petiolicola Bassett the gall is drawn out 

 to a decided tip in the region of the canal. These canals owe their origin 

 to the fact that the galls are of the "Umwallung" type, and the larvae 

 have been enclosed by the growth of the surrounding tissues. 



In some galls such as Dryophanta palustris O.S. a cambium is differ- 

 entiated at a very early developmental stage, and has a very marked in- 

 fluence on the general relation of the zones in the gall. This cambium 

 layer is shown in Fig. 49. The cells produced from the inside of this 

 cambial tissue constitute the nutritive and sclerenchyma zones, while 

 those given off from the outside form the parenchyma zone and epidermis. 

 The former that are under the immediate control of the larva and less 

 exposed to external conditions come to differ more markedly from the 

 normal than do the latter that are nearer the outside limit of the larva's 

 sphere of influence. 



