1912] Morphology and Biology of Insect Galls 367 



lining the interior of the larval chamber. None of the nourishment, 

 taken into the alimentary canal, passes from it as excrement; it is 

 either completely absorbed or remains in the digestive tract until the 

 completion of the larval stage. 



The invariable inert appearance and partially coiled condition of 

 the larva would seem to indicate inactive feeding habits, but the theory 

 of food absorption through the body wall is quite untenable; since the 

 complete digestive tract, containing often large quantities of nourish- 

 ment, as in Fig, 62, shows conclusively that the food enters the canal 

 through the mouth. 



Gall-Producing Stimulus . 



All actively growing tissues are capable of responding to a gall- 

 producing stimulus; the growth energy already present in them is con- 

 trolled and compelled to expend itself in a definite direction. These 

 abnormal tissues that result have the common characteristic of remain- 

 ing longer in a plastic state than if they had been produced under normal 

 conditions of growth. 



The stimulating influence produces an effect on tissues at a con- 

 siderable distance from the centre of application. Thus in the Acarina 

 galls this influence extends to tissues other than the epidermis on which 

 the mites are located, and in such a case as Stagmatophora ceanothiella 

 Cosens the epidermis of the stem undergoes division, although the larva 

 is feeding in the pith (Fig. 17). 



The power to stimulate tissues to abnormal activity is not confined 

 to gall-producing larvae. Certain inquilines likewise exhibit this ability 

 to a limited extent. By a fortunate chance I have been able to establish 

 this fact in the case of an inquiline larva found in the gall of Holcaspis 

 globulus Fitch. Reference to Fig. 65 will show that a nutritive layer has 

 been developed around the inquiline. That it possesses the power of 

 stimulation to a less extent than the producer is obvious from the fact 

 that it was unable to originate a cambium of its own, and in consequence 

 the nutritive zone is incomplete on che side opposite to the producer- 

 larva. This is shown in Fig. 65. Yet it is equally obvious that, feeding 

 as it was in proximity to the cambium of the producer, it was able to 

 excite that zone to the production of typical nutritive cells instead of the 

 parenchyma zone cells that would have resulted had the producer alone 

 been in control. Kiister^^ records a similar instance of inquiline-produced 

 galls in Rhodites eglantericz. 



KiJster^^ states that the excrement of the larval Pontania salicis is 

 capable of producing cell division. I have found this phenomenon 

 occurring also in Pontania pomum Walsh and particularly good examples 



