THE CANADIAN ENTOMOLOGIST. 381 



Hymenoptcra (Families Cynipidcc and TenthredinidcB). From the 

 Bryophytes to the Spermatophytes nearh- all plants are subject 

 to gall formations of this class. 



The type of gall produced by the orders Acarina and Hemip- 

 tera is simple in structure, consisting usually of a more or less pro- 

 nounced folding in the leaf of the host, often accompanied in the 

 former by an abundant production of trichomes. The Coleoptera 

 and Lepidoptera originate galls that show little dififerentiation of 

 tissues and an entire lack of a well-defined nutritive layer. The 

 Dipterous forms are in some cases as simple in structure as the 

 Acarina pouch galls, but in others are as complicated as any of the 

 highest types of galls. In the order Hymenoptcra are two families, 

 Cynipidae and Tenthredinid;r, the members of which produce 

 galls that are in marked contrast to each other. The sawi^y galls 

 are characterized by a very pronounced proliferation of tissue 

 without differentiation into distinct layers except at the xQvy 

 earliest stages of gall production. The Cynipid galls, by wa}- of 

 contrast, have invariably three distinct zones of tissues, and only 

 seldom is a fourth absent. These layers have the following rela- 

 tion to each other. Lining the larval chamber is the nutritive zone 

 with cells oriented usually in a radial direction. Bounding this 

 layer on the outside is situated the protective sheath, the zane 

 that is absent in a few types. Outside of that again the paren- 

 chyma or tannin zone is differentiated, passing out to the epidermal 

 layer. 



One fundamental and far-reaching principle of gall production 

 by insects is that the stimulus does not endow the protoplasm of 

 the host with power to produce new types of organs, tissues, etc. 

 Structures are in many cases originated that are not found on the 

 same part of the normal host, but invariably their prototypes are 

 present on another part of the plant or a nearly related species. 

 The protoplasm is so stimulated that not only are dominant 

 characteristics strengthened, but also in certain cases latent pro- 

 perties are called into activity, and thus the apparent new type of 

 production appears in the host. This principle can be illustrated 

 in the case of glands, trichomes and aeriferous tissue. 



It may be stated, as an unvarying rule, that when glands are 

 present in the normal tissue, they are always more plentiful or 



