1914 ENTOMOLOGICAL SOCIETY. 101 



brooms and the spherical stem swellings on the black spruce are due to the stimulus 

 of the dwarf mistletoe Arceuthobium pnsillam. The latter are incited by mites 

 (Acarina) and by insects in several different orders as follows: Hemiptera 

 (Families Aphididce and Psyllidcc), Diptera (Families Cecidomyidce and Trype- 

 tidce), Cohoptera (Families Buprestidre, CeranihycidcB and Curculionidce) , Lepi- 

 doptera (Families Gelechiidce, Sesiidcv, Tvieidre), Hymenoptera (Families Cyni- 

 pidce and Tenthredinidce) . From the Bryophytes to the Spermatophytes nearly all 

 plants are subject to gall formations of this class. 



The type of gall produced by the orders Acarina and Hemiptera is simple in 

 structure, consisting usually of a more or less pronounced folding in the leaf of 

 the host, often accompanied in the former by an abundant production of trichomes 

 ias on plate 2, Figs. 4 and 5. The Coleoptera and Lepidoptera originate galls that 

 show little differentiation 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 (Fig. 3), but in others are as complicated as any of the highest types 

 of galls. In the order Hymenoptera are two families, Cynipidge and Tenthre- 

 dinidse, the members of which produce galls that are in marked contrast to each 

 other. The sawfly galls are characterized by a very pronounced proliferation of 

 tissue without differentiation into distinct layers except at the very earliest stages 

 of gall production. These layers can be seen in Fig. 13. The Cynipid galls, by 

 way of contrast, have invariably three distinct zones of tissue, and only seldom is 

 a fourth absent. These layers have the following relation to each other. Lining 

 the larval chamber is the nutritive zone with cells oriented usually in a radial 

 direction (Fig. 8). Bounding this layer on the outside is situated the protective 

 sheath, the zone that is absent iii 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 proto- 

 plasm is so stimulated that not only are dominant characteristics strengthened, but 

 also in certain cases latent properties are called into activity, and thus the apparent 

 new type of production appears in the host. The 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 larger in the gall originating from 

 that tissue. This is exemplified in the galls produced by Eurosta solidaginis 

 Fitch (Fig. 2), Aulacidea nabali Brodie, and numerous other species. 



But glands also occur in certain galls on parts of the host that are normally 

 glandless. Thus they are plentiful in the gall produced by Neolasioptera perfoUata 

 Felt, on Eupatorium perfoliatum L., but are not found in the same location in the 

 normal, but are, however, present at the base of the stem. In E. urticcefolium 

 Eeichard they likewise occur in the transitional region between stem and root, 

 while in E. purpweum L. they are present in the roots, petioles and flowering axes, 

 as well as in the cortex and pith of the stem. In the case of gland production, it 

 is clear that not only have active characteristics of the protoplasm in that direction 

 been stimulated to an activity greater than the normal maximum, but nearly dor- 

 nuant properties have sometimes been aroused into action. 



8 E.S. 



