NATURAL HISTORY, TORONTO REGION 
leaf of the host, and often accompanied in the former 
by an abundant production of trichomes. The Cole- 
optera 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, but in others are as complicated 
as any of the highest types of galls. In the order 
Hymenoptera are two families, Cynipidae and Ten- 
thredinidae (Sawflies), the members of which pro- 
duce galls that are in marked contrast to each other. 
The Sawfly galls are characterized by a very pro- 
nounced proliferation of tissue without differentia- 
tion into distinct layers except at the very earliest 
stages of gall production. The Cynipid galls, by 
way of contrast, have invariably three distinct zones 
of tissues, 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. Bound- 
ing this layer on the outside is situated the protec- 
tive sheath, the zone that is absent in a few types. 
Outside of that again the parenchyma or tannin zone 
is differentiated, passing out to the epidermal layer. 
With respect to the stimulus that incites the 
abnormal production of tissues these galls are also 
markedly different. Sections of the Sawfly galls 
show the egg of the producer still unhatched even 
after gall production has advanced to a considerable 
degree, a very plausible argument that the stimulus 
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