NATURAL HISTOKY, TOEONTO EEGION 



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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