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NATURAL HISTORY, TORONTO REGION

 

leaf of the host, and often accompanied in the former by an abundant production of trichomes. 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, but in others are as complicated as any of the highest types of galls. In the order Hymenoptera are two families, Cynipidae and Tenthredinidae (Sawflies), the members of which pro-duce 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. 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. Bounding this layer on the outside is situated the protective 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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