CHAPTER XIII. 



INSECT GALLS OF THE VICINITY 

 OF TORONTO. 



By 



A. COSENS, M.A., Ph.D. 



THE term " gall " is applied to any enlargement 

 of plant cells, tissues, or organs induced by the stimu- 

 lus of a parasitic organism as a regular incident in 

 the life history of the parasite. 



Galls are divided into two classes according to 

 the agent tha't produces the stimulus, namely Phyto- 

 cecidia, those owing their origin to parasitic plants, 

 and Zoocecidia, those produced by animal parasites. 



From the Bryophytes to the Spermatophytes 

 nearly all plants are subject to gall formations of the 

 latter class. These are incited by mites (Acarina) 

 and by insects in several different orders as follows : 

 Hemiptera (Families Aphididae, Psyllidae), Dip- 

 tera (Families Cecidomyidae, Trypetidae), Cole- 

 optera (Families Buprestidae, Cerambycidae, Cur- 

 culionidae). Lepidoptera (Families Gelechiidae, Se- 

 siidae, Tineidae), Hymenoptera (Families Cynipidae, 

 Tenthredinidae) . 



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 

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