CHAPTER XIII. 
INSECT GALLS OF THE VICINITY 
OF TORONTO. 
By 
A. COSENS, M.A., Ph.D. 
Tus 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 that 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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