380 THE CANADIAN ENTOMOLOGIST 



INSECT GALLS. 



BY A. COSENS. TORONTO 



(Abstract of lecture, illustrated by lantern slides.) 



In the evolution of the study of galls there are different epochs, 

 each merging gradually into the following. From early historical 

 times these abnormal structures have excited attention. In the first 

 instance, this was in all probability due to the fact that they pre- 

 sented phenomena unusual and out of the ordinary. At this 

 earliest epoch witchcraft and like fanciful explanations were pro- 

 posed to account for their origin. Gradually, as they were better 

 understood and seen to involve a stimulus by a parasite and a 

 response by a host, the examination of them became more scientific, 

 and the hypotheses concering their causes, as a consequence, more 

 valuable. The problem presented was recognized as one of great 

 scientific interest, since it presented the unique feature of a foreign 

 organism stimulating and controlling for its owm benefit the growth 

 of a host. Within the last few years it has been shown that a close 

 relation exists between the structure of the bacterial crown gall 

 and certain malignant animal tumors. Thus the second epoch 

 with the subject of theoretical interest seems gradually to be passing 

 into a third in which it will rank as one of the greatest practical 

 importance. 



The term "gall" is applied to any enlargement of plant cells, 

 tissues, or organs induced by the stimulus 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, Phytocecidia, those owing their 

 origin to parasitic plants and Zoocecidia, those produced by animal 

 parasites. The former are caused by many different classes of 

 plants, myxomycetes, bacteria, algae and fungi. Even the flower- 

 ing plants are represented, among the gall producers, since the 

 witches' brooms and the spherical stem swellings on the black 

 spruce are due to the stimulus of the dwarf mistletoe Arceidho- 

 bium pusillum. The latter are incited by mites {Acarina) and by 

 insects in several different orders as follows: Hemiptera (Families 

 AphididcB and PsyllidcE), Diptera (Families CecidomyidcB and 

 Trypetidce), Coleoptera (Families BuprestidcE, CerambyddcB and 

 CurculionidfB) , Lepidoptera (Families Gelechiidce, Sesiid(e,TineidcB), 



November, 1913 



