Deformities Caused by Insects 



T. C. ALLEN 



THE hexapods gradually became adapted to a wide range of food 

 hosts. They can thrive on inorganic as well as organic matter, but 

 the greater proportion of them, however, feed upon plant life. In their 

 habit of feeding, insects may distuib the physiology of the plant by 

 causing a direct loss of tissue or its cell constituents by inoculating a 

 transmissible toxic pathogen or by causing a pathological condition 

 resulting from their feeding which is usually accompanied by toxic 

 salivary secretions. 



The latter group of insect feeders have been referred to as toxico- 

 genic insects, or those insects which cause a pathological disturbance of 

 tissue not ascribable to mere mechanical injury nor fulfilling the criteria 

 necessary to establish the presence of some microorganism. The greatest 

 number of toxicogenic insects belong to the orders Hemiptera and 

 Homoptera. 



Toxicogenic insects cause distinct deformative effects, and their ca- 

 pacity to produce such effects is often inherent in a particular species. 

 Most striking in this respect are the gall-formers. With them the 

 deformation of tissue is so specific that the species of insect concerned 

 can be identified by the type of gall formed. 



It is the opinion of many workers that gall formation is caused by the 

 introduction of some insect-produced toxic substance. Darwin and 

 writers before him frequently referred to chemical secretions injected 

 by the "gall mother." Early thinking in regard to the possible causes 

 of insect gall formation appeared long before our present knowledge of 

 auxin behavior. During feeding it is possible that an insect could inject 

 or withdraw active substances which would tend to increase or decrease 

 the activity of plant responses to hormones. 



