hegner] GALL INSECTS AND INSECT GALLS 211 



guillula) and fungi are also responsible for the development of 

 structures called galls. 



The Causes of Gall Formation 



One of the most difficult problems connected with the study 

 of insect galls concerns the causes of gall formation. What 

 stimulus causes the plant cells to increase in number and to form 

 a certain sort of gall for each species of insect? Some of the 

 suggestions that have been offered fall under the following cate- 

 gories : 



i. The stimulus is mechanical. 



2. The stimulus is chemical. 



3. The parent provides the stimulus. 



4. The offspring provides the stimulus. 



5. Combinations of the above. 



Many writers have been inclined to think that a poison is 

 introduced into the plant tissue by the insect when the egg is 

 deposited and that this poison stimulates growth. This may be 

 true for the galls that are made by certain species, especially by 

 the saw flies, for it has been observed by several that these galls 

 develop before the larva hatches from the egg. Many of the 

 Cynipida? have been observed to secrete a liquid at the time of 

 depositing the egg, but this does not seem to stimulate gall for- 

 mations because there is no increase in the number of plant 

 cells until the larva appears. 



If the stimulus is not a chemical one provided by the parent, 

 then perhaps it may result from the mechanical injury resulting 

 from the egg laying process. Here again the facts point to the 

 conclusion that it is not the parent but the offspring that initiates 

 the development of the gall. 



When the larva hatches from the egg it begins to feed upon the 

 plant tissues, a process that is accompanied by either mechanical 

 or chemical activities or by both of these. The mechanical irri- 

 tation set up by the larva may be the cause of gall formation, but 

 in at least some thoroughly studied cases chemical substances 

 produced by the larva provide the stimulus that causes the 

 rapid increase in the number of plant cells. Triggerson, for 

 example, concludes after careful observations and experiments 

 of the oak hedgehog gall that a secretion from the excretory 



