376 ELEMENTARY BIOLOGY 



414. Insect galls. The response of plants to special mechani- 

 cal and chemical disturbances is illustrated by the formation of 

 the so-called insect galls, some of which are shown in Fig. 196. 

 Many insects sting plants and suck juices from them for food. 

 The common mosquito is an example, and the plant lice are 

 sometimes parasitic to a degree that is very harmful. But 

 many insects sting plants with their egg-laying organs and 

 deposit the eggs in the tissues of the plant. Associated with 

 the process of egg-laying there is often a secretion of some 

 juice from the insect's body. The mechanical or chemical 

 injury thus produced is probably very slight ; but the young 

 that hatch from the eggs deposited in the tissues of the plant 

 begin to feed, and the injury that they do is likely to be of a 

 more serious nature. We find that many plants begin to grow 

 rapidly about a point at which insects have laid eggs, forming 

 casings of various shapes and structures about the mass of 

 eggs, and eventually about the young insects. Within these 

 galls the insect larvae find a limited amount of food, and they 

 are cut off from the rest of the plant. 



