HOW INSECTS INCREASE AND KEPT IN CHECK. 245 



destroyed by birds, caterpillars, flies and parasitic 

 fungi. 



Then the survivors of all this destruction may 

 not be able to find sufficient food to satisfy 

 their enormous appetites, and this again thins 

 them down. 



You therefore can readily understand why the 

 cabbage-white must leave an enormous number 

 of descendants, if it is not to be quite stamped 

 out of existence. 



You will find all through Nature that the 

 greater the risks anything is exposed to the 

 greater its power of reproduction. 



It is the large area of cultivated crops that gives 

 a ready and abundant supply of insect food. 



Weeds don't grow together as crops do, there- 

 fore when the weeds or small masses of weeds 

 are eaten, the caterpillar must die. Whereas 

 w r hen one portion of a crop is eaten, the cater- 

 pillar easily passes on to another portion of the 

 crop. 



The worst insects are those like the silver- 

 Y-moth and cockchafer, which are all-round 

 feeders, and will devour any crop or part of any 

 crop that you place in the ground. 



But there are others, arid these perhaps the 

 majority, that will only feed on a particular 

 crop, or one closely related to it. 



