CHAPTER xxi. 



BENEFICIAL INSECTS. 



N classing our insect life according to its food, we pointed out that 

 whilst a very great number of insects feed directly upon vegetable life, 

 a further large number derive their food from 

 other insects. The latter we may term 

 beneficial insects, since they are the greatest 

 natural check upon the increase of the des- 

 tructive insects which attack growing crops. 

 Beneficial insects play a very important part 

 in the economy of nature. Such is the rate of 

 increase of the majority of herbivorous insects 

 that they would, if able to breed continuously, 

 overrun the earth and devour all vegetable 

 matter. To this enormous increase is opposed 

 a variety of conditions and forces, which 

 result in regulating the increase of all classes 

 of life. Climatic changes periodically put a 

 check to the increase of animal life; the 

 absence of food is a check, which is however 

 largely inoperative under the conditions of 

 modern agriculture ; when these checks fail, 

 the action of the beneficial insects becomes 

 apparent, and we see one class of insects living at the expense of another 



and proving itself 

 directly beneficial to 

 man. These insects 

 can be regarded as 

 forming three large 

 divisions : the para- 

 sites are those which 

 live as larvae with- 

 in the bodies of 

 their hosts ; the 

 stinging predators 

 are those which store 



PIG. 321. 



An Ichneumon parasitic on 



large caterpillars. 



(Natural size.) 



FIG. 322. 



The parasite of the Moth-borer Caterpillar of Cane. 

 (Magnified.) 



insects for the 



