16 



INTRODUCTION. 



But these parasites are not always so successful as 

 this, for they frequently furnish a striking illustra- 

 tion of Dean Swift's oft-quoted couplet : 



The little fleas that do us tease 



Have other fleas that bite 'em, 

 And these in turn have other fleas. 



And so on ad infinitum. 



These parasites are frequently subject to the at- 

 tack of a still smaller para- 

 site which destroys them as 

 they destroyed their host. 

 In such cases the first men- 

 tioned species is called the 

 primary parasite, and the 

 other a secondary parasite. 

 There are also foes of 

 another kind from which 

 injurious insects often suf- 

 fer. These are the germs 

 ij» ■ of contagious diseases, of a 

 &£ n ^HP^ * bacterial or fungous nature- 



The Imported Cabbage 

 Worm, for example, is fre- 

 quently attacked by a bac- 

 terial disease — a sort of in- 

 sect cholera — that destroys it in great numbers. 

 Similar diseases affect the Army Worm, the various 

 cut-worms, and many other insects. Diseases of a 

 sowewhat different nature, due to certain fungi other 

 than bacteria, also attack many insects. For in- 

 stance, the Chinch Bug is frequently destroyed in 



Fig. 6. Chinch Bugs affected by 

 Entoatophora. 



