THE PRINCIPAL INSECTICIDES 17 



great numbers by a fungus that develops on the sur- 

 face of the bug as a dense, white covering. This dis- 

 ease is illustrated at Fig. 6 : a number of dead bugs 

 are shown on a wheat stalk on the left, while a sin- 

 gle bug, much magnified, covered with the fungus, 

 is represented at the right. This fungus belongs to 

 a genus of plants called by botanists Entomophora. 



THE PRIx\CTPAL INSECTICIDES. 



Insecticides, or the substances used for destroying 

 insects, may be broadly divided into two classes : 

 (1) internal poisons, or those which take effect by 

 being eaten along with the ordinary food of the in- 

 sect; and (2), external irritants, or those which act 

 from the outside — closing the breathing pores, or 

 causing death by irritation of the skin. Besides 

 these, however, various other substances are used in 

 preventing insect attack — keeping the pests away 

 because of offensive odors, or acting simply as me- 

 chanical barriers. 



The most important insecticides are the poisons. 

 Of these the most popular are the various combina- 

 tions of arsenic, known as Paris green, London pur- 

 ple, slug-shot, and a large number of patent insecti- 

 cides sold under various names. 



Paris Green is a chemical combination of arsenic 

 and copper, called arsenite of copper. It contains 

 about fifty-five or sixty per cent, of arsenic, and re- 

 tails at about thirty cents per pound. It is practi- 

 cally insoluble in water, and may be applied either 



