Insects on the Farm 163 



are many substances used to kill insects. They may be 

 grouped into three classes, according to the manner in 

 which they poison the insect. 



(a) Food, or Internal Poisons, are substances which 

 poison by being taken into the digestive tract of the 

 insect. This class includes various arsenical compounds, 

 such as Paris green, London purple, lead arsenate. 

 Poisons of this class are used for insects that chew their 

 food, as the leaf-eating forms, unless the use of the poi- 

 son render the plants dangerous for food, such as 

 cabbage. 



(b) Contact Poison. Substances that destroy by 

 attacking the body of the insect, such as washes of 

 caustic alkalies, oils, etc. They are used for sucking 

 insects, i. e., those having beaks, such as the San Jos6 

 scale. 



(c) Tracheal Poisons. Substances which enter the 

 breathing pores of the insect and cause death by poison- 

 ing or suffocation. Smoke, and the deadly hydrocyanic 

 acid gas, Pyrethrum, or "insect powder," and carbon 

 bisulphide, belong to this class. 



