2 5 6 



AGRICULTURE 



the insect breathes, which keep out the air and really 

 suffocate it. These are called contact insecticides. Kero- 

 sene emulsion is one of the most common of such sub- 

 stances. (See Appendix.) The second class includes 

 those gases that are either poisonous in themselves or 

 cause the death of the insect by replacing the air and 

 thus causing suffocation. 



EXERCISE. Watch caterpillars feeding on foliage, and mosquitoes 

 and flies sucking blood or sweets, and describe what you see them do. 



FIG. 167. LEAF-EATING CATERPILLARS AT WORK 



