44 INSECTS AND HUMAN WELFARE 



The proportion of these three classes of insects determine 

 to some degree the extent of the damage which the farmer 

 will find has been inflicted upon his crop by the end of the 

 season. 



In order to understand more fully the relations of these 

 classes, it is necessary to inquire rather closely into the degree 



FIG. 20. The European cabbage butterfly (Pontia rapae) and (be- 

 low) its American congener (Pontia philodice). Both have the same 

 food plants and the native butterfly has become more scarce as the 

 naturalized species has increased in abundance. 



of association exhibited by plant-eating insects to their host 

 plants. A few insects avail themselves of very many plants 

 as food, and such polyphagous forms like the locusts or grass- 

 hoppers, are a constant menace, whose varying abundance 

 depends upon factors not yet mentioned. Numerous other 

 forms which may be called oligophagous, depend upon several, 



