FOOD SUPPLY 



43 



changing, or temporarily shifting condition is reached 

 through the struggle for existence among the plants, and the 

 balance of nature maintained by the action of adverse con- 

 ditions in their environment, of which the insect population 

 is one factor. So far as the insects dependent upon plants are 

 concerned, the innovations of agricultural development 

 represent a cataclysm in their environment. When an ex- 



FIG. 19. Map showing the spread of the Mexican Cotton-boll Weevil (Anthonomus grandis), 

 introduced into the United States from Mexico in 1892. After about thirty years it had spread over 

 the most productive cotton growing areas of the United States; since 1913 its range has been 

 still further extended. The limits of the cotton belt are indicated by the heavy line. (After the 

 Bureau of Entomology, U. S. Dept. of Agriculture.) 



tensive area is planted to some particular crop, three alter- 

 natives present themselves to the original insect inhabitants 

 of the area. With their natural food plant eliminated more 

 or less completely, they may become suddenly extinct or 

 nearly so; this will be the fate of the great proportion and it is 

 of practically no human concern that it is so. Others (in very 

 rare cases, but nevertheless important ones) may turn their 

 attention to the newly arrived plant, and find it a satisfactory 

 substitute for their former diet. Still a third class, fortu- 

 nately a very small one, will find the crop plant closely 

 allied to, or even identical with their original source of food, 

 and consequently acceptable to their appetites. 



