62 INSECTS AND HUMAN WELFARE 



to a temperature that will destroy insect life, or fumigation by 

 some insecticide such as carbon bisulphide, carbon tetra- 

 chloride or hydrocyanic acid gas, after which they may be 

 marketed in sealed containers. In spite of the ease with 

 which these insects can be controlled, they entail an immense 

 economic loss which is probably in the neighborhood of $20,- 

 000,000 annually in the United States. Their origin as 

 enemies of stored products which are maintained under 

 purely' unnatural conditions, may at first sight seem a trifle 

 obscure, but they all undoubtedly represent species originally 

 enjoying very limited opportunities for multiplication, and 

 subject to great vicissitudes. Like agricultural pests they 

 now flourish with the barriers removed. 



