INSECTS IN RELATION TO HUMAN WEALTH 567 



The botflies are repre- 

 sentative of a large group 

 of insects that are often 

 injurious to horses and 

 cattle (see Fig. 239). 



The ox warble lays its 

 eggs on the cow. It is 

 not certain whether the 

 larvae work their way 

 through the skin or from 

 the alimentary canal. 

 They finally lodge under 

 the skin and thus ruin 

 millions of dollars' worth 

 of hides, besides making 

 the animals sick and re- 

 ducing their milk and 

 beef values. 



408. Fighting insects. 

 One of the first sugges- 

 tions that insects could 

 be controlled by encour- 

 aging other insects was 

 made about a hundred 

 years ago by two English 

 entomologists, who de- 

 clared that an increase in 

 the number of ladybirds 

 in greenhouses and fields 



Fig. 236. 

 dispar). 



The gypsy moth {Porthetria 

 (a, b, c, slightly enlarged ; 

 d, slightly reduced) 



This animal was introduced into this country 

 about 1869, in the course of some experiments 

 made to find a substitute for the silk moth, and 

 in twenty years it became so great a nuisance 

 that the legislature of Massachusetts made an (ap- 

 propriation for the study of methods to be used 

 in checking the insect. In ten years over a mil- 

 lion dollars was spent in the fight, but further 

 work was stopped by some of the legislators 

 whose regions had not been aft'ected. The insects 

 then multiplied to such an alarming extent that 

 in 1906 about a quarter of a million dollars was 

 again spent in the fight, a, male adult ; b, female ; 

 c, larva; d, pupa 



would clean out the 

 aphids, or plant lice, and 

 insure the hops against 



destruction (see Fig. 240). These predacious beetles do actu- 

 ally devour vast numbers of soft-bodied scale insects and plant 

 lice, which in turn live upon the juices of plants, sometimes 

 causing the destruction of an entire crop. 



