THE ARTHROPODS 393 



that efforts must be made to hold in check the ones which 

 are noticeably harmful. The ordinary insects which one 

 meets during a long walk in the country are not likely to 

 appear in such numbers or to develop such new habits as to 

 be of special economic interest. Hence there is no reason 

 why we should destroy them. On the contrary, this old 

 world is of greater interest because of their existence. As an 

 illustration, katydids and crickets do eat some leaves of 

 grass and other plants, but to many a person who is interested 

 in nature-study a mid-summer night's chorus by these insects 

 is worth far more than the trivial damage they do. 



The Control of Injurious Insects. The investigations by 

 entomologists in the past fifty years have made great prog- 

 ress towards controlling injurious insects. Knowledge of 

 habits and life-histories have made it possible to prevent 

 destruction of crops. Here are a few from hundreds of ex- 

 amples recorded in the large works on economic entomology : 

 The discovery that the fruit-moth (codling moth) lays its 

 eggs in the calyx of apple flowers after the petals have fallen 

 suggested the desirability of spraying the trees with arsenical 

 poisons before the Iarva3 hatch and burrow into the fruit. 

 Grasshoppers lay their eggs a few inches below the surface 

 of the soil, and hence shallow plowing in the autumn will ex- 

 pose the eggs to the winter storms. The scale-bugs and plant- 

 lice live by sucking sap from plants, and hence poisons like 

 arsenicals could not reach their stomachs, and the logical 

 conclusion is that they ought to be sprayed with lime, sul- 

 phur, or petroleum, which kills by contact. These cases are 

 simply illustrations of the fact that all the satisfactory meth- 

 ods of dealing with injurious insects are based upon careful 

 biological study of the species in question. 



329. Mosquitoes and Diseases. Careful investigations 

 made in recent years have proved that certain insects are 

 responsible for transmission of disease germs. The most 

 famous case is that of mosquitoes of the genus Anopheles 



