MOSQUITOES AND FLIES 



there are thousands of species of flies that do not affect us 

 in any injurious way; while, furthermore, there are 

 species, and many of them, that render us a positive serv- 

 ice by the fact that their larvae live as parasites in the 

 bodies of other injurious insects and bring about the de- 

 struction of large numbers of the latter. 



Scientifically, the Diptera are most interesting insects, 

 because they illustrate more abundantly than do the 

 members of any other order the steps by which nature has 

 achieved evolution in animal forms. An entomologist 

 would say that the Diptera are highly specialized insects; 

 and as evidence of this statement he would point out that 

 the flies have developed the mechanical possibilities of 

 the common insect mechanism to the highest general level 

 of efficiency attained by any insect and that they have 

 carried out many lines of special modification, giving a 

 great variety of new uses for structures originally limited 

 to one mode of action. But when we say that any animal 

 has developed to this or that point of perfection, we do not 

 mean just what we say, for the creature itself has been 

 the passive subject of influences working upon it or within 

 it. A fundamental study of biology in the future will 

 consist of an attempt to discover the forces that bring 

 about evolution in living things. 



[353] 



