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THE POPULAR SCIENCE MONTHLY 



There are numerous other flies and insects that serve as carriers, 

 and there are numerous diseases due to more or less uncleanly habits, 

 that are carried by insects ; but the process is so simple and direct that 

 it offers little of scientific interest. 



Much more complicated is that transmission of disease in which 

 the insects act as intermediate hosts, and here again the members of 

 the order Diptera lead the way. "We assume that man is highest in 

 the scale of vertebrate development, and it is no part of my thesis to 

 dispute this. It is equally assumed that the Diptera are most highly 

 specialized in their development among the insects, and it is significant 

 that the association of humanity with these flies has been so close and 



Fig. 5. Resting position of Culex — at left ; of Anopheles — at right ; after Howard, 



U. S. Department of Agriculture. 



so long continued that it has been possible for a common parasite to 

 develop whose continuance depends absolutely upon the intimate asso- 

 ciation of the two. 



If all insects were eliminated absolutely, typhoid, cholera and asso- 

 ciated diseases might still continue to plague mankind; but eliminate 

 Stegomyia calopus and yellow fever would be equally eliminated, because 

 without that particular species the parasite causing the disease would 

 find it impossible to continue its existence. Not that we know very 

 much about the " germ " of yellow fever, and its life-cycle is largely 

 guess-work ; but we do know enough, from direct observation and ex- 

 periment, to warrant the statements that I have just made. 



It is different in the case of malerial fevers and their association 

 with certain species of Anopheles. Here we do know from direct 



