Biology and Medicine 455 



required in the drainage of the extensive marsh areas. The 

 results were most gratifying. The Anopheles mosquitoes were 

 virtually exterminated, with a consequent reduction in the 

 number of malaria cases from thirty-three in 1905 to five in 

 1909, 7 while other mosquitoes also have been greatly reduced 

 in number. 



But insects play not only a necessary role in the spread of 

 disease; the incidental part which they take is on the whole 

 far more dangerous than the other. In its discovery of this 

 part biology has made one of its greatest contributions to 

 human welfare. The trail of the fly has been followed so 

 often and with such care in the literature of recent years 

 that it seems superfluous to repeat what is so well known. 

 And yet a few of the more striking facts concerning the 

 relation of flies and other insects to the spread of disease, and 

 especially the results of preventive measures may not be 

 amiss ; the more so since we still find in some quarters among 

 supposedly educated and intelligent people an almost total 

 disregard of the most common and fundamental laws of self- 

 defense against disease. 



While we are all sadly familiar with the reproductive 

 capacity of the fly, probably few of us realize the theoretical 

 possibilities of such increase theoretical, because, owing to 

 the inevitable loss of eggs and young, the possibilities are 

 never realized. They are interesting and instructive however 

 for if such possibilities exist theoretically, the realization must 

 at least be very great. A fly lays on the average 120 eggs 

 at one time, which come to maturity in ten days, and in the 

 latitude of Washington, D. C., there may be as many as 

 twelve broods in a season. If every other egg of every brood 

 gave rise to a fertile female (assuming an equality in the 

 number of males and females) and this in turn produced 

 broods of its own in due season, one mother fly would produce 

 2,568,034,296,513,029,664,000 8 flies, which if strung end to end 

 on a thread would reach some 400,000,000,000 times around 

 the earth. 



The fly's body is clothed with fine hairs and a single 

 fly has been estimated in some cases to carry more than 

 6,000,000 bacteria. 



In 1898 there were concentrated in army camps in the 

 South thousands of men gathered there for our campaign 

 against the Spaniards in Cuba. These men ate in unscreened 

 mess halls near which were the similarly unprotected latrines 

 of the camp, affording free passage for the millions of flies 



'Prior to 1905 there are no satisfactory data on the number of cases. 

 8 Computed by E. F. Chandler, Professor of Civil Engineering, Uni- 

 versity of North Dakota. 



