108 THE HOUSE FLY— DISEASE CARRIER 



"From the above table the bacterial population of 

 414 flies is pretty well represented. The domestic fly 

 is passing from a disgusting nuisance and troublesome 

 pest to a reputation of being a dangerous enemy to hu- 

 man health. A species of mosquito has been demon- 

 strated to be the cause of the spread of malaria. An- 

 other kind of mosquito is the cause of yellow fever, 

 and now the house fly is considered an agency in the 

 distribution of typhoid fever, summer complaint, chol- 

 era infantum, etc. 



*'The numbers of bacteria on a single fly may range 

 all the way from 550 to 6,600,000. Early in the fly sea- 

 son the numbers of bacteria on flies are comparatively 

 small, while later the numbers are comparatively very 

 large. The place where flies live also determines 

 largely the numbers that they carry. The average for 

 the 414 flies was about 1,250,000 bacteria on each. It 

 hardly seems possible for so small a bit of life to carry 

 so large a number of organisms. The method of the 

 experiment was to catch the flies from the several 

 sources by means of a sterile fly net, introduce them into 

 a sterile lx)ttle, and pour into the bottle a known quan- 

 tity of sterilized water, then shake the bottle to wash 

 the bacteria from their bodies, to stimulate the number 

 of organisms that would come from a fly falling into a 

 lot of milk. 



'*In experiments 'd,' 'e,' and 'f the bacteria were 

 analyzed into four groups. The objectionable class, 

 coli-cero genes type, was two and one-half times as 

 abundant as the favorable acid type. If these flies 



