The House-fly as a Carrier of Disease 151 



From a bacteriologist's viewpoint a discussion of the possibility 

 of a fly's carrying bacteria wovild seem superfluous. Any exposed 

 object, animate or inanimate, is contaminated by bacteria and will 

 transfer them if brought into contact with suitable culture media, 

 whether such substance be food, or drink, open wounds, or the sterile 

 culture media of the laboratory. A needle point may convey enough 

 germs to produce disease. Much more readily may the house-fly 

 with its covering of hairs and its sponge-like pulvilli (fig. 109) pick 

 up and transfer bits of filth and other contaminated material. 



For poptdar instruction this inevitable transfer of germs by the 

 house-fly is strikingly demonstrated by the oft copied illustration 

 of the tracks of a fly on a sterile culture plate. Two plates of gela- 

 tine or, better, agar medium are prepared. Over one of these a fly 

 (with wings clipped) is allowed to walk, the other is kept as a check. 

 Both are put aside at room temperature, to be examined after twenty- 

 four to forty-eight hours. At the end of that time, the check plate 

 is as clear as ever, the one which the fly has walked is dotted with 

 colonies of bacteria and fungi. The value in the experiment consists 

 in emphasizing that by this method we merely render visible what is 

 constantly occurring in nature. 



A comparable experiment which we use in our elementary labora- 

 tory work is to take three samples of clean (preferably, sterile) fresh 

 milk in sterile bottles. One of them is plugged with a pledget of 

 cotton, into the second is dropped a fly from the laboratory and into 

 the third is dropped a fly which has been caught feeding upon gar- 

 bage or other filth. After a minute or two the files are removed and 

 the vials plugged as was nttmber one. The three are then set aside 

 at roorn temperature. When examined after twenty-four hours 

 the milk in the first vial is either still sweet or has a ' ' clean' ' sour odor ; 

 that of the remaining two is very different, for it has a putrid odor, 

 which is usually more pronounced in the case of sample number 

 three. 



Several workers have carried out experiments to determine the 

 number of bacteria carried by flies under natural conditions. One 

 of the most extended and best known of these is the series by Esten 

 and Mason (1908). These workers caught flies from various sources 

 in a sterilized net, placed them in a sterile bottle and poured over 

 them a known quantity of sterilized water, in which they were shaken 

 so as to wash the bacteria from their bodies. They found the number 

 of bacteria on a single fly to range from 550 to 6,600,000. Early in 



