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 would 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 popular 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 flies are removed and 
the vials plugged as was number one. The three are then set aside 
at room 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 
