The method of feeding of the house-fly has an important bearing on the question of its ability to transmit pathogenic organisms. Graham-Smith (1910) has shown that when feeding, flies frequently moisten soluble substances with "vomit" which is regurgitated from the crop. This is, of course, loaded with bacteria from previous food. When not sucked up again these drops of liquid dry, and produce round marks with an opaque center and rim and an intervening less opaque area. Fly-specks, then, consist of both vomit spots and feces. Graham-Smith shows a photograph of a cupboard window where, on an area six inches square, there were counted eleven hundred and two vomit marks and nine fecal deposits. 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 gelatine 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 laboratory 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 garbage 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.