INSECTS AND DISEASES 407 



these figures show the enormous possibilities of these insects 

 for doing harm whenever injurious bacteria are exposed 

 within their reach. 



DEMONSTRATION 32 



Purpose. To show that flies carry bacteria. 



Materials. Two petri dishes, two tubes of culture medium. 



Directions. Melt some culture medium and pour into a 

 sterilized petri dish. Cover. When the medium hardens, 

 place a fly under the cover and allow it to walk on the medium. 

 Then let it escape and cover the dish. Prepare another dish 

 of culture medium and cover, but do not allow a fly to walk 

 on it. Allow the dishes to stand side by side for several days 

 and notice any differences. 



3. Some of the bacteria present on flies are the ones which 

 cause diseases. At various times the bacteria which cause 

 typhoid fever, tuberculosis, bubonic plague, and cholera 

 have been found either on flies or in fly specks. 



4. It is a fact only too well known that flies are abundant 

 in houses and stores and that they frequently alight on food. 



5. Investigations have been made which show a relation 

 between the number of flies and the number of deaths caused 

 by diseases which may be carried by flies. During the seasons 

 of 1907 and 1908 a committee in New York City made a 

 special study of the conditions found in that city with ref- 

 erence to typhoid fever and other intestinal diseases. To 

 obtain some idea of the relative abundance of flies at different 

 times of the year, fly traps were set in various parts of the 

 city, and the number of flies caught each day was counted. 

 Statistics were gathered regarding the number of deaths 

 due to the diseases mentioned above. On comparing the 

 records of the number of flies caught with these statistics 

 (taking into account the time necessary for the diseases to 

 develop), it was found that there were the most deaths during 

 that part of the year when flies were present in largest num- 



