350 Microbes and You 



carriers of microorganisms— both pathogens and non-pathogens. 

 The first serious attention paid to flies was in 1898 during the 

 Spanish-American war, when it was discovered that lime sprinkled 

 on open pit latrines was finding its way back to the kitchens in 

 which food was being prepared for the troops. The white tracks 

 left by the flies were identified as lime and indicated that these 

 insects were capable of transferring fecal matter and possibly 

 enteric pathogens from open latrines to food located at some dis- 

 tance from the latrines. 



Intestinal diseases have always presented serious problems 

 whenever large numbers of persons were obliged to live under 

 crowded conditions with inadequate sanitary facilities. As a direct 

 outcome of the studies conducted during the Spanish-American 

 war, better methods for the disposal of human wastes have re- 

 sulted, and increased attention has been focused on flies as vectors 

 in disease transmission. 



The common housefly, Miisca domestica, has no biting or suck- 

 ing mouth parts, and its food must therefore be lapped up. Its 

 nutritional requirements include proteins for growth and for 

 production of eggs, and sweets are needed for energy. Flies 

 dissolve sugar with their own saliva, and when the sugar is in 

 solution, the insects ingest the sweetened liquid. 



Examination of a common fly with a simple hand lens will 

 reveal the hairy nature which permits these insects to carry such 

 large numbers of organisms on the outside of their bodies. The 

 use of the low power of a microscope will make this point still 

 more apparent. The filthy living habits of a fly, which prefers 

 manure as a place in which to lay as many as one thousand eggs 

 during its lifetime, points out the potential danger that lurks when 

 flies gain access to uncovered food, especially food that is allowed 

 to ctand at room temperature for long periods of time. The few 

 hundred or few thousand bacteria deposited on the food by a single 

 fly can, if the medium is favorable, increase to millions or billions of 

 organisms when the temperature is favorable for their development. 



In spite of the usual statements found in textbooks that flies 

 are filthy insects and can spread disease, relatively little carefully 



