154 THE ECONOMIC IMPORTANCE OF ANIMALS 



pupae differ from the larvae? Watch to see the adults emerge 

 from the pupal case. How long does a complete life history take ? 

 Conclusion. — How many generations of flies might develop 

 during a hot summer? 



Problem 129 : To determine the harm done by the fly and 

 the way it does this harm. 



Material. — Sterile Petri dish containing culture medium. 



Method. — Allow a fly to walk over the surface of a sterile 

 Petri dish with culture medium within it. Cover the dish. After 

 three or four days examine the surface of the culture medium. 



Observations. — What do you see? 



NOTE. — Flies breed in manure, filth of all kinds, and human excrement as well. 



Study a diagram showing the relation of typhoid fever to open 

 toilets and flies in Jacksonville, Fla. (page 224, Civic Biology). 

 Why were there fewer cases when the toilets were screened ? 



Study the diagram below. What relation exists between 

 diarrheal diseases and flies? Explain. 



Examine the foot of a fly under the compound microscope or 

 study upper figure, page 223, Civic Biology. What adaptations 

 for carrying germs do you find ? 



Infant Mortality Curve. 



a, prevalence of flies; b, diarrheal under five years; c, deaths under one year; 

 d t mean temperature. 



