EXERCISE 139 



The spread of various disease germs has been charged against the common house fly. 



Problem. Does the fly actually distribute germs? 



What to use. Sterilized culture dishes ; living flies caught in various localities ; small scissors ; 

 clean hands. 



What to do. After catching the flies with clean hands, snip off the wings. Place one such fly in 

 each culture dish. Save one or more dishes for control. Apply fingers of the clean hands to the 

 gelatin or agar in other dishes. Label, and leave for twenty-four hours or more. 



Record. Note the number and arrangement of colonies in each dish. Describe results, draw your 

 conclusions, and give reasons. 



Questions. 1. How can you tell that any of the growths were due to disease germs? 



2. What is the idea of handling flies with clean hands ? 



3. Why cut off the flies' wings ? 



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