70 ' 



GENERAL ZOOLOGY 





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a few small hooks. The larvae feed for about a week, growing 

 rapidly and molting twice within that time, and then pass 

 into an inactive pupal stage within the larval skin. In a 

 week more the perfect insect appears by making a circu- 

 lar hole in one end of the pupal case by means of a large 

 bladder-like bulb, which swells out on the forehead and is 



later withdrawn in- 

 to the head. Thus 

 within about two 

 weeks the life his- 

 tory is completed. 

 As the imagoes soon 

 lay eggs, there may 

 be several genera- 

 tions in the course of 

 the summer. With 

 the approach of cold 

 weather most of the 

 flies die, although 

 some hibernate in 

 sheltered places. 

 Someone has made 

 the calculation that if all the offspring of a single pair of 

 flies starting to reproduce in April were to live, by August 

 there would be from this one original pair enough to cover 

 the entire earth with a layer of flies forty-seven feet deep. 

 The house fly has always been considered a nuisance 

 about the house; but a more serious charge is laid at its 

 door, — that of transporting on the pulvilli and proboscis 

 the germs of typhoid fever. With the knowledge of its 

 favorite breeding place it should be possible, by insisting 

 on cleanliness in stables, by the daily collection of manure 

 and garbage and their proper disposal, to mitigate greatly, 

 if not entirely destroy, this menace to health. 



Fig. 47. Metamorphosis of housefly. (Enlarged; 





