LIFE HISTORY 33 



cussion which followed to hear one of the most emi- 

 nent of America's scientific men (an astronomer, by 

 the way) ask Professor Riley, "It is true, of course, is 

 it not, that the little flies one occasionally sees on the 

 window-pane grow and become the large flies that are 

 so numerous?" 



No fly, after it issues from the puparium,. grows at 

 all ; no insects grow after the last molt ; in fact, insects 

 can grow only by casting their skins, and none of the 

 insects having what is called a perfect metamorphosis 

 casts the skin after reaching the imago or winged 

 stage. 



But some typhoid flies are larger than others, and 

 the explanation is a different one from that of the 

 growth of the winged form. The same thing is seen 

 with other insects, and it results as a rule from the 

 amount of larval food ; certain larvae stinted in their 

 supply of food transform to pupae when small and nat- 

 urally become small adults. There is a distinct con- 

 nection with them, as with human beings, in stint and 

 stunt, aside from the similarity of the words and their 

 origin. 



With the house fly, however, some exact observa- 

 tions have been made on this point by Griffith (1908) 

 and Packard (1874). Griffith found that when the 

 larvae were kept cool and the pupae warm all the flies 

 that came out were small. In fact, he found that it 

 was a rule that cold surroundings, even with plenty 

 of food, produced small flies. And he further states 

 that such small flies are incapable of reproduction. He 



