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 
