LIFE HISTORY 
37 
Farther south, however, where the summer is longer, 
and particularly where the climate is moist, there may 
be more generations than this. In India, for example, 
where Surgeon Major Smith made his observations 
showing a minimum rate of eight days to a generation 
and where the warm spell is very long, an extraordi¬ 
nary abundance of flies in the autumn, with proper 
conditions of moisture, is a certainty. No wonder 
that the punkah was invented in India! In the same 
way, as one goes north the number of generations per 
year is naturally smaller and the autumnal abundance 
of flies becomes greatly lessened in consequence. 
Forbes’s assistants in Illinois found the life round in 
midsummer to vary from nine to fourteen days. 
Possibilities in the Way of Numbers 
This number of generations has a direct bearing 
upon the number of flies, not only at different periods 
during the summer, but also in the early autumn, since 
there is, barring accidents, a constant and definite and 
enormous increase. Of course some summers are 
warmer than others and some are moister than others, 
and upon these two factors, taken in connection with 
that of available places for breeding, the number of 
flies must depend. 
Take, for example, the possibilities in Washington, 
and let us estimate—on the basis of the survival of all 
eggs and all individual flies—upon plenty of places for 
the insect to develop and for the larvie to feed, upon 
an average of ten days to a generation in midsummer 
