, 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 rnade 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 frorr 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 larvae to feed, upon 

 an average of ten days to a generation in midsummer 



