234 THE MOSQUITOES OF NEW JERSEY 



pools but maintains them over a period sufficiently long to permit the 

 completion of brood developments. 



The persistence of temporary breeding pools over sufficiently long 

 periods to permit completion of the mosquito life cycle is dependent not 

 alone upon rainfall, but also upon rate of evaporation. The two factors 

 — rainfall and evaporation — so completely underlie favorable or un- 

 favorable mosquito-breeding conditions that we can say that when the 

 weekly rainfall curve rises high above or falls below or scarcely above 

 the weekly evaporation curve the season will show many mosquitoes or 

 few mosquitoes. This conclusion applies to natural waters on the up- 

 land and only to a less extent on the salt marsh. It does not apply to 

 sewage-charged permanent waters or diked and drained salt marsh. The 

 seasonal distribution of relative rainfall and evaporation is an impor- 

 tant factor. 



Newly emerged mosquitoes, as has been noted in the last chapter, get 

 on the wing beginning in May and continue well into September, a pe- 

 riod of about five months. For the sake of studying rainfall effects it is 

 best to divide this period in three parts, each including fifty-one days. 

 The forepart of the mosquito season would thus run from May 1 to 

 June 20, the middle from June 21 to August 9, and the latter from 

 August 10 to September 29. The period required for mosquito develop- 

 ment in midsummer is covered by a ten-day period. It has been found 

 best to aggregate the rainfall and evaporation data in ten-day periods 

 and for comparison purposes to strike curves on that basis. 



When the evaporation curve is as high or higher than the rainfall 

 curve, the period covered by this condition is said to be dry. When, 

 however, the evaporation curve is much lower than the rainfall curve 

 the period is said to be wet : 



TABLE II 



