156 The Control of Mosquitoes 



of rivers. Hillside streams from distant hills 

 joined the quieter water near the camp. Larvse 

 and pupae were frequently carried by storm water 

 toward the settlement and remained to develop 

 in the quieter water in its vicinity. A rainstorm 

 thus produced a sudden influx of mosquitoes at 

 the settlement. This was prevented by installing 

 oil drips on all streams flowing toward settlements. 

 The drip may be used on temporary lagoons, as 

 an auxiliary to intermittent treatment. In the 

 rainy season in Panama lagoons are formed in 

 certain depressions from which drainage is difficult. 

 These lagoons rise and fall several feet in a few 

 days. Each fall of the water surface causes the 

 deposit of a large quantity of oil upon the banks, 

 where it is soon absorbed by the soil. After every 

 fall the oil film on the lagoon may have to be 

 restored by reapplication. This is, of course, a 

 great waste of labor and material. In many 

 instances labor and oil can be saved in such places 

 by introducing a few crude rafts carrying drips 

 and moored at suitable intervals in such a way 

 that they will readily rise and fall with the move- 

 ment of the water, and yet remain in about the 

 same position on the lagoon. The drips auto- 

 matically maintain the desired oil film, and are 

 replenished from time to time from a punt or boat. 



