58 The Control of Mosquitoes 



season and haul material over fields for construc- 

 tion purposes, areas of considerable extent are cut 

 up and the drivers seldom use the same route 

 twice; they find hauling easier over untrodden 

 ground, and temporary roadways may thus be 

 widened as much as a hundred feet. Near settle- 

 ments the matter is of svifficient importance to 

 make the filling of these new ruts necessary. It is 

 more economical to fill in than to treat with oil 

 weekly for an indefinite period. 



EFFECT OF LONG RAINY PERIODS AND FLOODS 



Floods occur during the rainy season and are 

 generally preceded and followed by showers; and 

 the rapid currents thus caused removed many if 

 not most of the larvas from the banks of rivers, 

 streams, ditches, and the pools or depressions near 

 them. Unfortunately the overflow reached depres- 

 sions on adjacent lands and caused new temporary 

 breeding places. When the flood was of short 

 duration and was followed by dry weather and 

 strong winds it was even advantageous. The 

 latter condition rarely happened on the Isthmus. 



Continuous rainy periods with daily showers 

 for twelve days or more, whether accompanied 

 by floods or not, are the cause of prolific mosquito 

 production on flat lands. Practically all Canal 



