40 The Control of Mosquitoes 



six or eight feet high, and the pools formed 

 were hidden. 



If inadequate provision is made for taking care 

 of natural watercourses blocked by uninterested 

 engineers, acres upon acres of new swamp lands 

 may be formed, which are often ideal breed- 

 ing grounds for mosquitoes. They contain few 

 aquatic enemies of mosquito larvee, plenty of 

 food supply, and the tangle of vegetable growth 

 protects the eggs and furnishes hiding-places for 

 larvae and pupae. Such places are ideal for mos- 

 quitoes as soon as made, and outrival the freshly 

 dug excavation, which is not frequently used by the 

 Anopheles for ovipositing until conditions, such as 

 protection for the young, food supply, and vege- 

 table matter, tempt their presence. 



It can be readily understood that if the area at 

 the rear of the new dump or blocked portion of the 

 drainage channel is flat or gently sloping toward 

 the dam, a much larger area containing vegetation 

 wiU become flooded. If the final water surface 

 does not rise above the vegetation, the mosquitoes 

 are better pleased. In many cases, nature lends a 

 helping hand, and the vegetation under the water 

 grows rapidly until it reaches and extends beyond 

 the surface. Under these conditions, aquatic 

 plants develop and fish find the larvae with diffi- 



