38 The Control of Mosquitoes 



wind. The rainfall tables show the number of days 

 a month on which rain fell. The depressions 

 referred to above take time to dry out, and mud 

 contains sufficient moisture to maintain life in 

 larvae of Anopheles and other mosquitoes. Admit- 

 ting that the Anopheles albimanus readily travels 

 half a mile or more, the numerous small depres- 

 sions hidden by vegetation within a circle of half 

 a mile radius are of importance, especially during 

 very wet periods. 



The climatic conditions of the Isthmus, except 

 during the brief period of a few weeks toward the 

 end of the dry season, are ideal for the propagation 

 and multiplication of Anopheles. It is difficult to 

 conceive of a locality where topographical changes 

 have been made more frequently and rapidly. 

 The cost of anti-malaria work depends largely upon 

 topographical and climatic conditions. 



Many miles of temporary track were frequently 

 changed and often each tie removed left a water- 

 holding depression. The excavation and trans- 

 portation of material excavated was continuous 

 for ten years. As the depth increased, more 

 water seeped into the new channels or benches 

 being cut by the steam shovels, and the former 

 ditch sites were excavated. 



As the bottom of the Canal was lower than the 



