144 The Control of Mosquitoes 



In wide ditches the stone or concrete floor lining 

 should slope toward the center. 



MAINTENANCE OF DITCHES 



Maintenance of a ditch impHes keeping it in 

 a condition to carry off water and not produce 

 mosquitoes. The work in Panama consisted of 

 keeping the bottom to proper grade, the cross 

 section of uniform width, the removal of all ob- 

 structions that affected the velocity of the water, 

 and also vegetable growth and algae that furnished 

 food and protection for the larvae. It included 

 treating the ditch with oil and other forms of 

 larvacide when necessary to destroy larvae, and 

 periodical inspection to make sure that the ditch 

 was in satisfactory condition and free from mos- 

 quito larvae. 



In comparing an open earth ditch with a con- 

 crete lined one, we found the following advantages 

 in favor of the lined ditch : The velocity of water is 

 increased to such an extent that mosquito larvae 

 cannot live in it but are washed away or destroyed. 

 The increased velocity prevents the deposit of silt, 

 etc. Each shower cleans the ditch out, and gen- 

 erally removes any debris that has collected. The 

 cross section of the ditch remains uniform. No 

 grass or other vegetation clogs the ditch. Algae 



