394 APPLIED BIOLOGY 



(Fig. 138), which inject the malarial organism ( 274) into 

 the human blood-system. Numerous experiments have 

 made it certain that one of these mosquitoes must first suck 

 blood from a person who has malarial parasites in his blood, 

 and it is now equally certain that a bite from such an infected 

 mosquito is the only way by which malaria can be acquired. 



It is also quite certain that another species of mosquitoes 

 is responsible for transmission of yellow fever, the germ of 

 which is still unknown. 



The discovery that mosquitoes are thus connected with 

 certain dreaded diseases has led to a study of the life-histories 

 and habits of these insects, with a view to reducing the num- 

 ber of cases of malaria and yellow fever. The following 

 rules are now agreed upon by competent entomologists. 



(1) Multiplication of mosquitoes should be checked by 

 destroying their breeding places; for example, by draining 

 swamps, ponds, and other places containing stagnant water. 

 Even a rain-water barrel, open cistern or tank, buckets, empty 

 fruit-cans, cavities in trees and stones, in short, any place 

 where a small quantity of water stands for a few days may 

 serve as a breeding place, producing thousands of mosquitoes. 

 As far as possible all such places should be arranged for per- 

 manent drainage, and others, such as cisterns and barrels, 

 should be tightly covered or screened with netting so as to 

 exclude mosquitoes which are about to lay eggs. 



(2) The larval stages of mosquitoes should be destroyed 

 when it is impracticable to follow the rule above. Many 

 streams, lakes, etc., cannot be drained ; but stocking them 

 with fishes will result in destruction of most of the mosquito 

 larvae. It is important in such cases that the banks of the 

 streams be freed from rubbish and graded so that there will 

 be no small depressions in which mosquitoes may lay their 

 eggs safe from the attacks of fishes. 



Oftentimes temporary relief, pending permanent drainage, 

 may be gained by spraying crude petroleum on the surface of 



