542 THE POPULAR SCIENCE MONTHLY 



that the blood of persons suffering from malaria contains the parasitic 

 organisms, and that these, on being taken into the stomach of the proper 

 kind of mosquito, undergo certain changes and later penetrate the wall 

 of the stomach to form vesicular swellings. Within these they multiply, 

 and finally on the bursting of the nodule are set free in the body cavity 

 and find their way to the salivary glands. After becoming infected, a 

 period of twelve to twenty days are required for these changes in the 

 mosquito. Then for a period of several weeks the virulent organisms 

 remain in the salivary glands and if the mosquito bites a second person 

 the parasites are introduced with the salivary secretion, through the 

 puncture into the circulation. Here they multiply and produce another 

 case of human malaria, which develops from ten days to three weeks 

 after inoculation. 



As previously stated, only certain mosquitoes can transmit malaria, 

 for when the parasites are ingested by other species of mosquitoes they 

 do not continue their development, but die without passing through the 

 stomach into the salivary glands. In the United States only one form, 

 Anopheles maculipennis, is capable of harboring malaria, but in other 

 parts of the world, especially in the tropics, other species of Anopheles 

 and related genera act as hosts for the several kinds of Plasmodium. 



Although in temperate regions the number of deaths from malaria 

 is rather small, in spite of the wide-spread occurrence of the disease, 

 the economic loss is very great, due to the debilitated condition which 

 invariably occurs in the population of malarial districts. In the 

 tropics, however, malaria in its various forms causes an enormous num- 

 ber of deaths and predisposes its victims to so many other dread diseases 

 that it ranks as perhaps the most important human disease. 



Fortunately prophylactic measures against malaria are not difficult, 

 although they have been shamefully neglected in our own country. 

 They consist in the elimination of anopheline mosquitoes, which is best 

 accomplished by the destruction of mosquitoes in general. The larval 

 or preparatory stages of anopheline, and of practically all other mos- 

 quitoes, are passed in the water of small quiet ponds, puddles, exposed 

 vessels containing water, rain-barrels, etc., and it is during this period 

 that they are most easily controlled. This is accomplished by oiling 

 the water with either crude or refined petroleum or with some miscible 

 oil. The petroleum forms a film over the surface of the water through 

 which the larvae can not extrude their breathing tubes and they are thus 

 suffocated. The application of miscible oils is efficacious, but attended 

 with some danger, since it destroys fish and predatory insects which are 

 themselves some of the most important natural enemies of mosquitoes. 

 Very frequently even oiling is not necessary, as much swamp land may 

 be permanently freed from mosquitoes by very simple systems of 

 drainage ditches which prevent the accumulation of the stagnant water 

 in which the larva? occur. 



