12 INSECTS AND HUMAN WELFARE 



Like other insect-borne diseases, malaria shows in its 

 seasonal prevalence a close relation to the seasons (Fig. 4), 

 undergoing a period of quiescence during the winter and at- 

 taining a sudden maximum in late summer, after which it 

 rapidly declines. This is in response to the increasing abun- 

 dance of Anopheles during the summer, coupled with a 



JAN FEB MCH APL MAY JUN JUL AUG SEP OCT HOV DEC 



Fia. 4. Seasonal Prevalence of Malaria in the United States. 



relatively high temperature, favorable for activity on the 

 part of the mosquito and for the development of the malarial 

 parasites in its body. It has recently been clearly shown by 

 Mitzmain that under the conditions of temperature prevail- 

 ing in the United States the malarial parasites do not persist 

 through the winter in hibernating mosquitoes, but winter 

 over in the human host from whence the Anopheles secure 

 them the following season. 



The larval or preparatory stages of anopheline, and of 

 practically all other mosquitoes, 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 



