8 



INSECTS AND HUMAN WELFARE 



through the puncture, into the circulation. Here they mul- 

 tiply 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. 



At the present time malaria in its several forms is the veri- 

 table scourge of the tropics. It also extends generally into the 



Fio. 2. Distribution of Malarial Mosquitoes in the United States. 



subtropics and warmer temperate regions and is prevalent 

 over a considerable part of the southern United States. In 

 these areas, as appears from investigations of the U. S. Pub- 

 lic Health Service, its range is roughly coincident with the 

 moist austral zones east of the 100th meridian as defined by 

 Merriam. Aside from this main area, there is a small one in 

 southern New England and another in central California; 

 there are also a few isolated localities scattered through the 

 country where malaria is thought to be endemic (Fig. 1). 

 At least three species of Anopheles are known to act as car- 

 riers of malaria in the United States and, from data given by 

 Howard, Dyar and Knab (Fig. 2) the distribution of these 

 taken together corresponds closely with that of malaria, al- 

 though slightly more extensive, especially along the Atlantic 



