490 PROTOZOOLOGY 



toes placed in genus Anopheles, while avian species of Plasmodium 

 are transmitted by those which belong to genera Culex, Aedes, and 

 Theobaldia. The chief vectors of the human malarial parasites in 

 North America are A. quadrimaculatus (eastern, southern and mid- 

 dle-western States), A. punctipennis (widely distributed), A. cru- 

 cians (southern and south-eastern coastal area), A. walkeri (eastern 

 area), and A macM/^pewn^s (Pacific coast). Boyd and coworkers ob- 

 served that (1) A. quadrimaculatus and A. punctipennis were about 

 equally susceptible to Plasmodium vivax; (2) A. qaudrimaculatus 

 was susceptible to several strains of P. falciparum, while A . puncti- 

 pennis varied from highly susceptible to refractory to the same 

 strains ; (3) A. quadrimaculatus was more susceptible to all three species 

 of Plasmodium than coastal or inland A. crucians. Thus A. quadri- 

 maculatus is the most dangerous malaria vector in the United States 

 as it shows high susceptibility to all human Plasmodium, it 

 possesses house-frequenting habits and furthermore it prefers human 

 blood to those of animals. A. pseud op unctipennis distributed from 

 southwestern United States to Argentina, A. albimanus occurring 

 in Central America, and A. gambiae, an African species, which sud- 

 denly appeared some ten years ago in the vicinity of Natal, Brazil, 

 and which now seems to be under control, are but one or two out of 

 many anopheline vectors of human Plasmodium in the areas indi- 

 cated. 



The time required for completion of sexual reproduction of Plas- 

 modium in mosquitoes varies according to various conditions such 

 as species and strain differences in both Plasmodium and Anopheles, 

 temperature, etc. Boyd and co-workers showed that when the ano- 

 phelines which fed on patients infected by P. vivax were allowed to 

 feed on other persons, their infectivity was as follows: 1-10 days 

 after infective feeding, 87.2%; 11-20 days, 93.8%; 21-30 days, 78%; 

 31-40 days, 66%; 41-50 days, 20%; and over 50 days, none. In a 

 similar experiment with P. falciparum, during the first 10 days the 

 infection rate was 84%, but thereafter the infectivity rapidly dim- 

 inished until there was no infection after 40 days. It is generally 

 known that the development of the parasites in mosquitoes depends 

 a great deal on temperature. Although the organisms may survive 

 freezing temperature in mosquitoes (Coggeshall), sporozoite-for- 

 mation is said not to take place at temperatures below 16° C. or 

 above 35° C. (James). According to Stratman-Thomas (1940), the 

 development of Plasmodium vivax in Anopheles quadrimaculatus is 

 completed within the temperature range of 15-17° to 30° C. It 

 varies from 8 to 38 days after infective feeding. The optimum tem- 



