Wenyon has found experimentally that mosquitoes which had fed on malarial blood and kept at incubator temperatures for a week to allow development of zygotes showed inhibition of development of zygotes when kept at temperatures corresponding to hibernating ones. This treatment did not kill the zygotes but complete development took place when subsequently the mosquitoes were again subjected to incubator temperatures. From this it would seem that the zygotes remain viable during the winter hibernation. This is at variance with Mitzmain’s views who regarded hibernation as destructive to zygotes.
Fig. 13.—Anatomy of the mosquito. No. 6 shows various types of scales.
The mosquito does not seem to suffer from her malarial infection—quite different from the serious affection that filariasis causes in the mosquito.
Epidemiology.—This matter has been considered rather extensively under the historical and etiological discussions.
It may be stated however that the requirements for the spread of malaria are: (1) Men who have sexual forms of the malarial parasite in their peripheral circulation; (2) efficient anopheline hosts, and (3) an atmospheric temperature above 60°F. (16°C.).
Fig. 14.—Anopheles maculipennis (quadrimaculatus), female. (Castellani and Chalmers, after Austen.) From P. H. Reports.
It is a well recognized fact in the tropics that the natives seem to have an immunity to malaria yet may carry parasites in their circulation and serve as carriers. The native children to a striking degree harbour parasites and to them malaria is a prime cause of death. After repeated infections, if not fatal, a temporary immunity is acquired. Many localities in the tropics owe freedom from malaria to an absence of anophelines, as for instance Barbadoes. Again malaria-bearing mosquitoes may acquire the habit of feeding on animal blood other than that of man. It is well recognized that rural populations are more liable to malaria than those of towns and as the population of a country moves to the industrial centres human blood may become difficult to obtain and the anophelines turn to other sources of blood supply. It has been suggested that mosquitoes may suffer from other infections which may be inimical to the development of malarial zygotes (black spores of Ross). Anophelines bite chiefly at sunset and at night from which fact there would seem to be some value in shutting the windows towards nightfall as is the custom in many malarious parts of the world.