IX] MOSQUITOES AND MALARIA 143 



Influences affecting Malaria through the Transmitting Host. 



The most powerful influences affecting Anopheles are tem- 

 perature, rainfall and humidity. Added to these must be 

 considered the physical characters of the country under con- 

 sideration, the nature of the soil, the prevalence of natural 

 enemies, and even the races of mankind inhabiting the countries. 



With regard to physical conditions, generally hot moist 

 climates are suitable, cold and also excessively dry climates 

 unsuitable^ to the development of Anopheles. In this respect, 

 however, the question of species and .even of special adaptive 

 habits becomes important. 



Temperature plays a double role. If low it not only delays 

 or prevents the propagation of Anopheles, but it even more 

 actively interferes with the development of the parasite within 

 the mosquito. A perennial low temperature may altogether 

 prevent the propagation of the parasites, for it is found that the 

 gametocytes cannot develop below a temperature of about 

 15 C. As a result, one can trace northern and southern limits 

 to the occurrence of malaria, corresponding to the mean sum- 

 mer isotherm of 15 to 16 C. Moreover, the different species 

 of malaria require different degrees of warmth in order to ensure 

 their development. Thus Plasmodium malaria develops best 

 at comparatively low temperatures, and consequently is found 

 in much colder regions than the other two species. On the 

 other hand P. falciparum will only develop at comparatively 

 high temperatures, and therefore in temperate regions it only 

 occurs during the summer and autumn months, whilst 

 throughout the tropics it is prevalent the whole year round. 

 Plasmodium vivax occupies a somewhat intermediate position, 

 for it is capable of development throughout a wide range of 

 temperature and, as one would expect, this species is the 

 most widely distributed, occurring in both tropical and tem- 

 perate regions. A perennial low temperature within limits, 

 however, does not prevent the propagation of certain species of 

 Anopheles, e.g. A. bifurcatus reaches a particularly large size 

 in the north of Scotland. Still less does a temporary or 

 seasonal period of low temperature destroy or permanently 



