144 MOSQUITOES AND MALARIA [CH. 



affect the prevalence of Anopheles, such periods being tided 

 over by " hibernation." It is thus easy to see how, as regards 

 the northern and southern limits of malaria, we may have 

 these fairly corresponding with the limits of the distribution 

 of Anopheles, and to understand how, even apart from any 

 other reason, a considerable altitude may be associated with 

 a freedom from malaria, even though Anopheles are still found. 



Rainfall, up to a certain point, is very favourable to Ano- 

 pheles, but if excessive it may have an adverse effect by washing 

 away the larvae and destroying them. Humidity would seem 

 to be entirely a favourable circumstance, and has been shewn 

 to affect favourably the development of the malarial parasite 

 within the mosquito. 



The nature of the soil may affect conditions as regards 

 Anopheles, not only by its power to retain surface water or to 

 allow of the formation of springs, but often by reason of its 

 chemical nature ; surface waters in some soils readily develop 

 a ferruginous scum or other conditions, unsuitable to the 

 development of mosquitoes. 



The occurrence of natural enemies, especially circumstances 

 which favour the general presence in surface waters of small 

 fish, is a most powerful influence checking the multiplication 

 of A nopheles . Under conditions of alternate rainfall and drought 

 many pools utilized by Anopheles are of too temporary a nature 

 for the presence of fish, or even of other natural enemies, such 

 as various predaceous insect larvae. On the contrary, in very 

 moist countries, even the smaller pools are usually stocked 

 with fish and other predaceous animals. There may thus often 

 be a very fortunate regulating mechanism which may com- 

 pletely reverse the normal order of affairs and make Anopheles 

 comparatively scarce under conditions that at first sight might 

 appear wholly favourable. 



Besides being open to attack by fish and various predaceous 

 aquatic insects, Anopheles are frequently the victims of parasites. 

 Among such may be mentioned acarid ectoparasites, an encysted 

 trematode, nematodes, gregarines, flagellates of the genus 

 Leptomonas, and at least one species of Nosema. They are also 

 liable to invasion by certain fungi. 



