INFLUENCE OF ENVIRONxMENT 233 



stage under four to nine weeks. Mosquito larvae of the species wintering 

 in the larval form are readily resistant to ice-forming temperature. In- 

 deed, they may be frozen in the ice and still live after the ice enclosing 

 them is melted. These larvae do not really freeze, because the concen- 

 tration of the body water depresses the freezing point. If the body water 

 really freezes the insect dies. 



WATER 



Natural water, adequately charged with food materials, is probably 

 next to temperature the most important factor. Acidity of natural wa- 

 ters appears to be a relatively unimportant factor because successful 

 breeding occurs in water well above neutrality (alkaline) to as far be- 

 low as pH 4.0. It is, however, possible to have such a concentration of 

 dissolved salts that mosquito larvae will die. Such cases are exemplified 

 by the clay pits where dissolved aluminum is probably the toxic agent 

 and by highly evaporated salt marsh pools in which salinity (salinom- 

 eter reading) reaches twenty-two per cent or above. 



Sea salt concentration appears to prevent the fresh water mosquitoes 

 from breeding on the salt marsh. Salt marshes from which salt water 

 has been long excluded show breeding of such fresh water species as 

 Aedes vexans. It appears also to govern the development of wrigglers 

 of certain salt marsh mosquitoes. In the spring the dominant mosquito 

 emerging from the salt marsh is Aedes cantator; later it is Aedes sol- 

 licitans. Aedes cantator emergences are normal throughout the season 

 from brackish marshes along the higher courses of the rivers. The same 

 salt marsh pool which turns out Aedes cantator in the spring may turn 

 out Aedes sollicitans later. In view of the fact that these species pass 

 the winter as eggs in the marsh mud, it seems likely that low salinities 

 encourage the hatching and larval growth of Aedes cantator, discour- 

 age hatching and growth of Aedes sollicitans, while high salinities en- 

 courage the hatching and larval development of Aedes sollicitans and 

 discourage the hatching and growth of Aedes cantator. 



To be successful breeders of mosquitoes, natural waters must not 

 only have adequate food supply but they must remain long enough for 

 the larval and pupal stages to be completed. In midsummer, under high 

 temperatures, species that complete the pupal stages quickly, such as 

 Aedes sollicitans, may complete pupal stages in the wet mud of drying 

 pools. 



There is no doubt that species of mosquitoes show greater success 

 during seasons of heavy and well distributed rainfall, because that type 

 of rainfall not only creates an enormous increase in number and size of 



