DISCUSSION 



G. SuRTEES: I have worked on West African mosquitoes, especially in 

 southern Nigeria, where one finds well-separated populations isolated in 

 village clearings. Such areas are often up to a quarter mile in diameter, and 

 within them the yellow fever vector mosquitoes breed only domestically, 

 in village water butts. There are no predators on Aedes aegypti in these 

 storage butts: their food is algae. The rainy season begins in March and 

 the rains increase to a June-July peak; are less in August, rise to a minor 

 September peak, and terminate in October. The mosquito larvae similarly 

 increase suddenly in March, at the start of the rains, when the eggs hatch 

 (Surtees, 1958, I959«). They reach a peak in May and by the end of that 

 month the larval populations slump to a very low level despite the abundance 

 of water. There is then a slow recovery. It might be thought that this slump 

 was due to some other organism affecting the adults, but the adult numbers 

 fall off six to eight weeks after the larval decline: the decrease in adults thus 

 seems to depend on that of the larvae. In the absence of predators taking the 

 larvae one must look for a direct effect involving them. Laboratory studies 

 show that cultures maintained at a certain high density level show a sudden 

 and high mortahty of larvae, and intraspecific larval competition therefore 

 seems to be involved as the control mechanism (Surtees, I959^)- 



J. A. Gulland: Is there any evidence for seasonal periodicity in egg 

 laying? 



G. Surtees: So far as we can see this does not alter significantly. Nor 

 have we been able to correlate larval mortality successfully with any environ- 

 mental factor. 



F. Raw: Many speakers have said that they believed they were dealing 

 with stable populations. Surely entomologists and terrestrial ecologists 

 would agree that such are exceptional. Why should aquatic populations be 

 more stable? 



E. D. Le Cren: I think that time relations are important in this. In 

 temperate regions, such as Britain, there is a pronounced seasonal cHmatic 

 variation and this makes it difficult to study species with a one-year or two- 

 year life cycle. Work is easier on species with a short life cycle, like Daplmia, 

 or a very long one, like a fish. In insects the marked seasonal effects may 

 give an impression of population instability. 



