﻿256 L. NICHOLLS — SOME OBSERVATIONS ON THE BIONOMICS AND BREEDING- 



General bionomics. 



A. argyrotarsis and albimanm appear to depend for their continued existence, 

 in any but very small numbers, upon their adaptability for being the first insects 

 to employ newly-formed pools for breeding purposes. In these, at first, they 

 have little or no struggle for existence, for an abundant food supply is present, 

 and natural enemies are absent ; soon, hoAvever, Neuroptera, Coleoptera, and 

 other creatures appear. As the pools become more crowded and stagnant, larval 

 diseases become rife. In permanent situations these adverse conditions are 

 usually present, the greatest destroyers of mosquito larva being small fish. 



The following is an example of conditions changing in a pool. A small 

 marshy swamp was formed in August 1911, and I examined 57 of the first 

 larvae to appear ; they were all healthy, and continuous search showed the 

 absence of natural enemies. In September the larvae were less in numbers, and 

 a percentage showed attacks of Vorti celiac and algae ; dragon-fly larvae had also 

 appeared. Owing to continuous rains the pool was still present in November 

 and December. The larvae were then far fewer ; all were diseased, and numerous 

 natural enemies were present. By the use of muslin tents covering 20 square 

 feet, it was found that the rate of emergence in September was 16 adults for 

 24 hours. During November and December between two and three was the 

 average for a similar time and area. 



An equal number of Stegomyia and Anopheles larvae have been placed in pools 

 containing natural enemies, in the form of "Millions" fish and dragon-fly larvae. 

 In 36 hours all the former larvae usually disappear, their active wriggling move- 

 ments quickly revealing them to the eyes of their enemies ; about half the 

 Anopheles larvae will remain, their less obtrusive position on the surface of 

 the water preventing other creatures from readily observing them, 



My conclusions are that the struggle against predaceous enemies is most severe 

 in — (1) permanent situations containing fish ; less severe in, (2) permanent 

 situations containing Neuroptera and predaceous creatures other than fish ; and 

 practically non-existent in, (3) non-permanent situations which have lasted for not 

 more than six weeks. 



On the other hand, the struggle against disease, caused by protozoa and algae, is 

 greatest in — (1) non-permanent areas of stagnant water, which are not spreading 

 further afield, and which have lasted for over ten weeks ; less in, (2) permanent 

 pools of stagnant water ; and absent in, (3) other permanent situations. 



Therefore, from the point of view of their natural struggle for existence, 

 recently formed non-permanent situations are indicated as the most favourable 

 breeding-grounds for Anopheles. 



The imagines are unobtrusive and purely nocturnal mosquitos, and bite all 

 through the night. On one occasion a friend and myself sat at night in a small 

 room in the neighbourhood of swamps carefully watching for Anopheles, and, as 

 they were seen, they were caught in nets. Females started coming in at 7 p.m., 

 but appeared in greater numbers after 10 p.m., and continued as numerous until 

 2 a.m., when we ceased observations. 



During the day the females can be found resting in their characteristic, 

 nearly-perpendicular attitude in any dark corners, or dotted over the walls of 

 labourers' huts, which are usually kept well shut up. 



