SALT ON THE LARVAE OF STEGOMYIA PASCIATA. 343 



that had pupated died on the third, fourth, and sixth days respectively. It was 

 evident therefore that alum in these concentrations had no peculiar action on the 

 larvae. 



In more dilute solutions in clean water the results were inconclusive. Of ten 

 mature larvae in a 0*01 % solution three pupated and hatched, and seven died as 

 larvae ; in a 0*02 % solution exactly the same results occurred ; and in a 0*03 % 

 solution two pupated and hatched and eight died as larvae. All had either 

 hatched or died by the eighth, ninth, and tenth day respectively. 



The action of a medium containing salt and alum in equal parts was tried in 

 another series of experiments. The results, however, were similar to those 

 obtained with a corresponding concentration of common salt alone. 



Summary of Results. 



The foregoing experiments would seem to prove that, in solutions of 2 X and 

 upwards, the action of common salt on the larvae of Stcgomtjia fasciata is due to 

 the hypertonicity of the solution. In more dilute solutions the destruction of 

 the natural food supply of the larvae may have some influence. Alum, however, 

 which clarifies water more efficiently than common salt, has no peculiar action 

 on the larvae. 



In Lagos the larvae of Stcyomyia fasciata are found most abundantly in water 

 contained in domestic utensils, and in the large pots in which the natives store up 

 water for drinking and cooking. It would be of great advantage if common salt 

 could be used as a larvicide in these cases ; for not only would the water not be 

 rendered unfit for use in cooking, as it is by the application of kerosene, but also 

 evaporation would tend to increase the strength of the larvicide, and repeated 

 applications would be unnecessary. It would also be possible to keep stores of 

 water in the compounds without danger. From the experiments described, it 

 would appear that sufficient salt would have to be added to each vessel to bring 

 the concentration of the solution up to 2 % NaCl in order to ensure the 

 destruction of the larvae. 



The Survival of Mosquito Larvae in Temporary Pools. 



It often happens that small pools of water, such as those illustrated by the 

 accompanying photographs (PI. XXXII), collect beneath the taps of tanks and at 

 the open ends of drains, and into these mosquito larvae are apt to be swept. Such 

 pools soon dry up, the water seldom lying in them for more than an hour or two, 

 and they may thus escape the attention of the Sanitary Inspectors. The soil 

 round them is, however, permanently wet ; and if, as is usually the case, the 

 pools are renewed once a day or more frequently by the tanks or drains being 

 used, it is quite possible that mosquito larvae might develop in them. In some 

 cases the finer soil washes away leaving a mass of irregularly-shaped stones in 

 the interstices between which water may lie concealed all day long. Such pools 

 may occasionally account for the presence of mosquitos iu a house near which no 

 breeding place can be located. 



With a view to determining to what extent mosquito larvae were capable of 

 surviving intermittent desiccation of this kind, the conditions obtaining under a 



