MACFIE, J. W. Scott
Bulletin of Entomological Research
GRAHAM found that the suspended matter in water in which the larvae of Pyretophorus costalis were breeding could be precipitated by the addition of 3 per cent. of common salt and that then the larvae fed on one another. He considered that this phenomenon was due to the larvae being .deprived of their natural food, the algae, which had been destroyed by the addition of salt. The author's experiments were undertaken to determine to what extent the action of the salt on mosquito larvae was due to the destruction of the natural food supply and to what extent to the hypertonic nature of the solution. The larvae were those of S. fasciata. Samples of water were obtained from the water-pots and domestic utensils in the native quarters at Lagos and an analysis was made of the amount of salt present. The average of six determinations was 0.012 per cent. NaCl. This percentage is therefore assumed to be that in which the larvae thrive best. Experiments were then made with solutions of 0.5 per cent. of NaCl and upwards and then with more dilute solutions.
" The experiments would seem to prove that, in solutions of 2 per cent. and upwards, the action of common salt on the larvae of Stegomyia fasciata[Aedes aegypti] 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 Stegomyia fasciata[Aedes aegypti] 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. . . . 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 per cent. NaCl in order to ensure the destruction of the larvae."
Some experiments were done to determine to what extent mosquito larvae were capable of surviving in temporary pools, such as those beneath the taps of tanks and the open ends of drains into which larvae are apt to be swept. These were not very conclusive. The author thinks it would be a wise precaution to treat these places with larvicides.
A. G. B.
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