339 



A NOTE ON THE ACTION OF COMMON SALT ON THE 

 LARVAE OF STEGOMYIA FASCIATA. 



By J. W. Scott Macfie, M.A., M.B., Cn.B., 

 West African Medical Staff. 



(Plate XXXII.) 



It has been pointed out by Graham" that measures designed to bring about the 

 destruction of mosquito larvae may be divided into (a) measures intended to 

 destroy the larvae, and (b) measures intended to destroy their food supply. He 

 points out that very little attention has been paid to the second method, which 

 entails an exact knowledge of the freshwater algae. 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 became cannibalistic and destroyed 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. " In 

 lesser concentration," he concluded, " salt appears to inhibit the growth of very 

 young larvae, probably by diminishing the supply of food, but the development 

 of fully-grown larvae appears to be hastened in a hypertonic medium, and they 

 pass into and through the pupal stage with unusual rapidity." 



The following experiments were undertaken with a view to determining to 

 what extent the action of 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 employed were those of Stegomyia fasciata. This species 

 was selected both because its breeding places would be most easily treated with 

 salt, should this substance prove of value as a larvicide, and on account of its 

 importance in a country in which yellow fever is endemic. 



Salinity of the Medium in which the Larvae were found. 



At Lagos the larvae of Stegomyia fasciata are commonly found in water-pots 

 and domestic utensils in the compounds of the native quarters of the town. A 

 number of samples of water were obtained from this source, and an analysis was 

 made of the amount of salt present in each. The percentage was found to vary 

 considerably, ranging from O'OOo % to 0*019 # NaCl, and the average of six 

 determinations was 0*012 % NaCl. 



This figure was found to be considerably below that for the water of the Lago> 

 lagoon at the same season (September). The salinity of the water flowing 

 beneath the Iddo bridge, to the North of Lagos, was found to be 0*026 % 

 NaCl ; whilst that at the Magazine bridge, considerably nearer the sea, was 

 0-112% NaCl. 



Bull. Ent, Res. Vol. i. p. 51^2, 1910. 



