342 



J. W. SCOTT MACFIE — A NOTE OX THE ACTION OF COMMON 



It will be observed that during the first 24 hours the effect of the saline 

 solutions in the natural medium, estimated by the death of the larvae, was pro- 

 portional to the strengths of the solutions used. In the experiment with clear 

 water the action was not marked until three days later. It is possible that the 

 injurious effects in the former cases may have been due to the clogging of the 

 tracheae of the larvae by precipitates. The deaths during the first 24 hours 

 cannot at any rate be attributed to starvation. 



All the larvae in these experiments had either died, or pupated and hatched, 

 by the 9th day. The number that died was roughly in proportion to the strength 

 of the saline solution. Thus in the 0*10 # solution none died as larvae, but in 

 the 0*30 % eleven of the original twenty died in this condition. 



NaCi. 



0T0 % 



0T5 % 



0-25 % 



0-30 % 



Hatched 



Died as pupae .. . 



Died as larvae ... 



17 

 3 

 



9 

 4 



7 



8 



1 



11 



8 



1 



11 



Very young larvae appeared to be more adaptable than mature larvae to a 

 change in the concentration of the medium. Solutions containing 0'10#, 0*20 #, 

 and 0*30 % NaCl did not appear to have any appreciable effect on them. 



Experiments with Alum Solutions. 



If, as has been suggested, the inimical action of common salt on mosquito 

 larvae is due to the precipitation of the minute algae on which the)' feed, it might 

 be supposed that alum would have a more pronounced effect owing to its well- 

 known clarifying action on impure waters. It was thought possible also that the 

 gelatinous precipitate might interfere with the respiratory processes of the larvae, 

 for it had been observed that in a pool containing innumerable larvae great 

 numbers had died when some toad spawn was deposited in it. The spawn floated 

 on the surface of the water and spread out like a veil, the eggs being connected 

 by fine threads of mucus in which the mosquito larvae had become entangled 

 and had died. 



Three experiments were carried out in which potash alum was added to turbid 

 water containing larvae so as to bring the strengths of the solutions up to 0*05 % , 

 0*10%, and 0*20% respectively. Ten mature larvae were used in each 

 experiment. 



The addition of the alum produced a copious precipitate Avhich slowly settled 

 to the bottom of the jars, but there was no immediate effect on the vitality or 

 activity of the larvae. In the jar containing 0*05 % solution five larvae pupated 

 and hatched, one larva died on the 7th day, and four that had pupated died on 

 the third and fourth days. The last pupa hatched on the 12th day. Alum in 

 this concentration appeared to have little or no direct effect. In the jar contain- 

 ing 0*10 % solution only one mosquito hatched, and the other nine larvae died 

 between the third and the seventh days. In the third jar containing 0*20 % 

 solution seven larvae died between the fourth and the seventh days, and three 



