SALT ON THE LARVAE OF STEGOMYIA FASCIATA. 



341 



the conclusion of the observations. The lowest concentration, 0*5 % NaCl, 

 seemed to have but little effect ; five larvae pupated and hatched, and live 

 remained alive at the end of the experiment ; none died. 



The rapidity with which the larvae were killed by the stronger solutions seems 

 to prove that, at any rate as low as 2 or 3 % NaCl, the action on the mature 

 larvae of S. fasciata is an osmotic one, and is not dependent on the destruction of 

 algae, nor on the deprivation of the insects of their natural food supply. 



The action of the salt solution is not, however, appreciably altered by the 

 presence of debris, etc., as is proved by the following experiment. A large 

 number of larvae and pupae in their natural medium were placed in a jar, and 

 enough salt added to bring up the strength of the solution to 2 % . Six hours 

 later many of the larvae were dead, and twelve hours after this all had perished. 

 A number of the pupae hatched during the first two days, but on the third day 

 all those that remained were dead. 



Experiments with more dilute solutions of salt. 



Although solutions of salt of 2 % and upwards appeared to produce an inimical 

 effect on the larvae of S. fasciata by direct osmotic action, more dilute solutions 

 in clear water had a much less pronounced effect. It was considered possible, 

 however, that the latter solutions might cause a precipitation of the organic 

 constituents of the natural fluid in which the mosquito larvae were found, and 

 might thus kill, or at any rate inhibit the growth of, the larvae by depriving 

 them of food as was suggested by Graham. 



Mature larvae were therefore distributed into glass jars, and the salinity of the 

 natural medium, which contained 0*012 % NaCl, was increased by the addition of 

 salt to 0-10 #, 0'15%, 0-20%, i)"25 % , and 0*30 # respectively. Larvae were 

 also introduced into a solution of 0*32 % NaCl in pure water for comparison. 

 The results of these experiments are shown below. 



Table II. 



NaCl. 



0T0 o/ r> 



0-15 o/ 



O"20 o/ c 



0-25 % 



0-30 >/r 



0-32 o/ c in 

 pure water. 



Oct. 



11. 



20 L. 



20 L. 



10 L. 



20 L. 



20 L. 



10 L. 



Oct. 



12. 



17 L., 3 P. 



13 L.. 2 P., 



5 L., 1 P., 



9 L.. 4 P.. 



9 L., 1 P.. 



7 L., 1 P.. 









5 dead L. 



4 dead L. 



7 died L. 



10 dead L. 



2 dead L. 



Oct. 



13. 



11 L., 9 P. 



9 L. r 4 P.. 

 2 dead L. 



2 L . 3 P., 



1 dead L. 



4 L., 6 P., 



3 dead L. 



8 L., 2 P. 



5 L., 3 P. 



Oct. 



14. 



8 L.. 12 P. 



3 L., 9 P.. 

 1 dead P. 



5 P. 



1 L., 7 P.. 2 M. 



2 L., 7 P. ; 1 M. 



5 L.. 3 P. 



Oct. 



15. 



2 L., 9 P., 8 M., 

 1 dead P. 



1 L.. 8 P. r 3 M. 



2 P., 3 M. 



1 P. ; 3 M., 

 1 dead L. 



7 P., 1 M.. 

 1 dead L. 



3 M., 5 dead L. 



Oct, 



16. 



1 L, ; 8 P., 2 M. 



3 P.. 3 M.. 

 3 dead P. 



2 Off, 



2 P. ; 2 M. 



2 P., 5 M. 





Oct. 



17. 



3 P.. 4 M., 

 2 dead P. 



3 M. 





1 M., 1 dead P. 



1., M.., 1 dead P. 





Oct. 



IS. 



1 P.. 2 M . 













Oct. 



19. 



1 M. 













L = Larva ; P = pupa ; M = mosquito. 



