TOXICITY OF ACIDS FOR MOSQUITO LARV^. 151 



Larvae in the CC>2 solution become motionless almost at once 

 and the movements of heart and alimentary canal also quickly 

 cease, while in HC1 of the same PH value they are apparently 

 unaffected for over 24 hours. The more rapid penetration rate 

 and mode of action of CO 2 as pointed out by Jacobs (9), doubtless 

 account for the differences observed in the effect of the two rea- 

 gents. 



It is also of much interest to know in what manner the acids 

 kill the animals, whether they enter the chitinous covering or 

 enter by the mouth or anus through the alimentary canal. By 

 using pupae, which are known not to eat nor to have external 

 openings as in the larvae, it is found that the acids do not kill 

 them for many hours, considerably in excess of the lethal ex- 

 posure for larvae. From this fact it seems reasonable to assume 

 that the larvae are killed by the entrance of the reagent orally 

 rather than cutaneously. The present discussion, however, deals 

 primarily with the resistance of the animals to the reagents 

 rather than with their mode of killing. Younger and smaller 

 larvae, and these doubtless have thinner chitin, are killed some- 

 what more quickly than older individuals. 



The general order of toxicity of the acids used for the larvae 

 is, salicyclic > oxalic > HC1 > butyric > acetic. This series 

 is strikingly similar to that found by Haas (10) for plants, by 

 Collett (n) for protozoa and by Bodine (8) for cysts of Colpoda. 



FATAL EXPOSURES IN MINUTES TO DIFFERENT PERCENTAGES OF HcCL2 



(TEMP. 25 C.). 



Per Cent. Time in 



HgCh. Minutes. 



0.05 305.0 



o.io 155.0 



0.50 48.3 



i. oo 26.5 



2.OO 2O. O 



Mercuric chloride in various concentrations was used and here 

 again the resistance of the larvae is of considerable interest. The 

 above table shows the effect of different percentages of this salt. 

 Honda (6) found that the free-living Nematode Rhabditis elegans 

 withstood 0.05 per cent. HgCl 2 for 60 minutes; Daphnia, 25 

 minutes and tadpoles, 5 minutes. Mosquito larvae are about 5 

 times as resistant to HgCl 2 as the most resistant form used by 

 this author. Sen (3), with other species of mosquito larvae (Ste- 



