THE LIMITATION'S OF KKKOSEXE AS A LARATCIDK. 285 



of the temperature towards evening? which niii.st be remembered. Five larvae that 

 were observed contimiousl}' were found dead after 10, 1()|, 12, 12, and 12^ hours 

 respectively. Three larvae kept simultaneously in the incubator at 37° C. were all 

 alive after 6, but all dead after 7 hours. 



There can therefore be no doubt that a lowering of the temperature prolongs and 

 a raising of it shortens the period of suirival of submerged S. fasciala larvae ; and 

 this one mav suppose is due to a variation in the activity of metabolism, that is in 

 the rate of ;ibsorption of ox}'gen. 



The Presence of Organic Mailer lit iJte Mediiiiii. The presence of organic matter in a 

 medium profoundly modifies the power of survival of larvae submerged in it. 



In one experiment conducted on the usual lines, fifteen very young S. fasciala 

 larvae (first phase) were used, five each in (A) a natural S. fasciala medium containing 

 vegetable debris, (B) tap-water containing a little organic matter, and (C) distilled 

 water. In (A), the natural medium, the larvae were found dead after 6, 7, 7, 7, and 8 

 hours respectively ; and in (B), the tap-water, after 10, 12, 13, 14, and 16 hours. 

 In (C), the distilled water, all the larvae were still alive after 16 hours, but after 24 

 hours, when the tubes were next examined, three were found dead ; the fourth cast 

 its first pelt and died between the 31st and 32nd hours, and the last larva was found 

 dead after 33 hours. 



In a similar experiment with Ci'lexfalif/aiis larvae in the fourth ])hase the results 

 were similar, but the periods of survival were, of course, shorter. In the natural 

 medium the three larvae used were all dead after 2 hours ; in the tap water one larva 

 was dead after 3 hours, and two after 4 hours ; and in the distilled water the larvae 

 were found dead after 4, 4, and 5 hours respectively. 



The presence of organic matter tindoubtedly materially shortens the period during 

 which mosquito larvae can survive when submerged in a fluid. As the larvae were 

 prevented from obtaining direct access to air by the conditions of tlie experiments, 

 it seems natural to suppose that the action of tiie organic matter is associated with 

 its effect in absorbing and using up the oxygen dissolved in the fluids. If then the 

 dissolved oxygen were to be expelled from the fluid by some other means, as 

 for example by boiling, we ought to province a similar effect on the larvae. 



A number of experiments were therefore carried out with boiled water, both 

 S. jasciata and C. faligans larvae being employed. In these experiments the water 

 was thoroughly boiled in the tubes, the paraffin added, and the tubes allowed to cool. 

 The medium was thus cooled out of contact with air and therefore could not have 

 rea.bsorbed oxygen. When the experiment was to be started the paraffin was quickly 

 pushed to one side w4th a glass rod, the lar\a introduced with a pipette, and the 

 paraffin melted just stifficiently to reseal the tube. As the larvae always sank to the 

 bottom of the tubes when they Avere first introduced, there was no danger of tlieir 

 being injured in the last operation. 



The results of these experiments were somewhat confusing owing to individual 

 lar\'ae suniving unexpectedly long, but for some of these variations the fall of the 

 temperature at nigh.t may have been resj^onsible. There could, however, be no doubt 

 but that the larvae in tap or tank water that had been thoroughly boiled suiA'ived a 

 much shorter time than did those in tiie control tubes of unboiled water, and this 

 applied equallv to S. fasciala. and C. faligans. and to l)ot!i young and fully developed 

 larvae oi both species. 



