288 J. W. SCOTT MACFIE. 



In the li'^lit of later observations it must I think be concluded that the tension of 

 the dissolved oxj'gen in the water drawn from the tap was already at the beji^iniiing 

 of the experiment so low as to be valueless to the lai"vae for cutaneous respiration. 

 In this case the tap-water used had been lying stagnant in the pipe for at least 

 twelve hours and nuicli of the oxygen originally dissolved in it must have 

 been absorbed. 



It is possible that some similar reason may account for the relatively short j»eriod 

 (6 or 7 hours) Sen found that submerged larvae might survive; for although lie 

 immersed his tubes in " large basins," he did not change the water, and if the amount 

 of dissolved oxygen were very small, it would be useless to the larvae no matter how 

 great the volume of the water. Indeed, in some circumstances the water may be able 

 actually to withdraw some of the oxygen from the bodies of the larvae and so shorten 

 the time of their survival, and in this case the larger the volume of water the mmv 

 rapid would be the death of the lan'ae. 



Probably Stegomyia J'asciato larvae require much less oxygen than larvae of species 

 of Culex. In some experiments with fully developed Cidiciomyia nebulosa larvae I 

 could detect no marked difference in the length of time they survived when submerged 

 in large and small volumes of thoroughly aerated tap-water. Indeed, they some- 

 times survived longer in the smaller volume, as for example in one experiment in 

 which a fully developed larva in 7"5 cc. survived between 2 and 21 hours, while one in 

 750 cc. of the same sample of water survived only between 1 and 1| hours. From 

 this it must be concluded that this species when fully developed is incapable of 

 accommodating itself to living without access to the external air even in thoroughly 

 aerated water. 



The Proportion of the Dissolved Oxygen available for Cutaneous Respiration. 



Theoretically at 30° C, which was about the laboratory temperature in the day 

 time, watei- sJiould dissolve 5'8 cc. of atmospheric oxygen per litre. The tap-water 

 used in my experiments was probably not fully saturated on the one hand, and 

 on the other the temperature was sometimes below 30° C, when more oxygen should 

 be dissolved, so that very roughly we may suppose that the water used contained 

 about 5 cc. dissolved oxygen per litre. In the 8 cc. in the small tube theie should 

 therefore have been 40 c.mra., and in the 1800 cc. in the large bottle 9,000 c.nnn. of 

 oxygen in solution. 



It has been calculated by Sen that a fully developed larva of C. m,icroaiiniduti(s 

 consumes oxygen at the rate of 1*1 c.mm. per hour. Assuming that this figure is 

 approximately correct, and that the larva of N. fasciala consumes oxygen at about 

 the same rate, there was enough dissolved in the water in the tube to keep one lai-va 

 alive for 36 hours if it had all been respirable. As a matter of experience, as has 

 already been shown, a fully developed larva of S. fasciafa when kept submerged 

 in tap- water sinvived only about 12 hours, and as in boiled water, which presumably 

 contained no dissolved oxygen, it survived say 4 hours, the oxygen in solution in 8 cc. 

 of water served only to keep it alive for about 8 hours. Adopting Sen's figure, this 

 means that only 8'8 c.mm. of the 40 c.mm. in solution was available for respiration, 

 or in other words only about one-fifth of the oxygen dissolved in water that hai been 

 thoroughly aerated is available for the larva for cutaneous respiration. If therefore 

 the water is less than four-fifths saturated with atmospheric oxygen, it Avill not be 

 respirable at all. 



