RESISTANCE TO DESICCATION 291 



hastens the hatching of the eggs, and of Jennings who has noted larvae in 

 water containing wood ashes. In this latter case, at Cartagena in Colombia, the 

 larvae were found in large numbers in an earthen pot containing a large quantity 

 of wood ashes in brackish water from a shallow well near the sea-coast. None of 

 the larv£e produced images. In such cases the potash contents, which are 

 variable with different kinds of wood, must be taken into account and also the 

 fact, already pointed out, that after some time the water loses its alkalinity. 



The degree of resistance to desiccation of the larvas and pupae of Aedes calopus 

 is important from the practical standpoint. The first yeUow-fever commission 

 to Vera Cruz found that in that dry climate larvae died very quickly when the 

 water containing them was poured upon the ground. In moister climates the 

 larvae may, under favorable circumstances, live out of water a considerable time 

 and the pupae show great resistance to desiccation. 



Peryassii records a series of experiments conducted on a large scale at Eio de 

 Janeiro. Placed upon filter paper none of the larvae lived nine hours. When 

 placed upon the moist ground, according to temperature and evaporation, they 

 survived as much as 13 days, and, when again put in water, developed to imagos. 

 In the experiments with larvae and pupae the following method was employed. 

 A determined number of larvae or pupae were put in a watch glass with a little 

 water and then emptied upon a cone of filter paper ; this latter was then placed 

 upon two other layers of filter paper, to absorb all the water from the first; then, 

 when the necessary time had passed, the paper with the larvae was placed in a 

 glass of water so that the number of dead or surviving larvae could be deter- 

 mined, as well as the time they required to return to a normal state. Tabula- 

 tions of the results are given and from these the following data are extracted. 



Larvae dried up to three hours developed normally when returned to the water 

 and it is only beyond this that they began to suffer. In the course of the many 

 experiments the longest periods during which calopus larvae survived were in 

 one case 8 hours and 20 minutes, and in another 8 hours and 55 minutes. It 

 must be noted that on the day on which these results were obtained it rained 

 copiously all day; the evaporation, in the shade, was 1.4 mm. and the relative 

 humidity 86.1. These results are based upon 64 experiments with 3685 larvae. 

 The larvae, when returned to the water, sometimes immediately resume their 

 activity, but most of them remain immovable for from 5 to 15 minutes. Gen- 

 erally the larvae that did not recover within this time would die. Drying during 

 periods of from three hours to four hours and fifty minutes was survived by 

 from 72 to 98 per cent of the larvae, and the number of these that reached the 

 imago state varied from 1.5 to 75 per cent. The time to transformation, after 

 being returned to the water, varied from 3 to 41 days. Experiments with drying 

 during 5 hours to 6 hours and 50 minutes gave the following results : The larv^ 

 survived in the proportion of 7.5 to 95.5 per cent; they produced imagos in from 

 2.5 to 62.5 per cent. The period to transformation varied from 3 to 42 days. 

 Experiments with drying for from 7 to 9 hours resulted as follows ; the larvae 

 survived in the proportion of from 4 to 97.77 per cent and produced imagos in 

 from 1.25 to 26.66 per cent; the variation in the time to transformation varied 



20 



