446 



MOSQUITOES 



water and even in brackish water, provided food material, in the 

 form of dead organic matter and the accompanying bacteria, is 

 present. Atkin and Bacot have recently shown that the food 

 consists almost, if not quite, exclusively of bacteria, and that 



when the larvae are present in large 

 numbers they exert a considera- 

 ble influence in the purification of 

 water. Often the larvae are over- 

 looked, since they immediately 

 wriggle to the bottom of their 

 dwelling place when approached, 

 and hug the bottom so closely that 

 even if a barrel containing thousands 

 of them is turned over on its side, 

 about 80 per cent will stay in the 

 little remaining water. The larvae 

 feed exclusively on the bottom and 

 can often be seen nibbling away 

 at a dead insect or bit of decaying 

 vegetation. With plenty of food 

 and at favorable temperatures the 

 larval existence may be completed 

 in four days, according to Bacot, 

 though it usually requires a longer 

 time than this, and may be drawn 

 out to two months or more. The 

 larvae are not resistant to dry- 

 ing, and die in a few hours in a 

 FIG. 204. Larva of yellow fever dry place, though capable of liv- 

 mosquito, Aedes caiopus. x io. ^ nearly two weeks on moist 



(After Howard, Dyar and Knab.) 



ground. 



The pupae (Fig. 205) transform, under normal conditions, in a 

 day and a half or two days. The entire cycle from egg to adult 

 seldom takes place in less than nine or ten days, and probably 

 12 or 15 days is more usual under ordinary conditions. As has 

 been shown above, the period erf development may be drawn out 

 over several months by unfavorable conditions. The adult mos- 

 quitoes may live for a considerable time, and apparently are 

 able to transmit yellow fever any time from 12 days after in- 

 fection to the end of their lives. Male mosquitoes ordinarily 



r-br.t. 



