LENGTH OF EARLY STAGES 231 



RESISTANCE TO DESICCATION. 



One of the points of importance in considering the eggs and larvae of the 

 malarial mosquitoes is their possible resistance of desiccation. Exact observa- 

 tions on this point are still needed. Instances of supposed resistance on the part 

 of larvas must be studied from every side with care before forming conclusions. 

 Nuttall and Shipley give an account of two observations made at Gainsborough, 

 where many fully developed larvae and pupce were encountered in ditches which, 

 according to persons living in the vicinity, had been filled with river water after 

 having been dry from four to seven days. It seemed to the authors at first that 

 the larvae might have been carried in with the water from the river, as larvae were 

 also found there, but the larvae in the ditches were so numerous as to render this 

 explanation incompetent for a large part of them. In all probability the ditches 

 in question had not been completely dried out. The authors call attention to 

 the fact that Christophers and Stephens in Africa found that no large larvae 

 reappeared in a pool which had been dried up for two days and then refilled with 

 rain-water. The larvse which did appear were very small, and issued from eggs 

 which apparently had resisted desiccation for the two days in question. 



James and Liston point out that when Anopheles eggs are laid on water they 

 almost invariably hatch in 48 hours, but that when laid upon damp mud the 

 development of the embryo goes on and when water is added the larva hatch out 

 almost immediately. They point out that in Mian Mir Christophers found 

 that after the water in an irrigation canal or pool had been emptied out the 

 Anopheles still laid their eggs on the soft mud and that if water entered before 

 the mud had become completely dry young larvae would almost immediately be 

 found. After giving the details of his observations, James and Liston state, " It 

 will be seen therefore, that pools which are, to all intents and purposes, quite dry, 

 may contain numerous larvae almost immediately after a shower of rain." 

 Christophers also found that larvae may remain alive on soft mud even after 

 exposure to the sun for several hours, but were killed when the mud had so far 

 dried as to lose its glistening surface. 



The observers of the Instituto Oswaldo Cruz at Rio de Janeiro made experi- 

 ments with Anopheles alhimanus and A. argyritarsis, the two species of greatest 

 economic importance in tropical America. When the larvae were dried on filter 

 paper, in the laboratory, they were still alive after six hours. On moist mud 

 they remained alive six days. 



LENGTH OF EARLY STAGES. 



The duration of the early stages of Anopheles varies according to the tempera- 

 ture and food-conditions. With the heat the growth is faster ; food supply also 

 governs the duration of the larval stage. In India Liston found that the larval 

 state might last but a week, but with a deficient food supply it might last for a 

 month. Foley and Yvernault record the development of Anopheles chaudoyei 

 from egg to imago within 13 days. The early observations made by one of us 

 (Howard) on Anopheles quadrimaculahis at Washington showed a minimum 

 larval period, in May and June, of sixteen days, and a pupal period of five days ; 

 eggs, three days — these periods undoubtedly being reduced later in the summer. 



