RESISTANCE OF THE EGGS OF STEGOMYIA FASCIATA. 83 



The reverse idea then suggested itself, namely, that not until the egg had been in 

 contact with water for some time and the embryo partially developed was the shell 

 rendered resistant to drying. The results obtained in earlier experiments, when the 

 leaves from West Africa were placed on water, in so far as the larvae hatched within 

 a few hours, also favoured this view, as showdng that the embryo was well developed 

 in the resistant eggs. 



Ten fertilised and fully fed mosquitos were therefore placed in a small cage con- 

 taining the usual petri dish of water on which they might oviposit. On the night 

 of the fourth day after feeding, large numbers of eggs were laid, and these were divided 

 into ten approximately equal batches at 10 o'clock the next morning. Oviposition 

 had not commenced by 6 o'clock on the night of the fourth day when I left the 

 laboratory, and thus the maximum time that the first eggs could have remained on 

 the water ere they were divided into equal batches, may be roughly estimated at 

 15 hours. 



Each of the 10 batches of eggs was allowed to remain in water for different periods, 

 and was then collected on filter-paper and allowed to dry slowly at the temperature 

 of the laboratory (64° F.). The following table gives the results :— 



Contact with water for some time is clearly shown in the above table to be necessary 

 in order that the eggs may become resistant to desiccation. Whether this is due to 

 some change in the egg-shell itself or to development of the embryo and its enveloping 

 membranes is a subject of study at the present time, as is also the question of the 

 longest period that eggs will resist desiccation. The latter is a point of some doubt, 

 and has been put at various lengths of time by different investigators. Patton and 

 Cragg state, " Francis has shown that the eggs of S. fasciata may remain viable for 

 as long as six and a half months when they are kept dry ; they do not however hatch 

 after two years. Peryassu found that after five and a half months the eggs would 

 not hatch."* Also it has been shown by Theobald that they would survive out of 

 water for a considerable time, and in an exact experiment by Newstead, with eggs 

 that were sent fi'om Manaos, South America, to England, that they could remain 

 dormant for forty-two days, while in the paper already referred to I have shown that 

 thev survived for at least three and a half months. 



*"A Textbook of Medical Entomology," by Patton and Cragg. 

 <C250) 



