X] LIFE-CYCLE 173 



excretory matter. In addition the egg is surrounded by a series 

 of small air-chambers which help to keep it afloat. 



As each egg is laid by the female a small drop of liquid is 

 also extruded on the surface of the water and may help to keep 

 the egg floating, as this secretion is of somewhat oily consistency. 

 Certainly a slight agitation of the water is sufficient to cause 

 the eggs to sink, but development proceeds almost as well 

 beneath the water as at the surface. In the former case, 

 however, the eggs are more liable to the attacks of bacteria, 

 etc. Under favourable conditions the eggs hatch out in about 

 two or three days. In the Amazons the normal incubation 

 period is said to be from three to eight days, but its duration 

 depends almost entirely on the temperature. If this falls 

 below 20 C. the eggs are unable to develop, and if the ther- 

 mometer falls to this temperature for a few hours each day 

 the development is greatly prolonged. 



The eggs are very resistant and can withstand long exposures 

 to cold and dryness. They may be exposed to a temperature 

 of 37 C. for one hour, or kept at o C. for many days, without 

 their vitality being affected. Eggs have been maintained for 

 several months between 10 C. and 20 C. and when the tem- 

 perature was raised, about one-twentieth of them hatched out. 



The most striking results, however, are those obtained 

 with eggs that have been dried. These may be kept for three 

 months without more than 40 per cent, of them dying. Francis 

 has also shewn that they may remain alive for six and a half 

 months if kept dry. Newstead describes an experiment with 

 eggs of S. fasciata that had been sent from Manaos on the 

 Amazon. The eggs had been laid on moist white filter paper ; 

 they were then dried in air and subsequently for 24 hours 

 over calcium chloride. They were then packed in tubes and 

 sent across to England. Forty-five to 47 days later, on their 

 arrival at Liverpool, they were placed in water at 23 C. and 

 several larvae hatched out within 12 hours. Moreover, these 

 larvae developed very rapidly and the adult insects emerged 

 within 12 days. From these results it is evident that the eggs 

 of S. fasciata under certain conditions may remain alive for 

 considerable periods without developing. 



