316 NATURAL HISTORY OF AQUATIC INSECTS CH. 



act of expulsion. The air, which dilates the vesicles 

 and expels the eggs, may be taken in by the two 

 pairs of large thoracic stigmata. 



" Ephemerae which are not dazzled by light or 

 otherwise led astray skim the surface of the water, 

 and support their bodies upon it by means of the 

 tail-filaments while engaged in egg-laying. The 

 specific gravity of the eggs is higher than that of 

 water, and they fall at once to the bottom. There 

 the eggs soon become scattered, for the jelly in which 

 they are imbedded is soluble in water. I have placed 

 several egg-masses over night in vessels of water ; 

 next morning the eggs were spread over the bottom, 

 quite loose, though in regular groups. If placed in 

 alcohol they cohere, for alcohol does not dissolve the 

 jelly. 



" How are the eggs fertilised ? How can time be 

 found for their fertilisation in the brief space during 

 which the winged Ephemeras are active ? I have no 

 precise information to give on these points ; the time 

 of emergence renders it hard to observe directly what 

 happens. 1 



" Swammerdam has observed the habits of an 

 Ephemera (Palingenia longicauda) which emerges 

 earlier in the day than Polymitarcys, and takes to 

 flight more than two hours before sunset. He thinks 

 that in this species the eggs are fertilised by the 

 males in the water, after the manner of Fishes. But 

 it is hard to understand how eggs which, like those 

 of Polymitarcys, fall to the bottom as soon as they 



1 De Geer has cleared up this difficulty, as will be seen 

 further on. 



