282 INDIRECT BENEFITS DEHIVED FROM INSECTS. 



about and walk upon the bed of the stream, or conceal 

 themselves under stones or upon pieces of stick. Though 

 their life, when they assume the perfect state, is usu- 

 ally extremely short, some being disclosed after sun- 

 bet, laying their eggs and dying before sun-rise ; and 

 many not living more than three hours; yet in their 

 preparatory state their existence is much longer, in 

 some one, in others two, in others even three years. 



The different species assume the imago at different 

 times of the year ; but the same species appear regu- 

 larly at nearly the same period annually, and for a cer- 

 tain number of days fill the air in the neighbourhood 

 of the rivers, emerging also from the water at a cer- 

 tain hour of the day. Those which Swammerdam ob- 

 served, began to fly about six o'clock in the evening, 

 or about two hours before sun-set ; but the great body 

 of those noticed by Reaumur did not appear till after 

 that time; so that the season of different harvests is not 

 better known to the farmer, than that in which the 

 Ephemerae of a particular river are to emerge, is to the 

 fishermen. Yet a greater degree of heat or cold, the 

 rise or fall of the water, and other circumstances we are 

 not aware of, may accelerate or retard their appearance. 

 Between the 10th and 15th of August is the time when 

 those of the Seine and Marne, which Reaumur de- 

 scribed, are expected by the fishermen, who call them 

 manna: and when their season is come, they say " the 

 manna begins to appear, the manna fell abundantly such 

 anight;" — alluding, by this expression, either to the 

 astonishing quantity of food which the Ephemerae af- 

 ford the fish, or to the large quantity of fish w hich they 

 then take. 



Reaumur first observed these insects in the year 1738, 



