280 INDIRECT BENEFITS DERIVED FROM INSECTS. 



tremely short, some being disclosed after sun-set, laying 

 their eggs and dying before sun-rise; and many not liv- 

 ing 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 regularly 

 at nearly the same period annually, and for a certain 

 number of days fill the air in the neighbourhood of the 

 rivers, emerging also from the water at a certain hour of 

 the day. Those which Swammerdam observed, 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 

 deo-ree 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 andMarne, 

 which Reaumur described, are expected by the fisher- 

 men, who call them manna .- and when their season is 

 come, they say " the manna begins to appear, the manna 

 fell abundantly such a night;"— -alluding, by this expres- 

 sion, either to the astonishing quantity of food which the 

 Ephemeras afford the fish, or to the large quantity of fish 

 which they then :ake. 



Reaumur first observed these insects in the year 1 738, 

 when they did not begin to show themselves in numbers 

 till the 18th of August. On the 19th, having received 

 notice from his fisherman that the flies had appeared, he 



