INDIRECT BENEFITS DERIVED FROM INSECTS. 283 



millions emerge at a certain season of the year from some 

 of the rivers in France, which, as it is well worth your 

 attention, I shall abridge for you. 



These insects in their first and intermediate state are 

 aquatic : they either live in holes in the banks of rivers 

 or brooks below the water, so that it enters into their 

 habitations, which they seldom quit ; or they swim about 

 and walk upon the bed of the stream, or conceal them- 

 selves under stones or upon pieces of stick. Though their 

 life, when they assume the perfect state, is usually ex- 

 tremely short, some being disclosed after sun-set, 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 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 

 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 described,' are expected by the fisher- 



