INDIRECT BENEFITS DERIVED FROM INSECTS. 239 



But to revert to insects as indirectly advantageous to us, by 

 furnishing food to fishes and birds, beginning with the former. 



Our rivers abound with fish of various kinds, which at 

 particular seasons derive a principal part of their food from 

 insects, as the numerous species of the salmon and carp 

 genus. These chiefly prey upon the various kinds of Tri- 

 choptera, in their larva state called case- or caddis-worms, 

 and in their imago may-flies (though this last denomination 

 properly belongs only to the Sialis lutaria, which generally 

 appears in that month) and EphemercB. Besides these, the 

 waters swarm with insects of every order as numerous in 

 proportion to the space they inhabit, as those that fill the air, 

 which form the sole nutriment of multitudes of our fish, and 

 the partial support of almost all. 



Beaumur has given us a very entertaining account of the 

 infinite hosts of Ephemerce that by myriads of 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 themselves under stones or 

 upon pieces of stick. Though their life, when they assume 

 the perfect state, is usually extremely short, some being 

 disclosed after sunset, laying their eggs and dying before 

 sunrise ; 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 diflerent species assume the imago at diflerent 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 sunset; but the great 

 body of those noticed by Beaumur did not appear till after 

 that time ; so that the season of different harvest is not better 



