162 INDIRECT BENEFITS DERIVED FROM INSECTS. 



Our rivers abound \v\thfah 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 Trichoptera, 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 

 Ephemera. 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. 



Reaumur- 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 

 vear 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 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 sunset ; 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 fisherman. Yet 

 a greater degree of heat or cold, the rise or fall of the water, and other cir- 

 cumstances we are not aware of, may accelerate or retard their appearance. 

 Between the 10th and loth of August is the time when those of the Seine 

 and Marne, which Reaumur described, 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 a night; " alluding, by 

 this expression, either to the astonishing quantity of food which the 

 Ephemerae afford the fish, or to the large quantity of fish which they then, 

 take. 



Reaumur first observed these insects in the year 1738, when they did 

 not begin to show themselves in numbers till the 18th of August. On the 

 19th, having received notice from his fishermen that the flies had appeared, 

 he got into his boat about three hours before sunset, and detached from 

 the banks of the river several masses of earth filled with pupa?, which he 

 put into a large tub full of water. This tub, after staying in the boat till 

 about eight o'clock, without seeing any remarkable number of the flies, and 

 being threatened with a storm, he caused to be landed and placed in his 

 garden, at the foot of which ran the Marne. Before the people had landed 

 it, an astonishing number of Ephemeras emerged from it. Every piece of 

 earth that was above the surface of the water was covered by them, some 



