162 INDIRECT BENEFITS DERIVED FROM INSECTS. 
Our rivers abound with jish 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 
Ephemere 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 
erfect state, is usually extremely short, some being disclosed after sunset, 
aying 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 Ephemere 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 15th 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 
Ephemere 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 Ephemere emerged from it. Reka piece of 
earth that was above the surface of the water was covered by them, some 
