middle of the day, in bright days, or when there is sunshine. These are
generally black, and they escape the influence of the frost by the
effects of light on their black bodies, and probably by the extreme
rapidity of the motions of their fluids, and generally of their organs.
They are found only at the surface of the water, where the temperature
must be above the freezing point. In February a few double-winged
water-flies, which swim down the stream, are usually found in the middle
of the day, such as the willow-fly; and the cow-dung-fly is sometimes
carried on the water by winds. In March there are several flies found on
most rivers. The grannam, or green-tail-fly, with a wing like a moth,
comes on generally morning and evening, from five till eight o'clock,
A.M. in mild weather, in the end of March and through April. Then there
are the blue and the brown, both ephemerae, which come on, the first in
dark days, the second in bright days; these flies, when well imitated,
are very destructive to fish. The first is a small fly, with a palish
yellow body, and slender, beautiful wings, which rest on the back as it
floats down the water. The second, called the cob in Wales, is three or
four times as large, and has brown wings, which likewise protrude from
the back, and its wings are shaded like those of a partridge, brown and
yellow brown. These three kinds of flies lay their eggs in the water,
which produce larvae that remain in the state of worms, feeding and
breathing in the water till they are prepared for their metamorphosis,
and quit the bottoms of the rivers, and the mud and stones, for the
surface, and light and air. The brown fly usually disappears before the
end of April, likewise the grannam; but of the blue dun there is a
succession of different tints, or species, or varieties, which appear in
the middle of the day all the summer and autumn long. These are the
principal flies on the Wandle--the best and clearest stream near London.
In early spring these flies have dark olive bodies; in the end of April
and the beginning of May they are found yellow; and in the summer they