their maturity depending upon the temperature of the season, their quantity of food, &c.  They are at least six weeks or two months after they have spawned before they recover their flesh; and the time when these fish are at the worst, is likewise the worst time for fly-fishing, both on account of the cold weather, and because there are fewer flies on the water than at any other season.  Even in December and January there are a few small gnats or water-flies on the water in the 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 become cinnamon coloured; and again, as the winter approaches, gain a darker hue.  I do not, however, mean to say that they are the same flies, but more probably successive generations of ephemerae of the same species.  The excess of heat seems equally unfavourable, as the excess of cold, to the existence of the smaller species of water-insects, which, during the intensity of sunshine, seldom appear