Because it lives but a very short time in its perfect state ; often only for a few hours. Day-flies live in water some years as larvae. After that time, in many places, millions of the perfectly-formed insects make their appearance from the water, within a few days, in the middle of summer; they then also, contrary to what happens to other insects, cast their skin again. - Blumenbach.
Because of the diminution of rivers and streams, and the cultivation of the country. " Thus," observes Sir H. Davy, in his Salmonia, " most of the bogs or marshes which fed considerable streams are drained ; and the consequence is, that they are more likely to be affected by severe droughts and great floods; the first killing, and the second washing away, the larvae and aurelias. May-flies, thirty years ago, were more abundant in the upper part of the Teme river, in Herefordshire, where it receives the Cluer ; they are now seldom or rarely seen ; and most of the rivers in that part of England, as well as those of the west, with the exception of those that rise in the still uncultivated parts of Dartmoor and Exmoor, are often rapid and unfordable torrents, and in dry summers little more than scanty rills."