But when casting over a rising trout everything is certain and well-defined. You know where the fish is located, or at least where he came up; you generally have a pretty fair idea of his size; if duly observant you can guess closely to what sort of natural fly the fish rose, everything is sure save the eventual capture of that particular trout. You are fairly certain that if the right fly is put over the fish in the right way success will follow. It is up to you.
To cast with some understanding to a rising trout, it is very necessary that the angler be somewhat familiar with the habits of the fish when feeding upon the floating insect and also be fairly conversant with the life histories of what may be termed the fishing flies. That rises occur when the fish are not feeding, that sometimes the trout roll up to or leap above the surface, is well known to the experienced stream fisherman. With this feature of the matter we are not here concerned; the habit has been variously accounted for by anglers and icthyologists but the motive of the fish in thus acting is still debatable. However this may be, the angler may safely conclude that any visible rise--save generally a clear leap above the surface--is a rise to the natural fly by a feeding trout until the contrary may appear from the attendant circumstances.
It is with the _bona fide_ rise of a trout to the floating natural fly that the dry fly caster is chiefly concerned. But in this connection it should be noted that the feeding of trout upon the natural insect is by no means confined to the time of the latter's appearance strictly on the surface. Of the water-bred insects the _Ephemeridae_, called "duns" when in the sub-imago state, occupy the place of greatest importance in the entomology of the dry fly fisherman. In a later chapter something is said of the commoner insect life of the stream; it should here be noted, however, that trout feed upon the _Ephemeridae_, for instance, at all stages of their existence.
From the eggs deposited upon or in the water by the adult insect, or "spinner," in due time the nymphs are hatched. Upon these the trout feed at times on the stream-bed and in the weeds, nosing upon the bottom and in the aquatic vegetation in somewhat the same way as the common sucker or the German carp go about their business of drawing sustenance from the muck and weeds of the stream-bed. This habit of the trout, when followed in shallow water, results in an occasional disturbance of the surface by the tails of the fish and is called "tailing" in the nomenclature of the English dry fly fisherman. In this connection it should be noted, however, that the nymphs of the _Ephemeridae_ which burrow under rocks and in the stream-bed and there remain until about to assume the first winged, or dun, state are practically inaccessible to the fish, although doubtless taken at times. Tailing trout are usually feeding upon caddis and other larvae.