In fact it would seem that when the duns are hatching many, perhaps most, rises are to the floating nymph and not to the winged insect. Autopsy shows a marked preponderance of larvae and nymphae about to change to the winged state over winged insects in the stomachs of trout taken under natural conditions. Furthermore, I might quote Mr. Halford when, in discussing bulging trout, he says: "Fish, when feeding on larvae and nymphae, at times rise quietly, without moving about from place to place. It is almost impossible under these circumstances to distinguish the apparent from _bona fide_ rises."
All of which does not militate in the least against the theory that the artificial fly should correspond with the natural; when a decided hatch is on the trout are fully aware of the nature of the prevalent fly and if feeding upon it are quite likely to notice no other either natural or artificial. But the theory does, indeed, explain some phases of dry fly fishing which otherwise are quite inexplicable; for instance, inability to induce a rise to the properly fished winged artificial when its corresponding natural is hatching and "apparent" rises to it are evidently common. It seems fairly certain that at such times the fish are feeding exclusively on the floating nymphs, taking them on the surface in practically the same way as the winged dun is taken. Also fish thus feeding would hardly come within the technical definition of bulging trout as that term is generally understood.
Having assumed the first winged state, scientifically the sub-imago, the duns thereafter are upon the water more or less during its continuance, sometimes blown upon the water or descending to the surface without apparent reason, and the trout rise to and feed upon them when in the mood. In a short time the dun or sub-imago undergoes another metamorphosis to the imago or "spinner"--the adult insect. The male spinners are subsequently upon the water in a spent or practically lifeless condition following the completion of the act of coition. The latter takes place over the stream to which the female spinner then descends to void her eggs. This, with some species, is done upon the surface, the fly floating downstream the while eventually to rise again and fly, generally it would seem, upstream--unless the program is incontinently halted by the accurate rise of a trout. The spent spinners ("spent gnats") also serve as food for the trout--the male when it has completed the act of procreation and falls to the stream, the female when all the eggs have been voided.