Returning mayfly spinners or egg-laying caddisfly adults can cause intense feeding by the trout, but this situation can be misleading for fly fishermen. Because the spent insects are lying prostrate on the surface, nothing sticks up above the water, and they’re difficult to see. The secret is to look up. Aquatic insects can hatch over spread-out periods of time, but they must all mate at the same time. They form mating swarms, which hover and dip above the stream, starting at treetop level and gradually working their way down to the surface of the water. (See the video below.) If the flies are still pretty high and the fish are rising, they’re probably rising to something else; but when the flies get lower, peer closely at the surface. You should be able to see the dying flies lying with their wings spent, half spent, or fully upright.
Trout may prefer the spinners with either upright or fully spent wings—I’ve never seen them actually selectively feeding on half-spent flies. If you can’t see what they’re taking, it’s probably a spent spinner; if you see flies disappearing into the rises, they’re taking the insects whose wings haven’t collapsed yet. The one hallmark of fish taking spent flies is a very steady, deliberate rise. The trout seem to sense that the flies won’t get away and they can take their time. The only exception is at the very beginning of a fall of spent insects, when overeager trout (usually smaller ones) slash at the flies just as they touch the water. Small trout may even clear the water in an attempt to catch the flies in midair—a popular theme with calendar art, but in real life the big fish wait until the flies are trapped in the surface film and are an easy meal.
Mayfly spinners with wings that are still upright are easy to match—just use a standard adult pattern of the correct size and color. After the flies are spent, though, not only are size and color critical, but the fly must lie flush in the surface film, too. This is the time for a spentwing dry fly, such as a hackled spinner or polywing spinner. In a pinch, you can also trim all the hackle from the top and bottom of a standard dry fly with a pair of scissors or your angler’s clips.
Excerpted from The Orvis Fly-Fishing Guide, by Tom Rosenbauer.