WARM SPRINGS, Ore. — The sky was not exactly dark in a blotting-out-the-sun sense, but the salmon flies were certainly thick above central Oregon’s Lower Deschutes River. Thousands of female specimens circled 30 feet above the water’s surface, preparing to descend and drop their eggs. Occasionally, a bug would spiral slowly down to the river, flutter awkwardly on the surface, then disappear in a sudden splash.

The Deschutes’s native rainbow trout take notice of the salmon flies’ arrival among the river’s rimrock walls. And anglers do, too.

For most trout enthusiasts, dry fly fishing represents the pinnacle of the pastime. Mayflies, caddis flies and a host of other minuscule insects emerge from the river bottom and make their way to the surface. The trout, which have been feeding on the nymphal forms of the insects lower in the water column, shift their focus to the top, where the bugs float at the whim of the current. Observant anglers can identify the kind of bug that is emerging and choose a fly pattern that emulates the natural insect — matching the hatch, in their parlance — then, if all goes right, enjoy the visceral thrill of watching the trout rise and take the fly.

The challenge, at least for anglers of a certain age, is that many aquatic insects and the flies that imitate them are considerably smaller than a pinkie nail. Identifying one’s fly on the water among the many natural bugs — or tying it on the line — can be frustrating.