Over time anglers find methods to select a fly, whether it is a dry fly, nymph, or streamer. There is no such thing as the perfect fly, but an understanding of entomology can help identify insects in a particular stream. How do you know what insects inhabit the stream you are fishing?
Adult aquatic insects emerge from the water during a hatch and often stay close to the water to mate and lay eggs, making themselves available to land on your hat or be caught from the air. The term matching the hatch comes from selecting a fly that resembles the size, color and profile of the insects landing on the water that the fish are feeding on. Once you catch a fly, you match it as closely as possible to the flies you have in your fly box. Adult mayflies have upright wings that look like a sailboat while stoneflies and caddisflies have wings that fold down over their backs like a ramp.
Nymphs take a little more effort to collect and identify. Habits, superstition, recommendations from fly shops, or a wild guess often can lead an angler to select a nymph. Managing weight for the velocity and depth is critical to making a desirable presentation, but otherwise, an angler has a myriad of choices for the size and color of a nymph. Experts will often recommend reaching into the stream to dislodge a rock approximately the size of your hand, to lift it out of the water to see what insects are on the underside of the rock. In a healthy stream, rocks in riffles will be covered with insects of various sizes and shapes.
Caddisflies typically build cases and attach them to the rocks using a silk substance they produce. Insects without protective cases have body shapes that are adapted for a life in moving fluids. Flattened body types are suited for clinging to rocks in areas of higher velocities. Rounded bodies are adapted for slower moving water and aid the insect in swimming to navigate the water.
Inspecting the submerged stream rocks provides the opportunity to pick nymph flies that match the size, color and shape of what is present in the stream. Historically successful flies like the pheasant tail or hare’s ear mimic the color, shape and filamentous gills of many aquatic insects. In my experience, insects closely resemble the color of the stream bed rocks. It is important to note that macroinvertebrates available as a food source to fish don’t just live in riffles. They are present in slow back eddies, pools, runs, log jams, and aquatic vegetation.
Many states collect data on stream health, Maryland has a wonderful program, The Maryland Biological Stream Survey. The Department of Natural Resources collects data statewide on fish, macroinvertebrate, habitat and water quality to evaluate stream health. Their website also curates data from trained Stream Waders at more sampling stations. This interactive data set can provide an idea of all the macroinvertebrates sampled within the various stream habitats. In addition, crayfish, baitfish and terrestrial insects also can be a preferred food choice at any time.
One of my most successful flies is a mop fly and the prince nymph is a fantastic fish catcher, and you’ll never see a bug that looks like either of those flipping over a rock in a stream. Attractor type flies can work in any setting and anglers catch many fish without any knowledge of aquatic entomology.
It’s not necessary to flip over rocks to match nymphs to catch fish. Like tying flies, I think knowledge of aquatic insects has increased my choices, strategies, knowledge, and enjoyment of fly fishing. In some cases, the insects identified on the rocks may not easily be dislodged to be available for fish to eat. If matching the nymphs builds confidence in your fly selection or provides knowledge that is helpful for another time on the river when those insects may be hatching, then it is valuable. Learning more about the streams we fish makes an angler versatile and connected to the ecosystem. Anything that builds that connection seems like a good idea to me.