By Leslie Mertz

On a cold August afternoon, Jay Buchner leads three novice fly fishermen — two men and a woman — through a mountain-ringed meadow and to the quickly flowing Granite Creek near Jackson, Wyoming. He studies the rushing water and sees fish rising to the surface in a slightly calmer section on the far side.

“OK now, what are you eating?” he asks the fish, as he scans the water moving past. He spots a few caddisflies, helps his students select tiny lures that approximate the insects, and then steps back to let them try out their new casting skills.

Leslie Mertz

After just a few minutes of fishing, one of the men reels in his first Snake River cutthroat trout, a striking fish with a bright red, gash-like stripe on its lower jaw. Shortly afterward, the other two students also bring in trout of their own. The students are laughing, and behind his bushy, white mustache, Buchner is beaming.

That excursion, offered during a three-day fly-fishing workshop organized by the American Wilderness Leadership School last year, was one of many Buchner has headed as a fishing guide. In that workshop and on many of his fishing forays, he takes time to talk about insects.

“In fly fishing, you can be successful to a certain degree knowing nothing about entomology, but if you embrace fly fishing and would like to improve your skills, knowing about insects and understanding their aquatic life cycles and their behaviors will help you do that,” he said.

For the most part, fly fishermen concern themselves with four main insect groups: stoneflies, caddisflies (also known to anglers simply as caddis), mayflies, and midges. That’s because these groups are a big part of a fish’s diets. Whether living underwater as aquatic larvae, rising through the water column as they metamorphose into winged adults, or floating on or flying near the surface, the insects are major prey food for trout and many other sport fish.

An adult Drunella grandis, also known as a spinner green drake to fly fishermen, is mimicked in this artificial lure with an “extended body fly pattern,” says Buchner who tied the fly. Photo by Jay Buchner.

Buchner prepares to return a Snake River cutthroat to the fast-moving water after a student has caught it on an insect-imitating fly. Buchner is both a fly-fishing guide and former member of the U.S. Fly Fishing team, which participates in international, Olympics-style competition. Photo by Leslie Mertz, inset courtesy of Jay Buchner.

Professional fly crafter Julie Nielsen adjusts a fly, tied by one of the participants in a workshop she conducted during the Fly Fishing School and Fair at the MacMullen Conference Center on Higgins Lake, Michigan, on June 12. Organized by the Great Lakes Council of the Federation of Fly Fishers, the event includes sessions on entomology and on tying insect-like flies. Photo by Leslie Mertz.

Nielsen’s gray drake spinner pattern uses thread cross-hatched over caribou hair to simulate the body segmentation of an adult early brown quill mayfly (Siphlonurus quebecensis).

Nielsen’s “salmon fly” mimics a stonefly with its salmon-colored abdomen. Photo by Leslie Mertz.

Besides insects, flies are designed to look like other invertebrates, including Mysis shrimp. Nielsen crafts this version to match the shrimp that are a favored food of salmon in the so-called “Toilet Bowl” area of the Frying Pan River in Colorado. Photo by Leslie Mertz.

These flies, tied by Buchner, imitate the common burrower mayfly (Ephemera simulans) and other mayfly species, including a great red quill (Timpanoga hecuba) shown in the inset photo, in either the adult or subimago/dun forms. The fly on the left is an extended-body brown drake pattern, and the right is a parachute hare’s ear pattern. Photos by Jay Buchner.

Rather than using actual insects for bait, fly fishermen usually opt for artificial lures, or “flies,” especially those that look like different insects. Crafted from bits of feather, fur, chenille, and other materials lashed onto a hook, ties can imitate the body size, shape, and color of an insect, including such details as the wings, the threadlike tails (or cerci), and legs. The two primary types of insect-mimicking flies are dry flies that imitate emerging or flying insects at the surface of the water, and nymphs, which are underwater flies that copycat immature insects, including mayfly and stonefly nymphs, as well as midge and caddisfly larvae.

“Imitator flies are specifically designed to look like whatever the fish may be keying on, because fish can be extremely selective,” said professional fly crafter Julie Nielsen, who is taking a break after wrapping up a fly-tying workshop in northern Lower Michigan. Although several types of insect may be present on a stream, she said, the fish may only be feeding on a certain life stage of one of them. For example, trout may be dining on what Midwestern anglers call emergers or dun Hexes, which are the newly winged, subadult (subimago) mayflies of the species Hexagenia limbata, so they’ll turn up their noses at anything but a near-perfect imitation fly.

“You can see it happen,” she said. “The fish scream toward the fly, and you’re thinking ‘Oh boy, oh boy, oh boy!’ but then – screech! – it stops and swims away. That’s how you know that something’s not right about your fly compared to the natural.”

When tying a fly, the most important parameter is size, Nielsen said.

“Oftentimes, the biggest mistake people make is to use too much material. Think about the tiny biomass of a midge. Just with the hook shank, the fly can be almost too big to start with. So you want to tie very light.”

The next criterion is silhouette.

“The adult or dun mayfly silhouette looks like a sailboat floating down the river; caddis wings are shaped like a pup tent when they’re folded together; and stonefly wings are stacked,” she said.

Finally, there’s color, which includes body hue and translucency of the wings.

Of course, crafting the perfect fly is just one part of successful fly fishing, according to Nielsen. The angler also has to be able to match the fly to the item fish are eating. One way is to spot a fish taking an insect, but that only works when the insects are large enough to see and are at the surface.

Another approach is to observe the so-called “rise form.” She explained that fish rise to the surface differently depending on what insect they’re after. For instance, fish are slower and more methodical when nabbing a newly emerging mayfly, because that insect must sit on the surface while its wings dry before it can take off. On the other hand, caddisflies don’t need the drying time.

“Caddis come racing up off the bottom of the river, break through the surface film, and keep on going. The fish, after years and years of evolution, know they have to move fast to get that caddis, so their rise form is a lot more splashy.” Astute anglers can pick up clues like that, she said.

Knowledge of insect behavior is also critical, according to Michael Parrella, professor and chair of the Department of Entomology and Nematology at the University of California Davis, an avid fly fisherman, and author of “A History of the Entomology of Fly Fishing,” which appeared in the January 2013 issue of American Entomologist. For instance, often after casting, the angler has to “mend” the line, lifting and looping the line upstream to allow a fly to drift naturally in the water. Without mending, the line moves downstream past the fly and tugs it downstream.

“If you don’t have a perfectly naturally drifting nymph, for instance, I don’t care how good your fly is, you’re probably not going to take fish,” he said. “I think 90 percent of the battle is delivering that fly: casting, mending the line, and mimicking the behavior of the arthropod. The fish are keying on the behavior of the insects, so you really have to know what the insects are doing, whether they’re on the surface of the water or under the water.”

Fly fishermen certainly learn about behavior from the work of entomologists, and in addition they have made contributions of their own, Parrella said. Back in 1971, Carl Richards and Doug Swisher wrote Selective Trout, followed by Emergers. The two fly fishermen not only observed insects in their natural environment, but they also collected, reared, and studied insects in aquariums.

“That approach opened up a lot of eyes, and that kind of work continues now, as fishermen use GoPros to go underwater and look at the behavior of the insects,” Parrella said.

Fly fishing is definitely a combination of skill and knowledge, and that’s what makes it so much fun, Buchner asserted.

“You have to identify where those fish are in a stream, determine which aquatic insect and what life stage of that insect they might be feeding on, select the correct fly, make a good cast and a proper delivery of the fly without spooking the fish with shadows, and then hopefully catch a fish. There’s a lot of craft to put together with the entomology.”

Fly fishing is a lifetime endeavor, he added.

“I’ve been fly fishing for some 60 years, and I still have an opportunity to discover something new every time I go out, whether it’s about the fish and they way they behave, or about insects and their life cycles and they way they behave. There’s always more to learn, and that’s what makes fly fishing so enthralling and absorbing.”

Leslie Mertz, PhD, teaches summer field-biology courses, writes about science, and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.