Designing Trout Flies

by Gary Borger

Chapter 1: Designing Trout Flies

Fly tying has as its basis the intent of deceiving the fish by designing and creating imitations of its food organisms. To produce consistently successful designs, then, the fly tier must understand both the fish and its food organisms. In addition, the tier must have a good working knowledge of the materials used to construct the artificials; information such as color, texture, light transmitting or reflecting quality, and durability. And, the tier must be acquainted with a range of techniques for applying the materials as well as the angling techniques that will be used to fish the fly. For example, if the artificial is to be skated on the surface, then it must consist of materials that represent the natural while simultaneously helping the fly to float, and it must be shaped in such a way so that it not only imitates the natural but will skate as well

I begin all my designs with the fish. Like other animals, they can see, hear, taste, smell, and touch. There are salmon fishers on the West Coast that take great pains to minimize human odor on their flies, but taste and smell are not of concern in the design of flies. Touch becomes important only if the fish has an opportunity to chew the fly before the angler sets the hook. Basically, trout and salmon are sight-oriented hunters that occasionally also rely on their hearing. It is these two senses that the fly tier must understand when striving to produce consistently successful designs.

What the Trout Sees

The photoreceptive layer, or retina, of the trout's eye contains both rod and cone cells. The rods form only a black and white image; the cones are sensitive to color. The quality of the image produced by the retina depends upon the packing of the rods and cones; the denser the packing, the more detailed the image. In the trout's eye, the cells are packed rather loosely, and the image is of what we would consider rather poor quality. In fact, recent scientific experiments indicate that the human eye has about 14 times the resolving power of the trout's eye (see Byrnes, 1990). And thank goodness for that. If its vision were as good as ours, the fish would never take even our most perfectly crafted flies because no real creature has a hook dangling out of its rear and a long chunk of monofilament sticking out of its nose. The trout's eye can detect relative size, overall silhouette (shape), and broad color patterns. But while the retina does not form a highly detailed image, it is extremely sensitive to contrast and motion. Such sensitivity can be crucial in fly fishing: a trout will often take a rather crude-looking dry fly if it's presented drag free but reject the same fly if it's dragging. And, such sensitivity can be significant in fly design. A trout may not recognize the exact shape of a mayfly nymph's gills, but it is highly attuned to the movements of these appendages.

The spring creeks near Livingston, Montana, provide an excellent laboratory for observing the ability of feeding fish to visually discriminate between the real and the artificial. At the beginning of the season, before they have been subjected to every deception of the fly fisher, the trout don't spend much time scanning a dry fly. If it's a relatively close match, they take it confidently. By the end of the season, however, they drift backward under both natural and artificial, watching closely for something, anything, that will give the fake away. If they had excellent eyesight, only a glance would be needed to tell the artificial from the real thing. But because the fish's eye forms a poor image, it has a difficult time separating the appearance of real from fake. In this process, the trout become extremely sensitive to drag. Let your artificial move in the slightest unnatural way and it's rejected instantly. Frequently, I've seen these trout reject a natural that fluttered or was blown off course by the wind. For the fish, eating becomes a real craps shoot. Numerous times, I've watched in amazement as a trout snatched a real insect off the surface, shook its head, and dashed off a short distance as if expecting to be hooked. The fish has to eat, but it's eyesight is too poor to separate a good imitation from the natural. For this reason, many of the late-season fish simply stop feeding on dry flies and concentrate on nymphs. The heavy angling pressure of the season has trained them to avoid dry flies.

Impressionism in Design

Because the trout's eye produces such a crude image (relative to our eye), highly realistic flies that satisfy the human desire for perfection in detail are not essential for consistent angling success. For angling purposes, the goal of fly tying, therefore, becomes to create an impression of the food organism, not a carbon copy of it. Fly fishers have wrangled with this concept for centuries. One of the earliest discussions that clearly put the idea of impressionism into perspective, so to speak, occurs in G. P. R. Pulman's book, Vade Mecum of Fly Fishing for Trout published in 1851. He describes the need to suggest size, color, and form which he states together... "constitute the character of the insect...," and goes on further to say that the character "...can be represented without counting the exact number of legs, or microscopically examining the fibers of the wings; on the same principle that, in individual portraiture, what is alone sought to be attained is not minute imitation but individual character and expression."

Perhaps the most eloquent and thoughtful arguments for impressionism in flies were put forth by Jack Atherton in his book The Trout and the Fly. Atherton focused the ideas of many writers; he gave us theory to explain our experiences. Like Pulman, Atherton drew a parallel between fly construction and the goals of portraiture; specifically the work of impressionistic painters such as Renoir and Monet. As the name implies, the impressionists were not interested in a photographic representation of the subject. Rather, they tried to capture the essence of the object: that which made it recognizable. Atherton applied these concepts to fly design. Instead of trying to create patterns that were exact copies of the insect, he sought to represent the essence of the insect; to create an impression of life. And always, it was the impression that the pattern made on the trout, not the impression that it made on the angler, that was important to Atherton. He said: "The flies used for so discriminating a fish as the trout should, first of all, have the appearance of life."

Scientific findings have reinforced the thoughts of Atherton and those before him, shoring up the empirical theories resulting from nearly five centuries of careful observations by the best original thinkers in fly fishing. In addition to investigating the visual acuity of trout, scientists have also studied the ability of the fish to respond to various traits of the food item. It is eminently clear from these technical experiments, as well as observations made by fly fishers, that the four characteristics which most strongly represent the essence of life, and therefore trigger the trout's feeding response, are size, shape, color, and behavior. Trout respond to these four traits of the food item whether feeding opportunistically or selectively.


Opportunistic feeding periods can occur at any time, day or night and are marked by a lack of insect hatches and no noticeable, regular feeding activity on the part of the trout. The fish are simply watching for and eating whatever food items come their way. And, while trout respond to size, shape, color, and behavior during opportunistic periods, the fish are not looking for a specific size, a specific color, and a specific shape all coupled with a specific behavior. During selective feeding they are.

When in the opportunistic mode, the trout samples any item in the water column that is suggestive of food: a piece of twig, a pollen cone, a cigarette butt, a strike indicator. But fish are far more likely to grab items that strongly suggest life.


Movement is the single characteristic that most strongly suggests life, and during opportunistic periods, is the primary trait that triggers the trout's feeding response. A moving pile of junk is often better than a perfectly crafted, but static, imitation. When I was ten, a friend and I were bumming along the stream near home, fitfully attempting to lure some stocked rainbows onto our lines. We began talking with a group of spin fishers who were having lunch along the stream. As a joke, one of them hooked a strip of banana peel on a Mepps spinner and flipped it out into the currents. A 27-inch brown immediately seized the undulating concoction and was summarily wrestled to shore before our coveting eyes and gaping mouths. It wasn't that the trout preferred bananas, it was the undulating movement of that strip that rang the fish's dinner bell. Thus, during opportunistic periods, the fly fisher could do a lot worse than to select a fly which has plenty of movement: fuzzy nymphs, soft hackle flies, the Strip Leech (my favorite), marabou patterns, and the like.


Color can be another important trigger during opportunistic times. Color vision in trout allows the fish to distinguish food items against the background color of the water. Called background space light, this color results from the scattering of light rays by the water molecules. In very clear water, blue light is scattered the most and the water appears blue (this is the same reason the sky appears blue). When free-floating algae (phytoplankton) are present, they absorb red and blue light and the background space light appears greenish yellow. The dissolved organic humus in tea-colored bog water absorbs the blue end of the spectrum, making the background space light appear reddish orange. Underwater flies tied with flashy materials (such as tinsel or Flashabou), iridescent feathers such as peacock herl, or a splash of fluorescent marabou or hair are very successful during opportunistic times because they stand out strongly against any of the background space light colors.

In addition, with increasing depth, water very rapidly absorbs all light. Red light disappears below about three feet; with increasing depth, orange, yellow, green, and blue are absorbed, in that order. In most freshwater systems, all light disappears by a depth of about 30 feet. Usually this is not of much concern to the trout fisher because most of the time the fish are feeding in water less than three feet deep. However, it can be of importance on those occasions when it's necessary to fish deeper water. Standard dyed red flies become gray and eventually black as they sink below three feet. But again, flashy materials (that reflect any available light), iridescent feathers (that refract any available colors), and fluorescent materials (that absorb available light and then convert and give off the energy as a different color) are all readily visible to the lowest limits of light availability.

Color can also be important for dry flies fished during opportunistic times. Patterns like the Royal Wulff and Adams are such superb fish getters because they are easily seen by the fish and look like something good to eat. Many times, I've had trout rise to my fluorescent orange strike indicator; during opportunistic times, a fluorescent spinner pattern can be a very effective fly.


Size can be a determining factor during opportunistic feeding. During these times the old axiom "big fly, big fish" was never truer. The fish are not concentrating on any one food item, they just want something to eat, and because a large item contains so many more calories than a small one, a big fly is often better than a small fly. But be warned, by a big something, I mean flies in the four- to six-inch range or longer. One does not catch small trout on them. And too, when you fish big flies, you don't catch large numbers of fish, but boy! when you do catch one, it will be exciting. If you suspect that big fish are not on the prowl (as during the middle of a bright sunny day), you may prefer to use smaller flies and enjoy the pleasures offered by more average-sized fish.


Behavior can also be significant in the non-hatch periods of opportunistic feeding. Skater flies, teased and danced over the surface, can produce explosive rises. Minnow and leech patterns are best fished with an erratic, jigging, twitching, jerking movement that suggests a crippled or weakened prey item. Obviously the design of the fly is important to the type of action trying to be imparted.

What the Trout Hears

During opportunistic times, fish will often use their sense of hearing in conjunction with their vision to find food items. The trout's lateral line mechanism is a very sensitive sonar device that runs along its flank and forward around its mouth and eyes. So sensitive is it that the fish can hear (feel) a nymph swimming or a dry fly drop to the surface. A blind trout placed in a tank with minnows can easily catch and eat them all, solely by the sound of their swimming. Fly designs such as the Marabou Muddler and Strip Leech are so extremely effective because they not only have a great deal of motion in them, but they send out attracting sonic vibrations that the trout can detect with its lateral line mechanism. Sound can also be significant when fishing dry flies during opportunistic periods. Splatting a big hopper imitation down next to an undercut, grassy bank on a breezy afternoon can sometimes produce startling results, and chugging a big mouse fly across the surface after dark can be absolutely smashing.

Dry or Wet?

Trout often move from sheltering lies and into the shallows at the edges and tail of a pool, up into the riffles, along the banks of deep runs, or near the shoreline of lakes to feed opportunistically. For the most part, the fish will be concentrating on underwater items for the simple reason that there are usually far more of them. So many times, during opportunistic periods, I'll use an imitation of a sub aquatic invertebrate that I know is prevalent in the stream or lake I'm fishing. It's a good strategy because it suggests to the fish a commonly eaten prey item.

Once, while fishing the high country of New Zealand with my friend Mike Allen, I located a very large brown trout lying in a foot of water and only two feet from shore. A feeding movement by the fish indicated it was taking nymphs, but I wanted to see that big brown come up for a dry fly. A half dozen perfect floats later I was sure the trout wasn't interested in anything on the surface. I replaced the Royal Wulff with a Hair Leg Nymph to suggest a locally prevalent mayfly, and on the first drift, the fish darted forward and grabbed the sunken imitation.

But by the same token, trout holding in feeding lies during opportunistic periods may be watching the surface and will respond very well to the dry fly. This is especially true when there is a continual dribble of various insect species floating by. If I'm fishing and see a mixed bag of insect flotsam, I'll often tie on an easily seen, buggy fly like the Royal Wulff or the Adams. And often the fishing is just great. In meadow streams the fish are often very surface conscious because terrestrial insects such as hoppers, beetles, and ants are always being blown into the water, and imitations such as the Bow Legged Hopper, Foam Beetle, or Para Ant are excellent during opportunistic periods.

Thus, during opportunistic periods, it's dealer's choice. Pick a fly you like. Fish wet or dry. Go with a fly for leviathan or chase the average fish. Angle in the daytime or haunt the dark hours. Be serious or make a light time of it. You'll usually find fish to fit your mood.

Selective Feedings

Selectivity is a different story. Selective feeding occurs when there is a great abundance of one food item in or on the water. An insect hatch most often produces selective feeding, although a great abundance of non-insect organisms such as scuds or snails can occasionally cause the fish to become selective. During such periods, the fish feed heavily on the prevalent food organism and ignore all others.

Understanding Selectivity

At first, selectivity seems like a trick of mother nature. Why should the trout be locked in to eating tiny insects and pass up a juicy minnow or perhaps an occasional, larger insect? After all, survival for any wild animal requires that it maximize food intake while minimizing energy expenditure. Upon careful examination, however, it's clear that selectivity allows the fish to do just that. Instead of sampling everything that comes along (and wasting energy examining non-food items), the trout sticks with a prevalent, known food source; in a short time with very little energy expended, the fish can fill its gut.

Selectivity can cause the fly fisher great pains, however, because the fish become so tightly fixated on a single food item. Not just a particular type of insect, not just a particular species of a particular type of insect, but a particular stage of the life cycle of a particular species of a particular type of insect. For example, it's common for trout to lock in on the emerging mayfly dun, or egg laying damselflies, or diving caddis, and so on. Selective feeding periods are therefore the time when the correct fly design is essential. But if the match is correct, it is also the time when the angler has the greatest opportunity to take fish.

Since the 1800's, when the dry fly came into vogue, there have been fly fishers who have suggested that trout are more selective to dry flies than to nymphs, implying for the floating fly mystical properties that simply do not exist. When a trout is selective, it is selective, period. I've seen trout on spring creeks in Pennsylvania, Montana, Colorado, Idaho, Wisconsin, and other places feeding exclusively on nymphs just under the film. They would accept only the correct fly presented in only the correct manner. Nothing else would do. It's not that trout are more selective to dry flies, it's just that we as fly fishers have not always clearly understood, nor represented in our dry flies, what the trout sees when it is looking up at insects floating on the surface. In recent years, however, much of the dry fly puzzle has been unraveled, and we now better understand how to dress floating flies to fool highly selective, surface-feeding fish (see Gary LaFontaine's book "The Dry Fly").

Nor is selectivity a matter of choice for the trout. The fish doesn't think "Well today I'm going to eat only size 18 Baetis emergers - that ought to drive the anglers crazy." The animal mind doesn't work that way. Rather, as the fish feeds on the very abundant food organism, it becomes entrained on the prey's characteristics. Selectivity, therefore, is not a conscious decision for the trout; it is a trained response. Such training in animals can be very precise. For example, researchers once taught chickens to work as quality control inspectors in a vitamin manufacturing facility, selecting out imperfectly coated pills. But the birds worked too well, removing every pill except the absolutely perfect ones. They were unable to make independent judgments on the degree of imperfection. The pills were either perfect or not perfect. It's the same for the selectively feeding trout. The fish cannot consciously overrule its training. Once locked onto size 18 Baetis emergers, the fish is unable to select anything else.

Thus the fly fisher should not think of the trout as a sly, crafty opponent, but rather as a creature that is a victim of its own genetic programming. To truly understand selectivity, then, we must understand the characteristics of the food organism that entrain the fish's feeding response.

Triggering the Fish's Selective Feeding Response

Selectivity is basically a shallow water phenomenon, and so the fish can clearly see the four major characteristics of the food organism: size, shape, color and behavior. During selective feeding, one of these characteristic (the primary trigger) usually trips the fish's initial, investigative response. Some angling authors have suggested that one of the traits (size, for example) is always the most important and have even ranked the four characteristics in order of importance. In reality, any one of the four characteristics may serve as the primary trigger; it is usually that characteristic which is most obvious or unique in the food organism.

But while one trait serves as the primary trigger, the other three (the secondary triggers) must also be present if the fly is to be consistently successful. For example, some caddis will run over the water after hatching or during egg laying. Trout crash after them recklessly. In this circumstance, the behavior of the insect is the primary triggering characteristic. But while such fish will readily investigate any dragging fly, most will refuse it unless it is also the correct size, shape and color. Then again, when mayfly spinners fall heavily, the trout become fixed on the shape of the spent flies and steadfastly reject any other shape. Obviously, shape becomes the primary trigger in this instance. But again, if the fly is of incorrect size or color, or if it is dragging rather than floating dead drift, the fish will usually refuse it. During hatches of bright green midges, the fish become quite sensitive to the strong chroma of these insects and scrutinize any item of similar color. However, once the fish gets close, it also wants to see an object of correct size and shape doing what midges are supposed to be doing. If not, the fish rejects the artificial. Size can also be the primary trigger. When the Lilliputian Trico mayflies are on the water, the trout want something of definite diminutive form, but they also expect to see the other three characteristics.

It is this primary trigger, secondary trigger sequence that often causes the trout to reject the fly at the last possible moment. Some authors have suggested that the trout's eye sight is so bad that it affects the fish's aim - in other words, the trout tries but simply misses the fly. Well, just take a few moments to watch a trout feed on naturals and you'll realize that they don't miss. No, a "false rise" is a last-second rejection by the trout. The animal's eye achieves maximum resolution when very close to the imitation, so as the fish draws near, it is able to carefully examine all the traits of the fly. If one or several characteristics of the pattern are incorrect, the trout refuses it. Such false rises should be a tip off that one (or more) of the secondary triggering characteristics is incorrect.

The trout's ability to discern both primary and secondary triggers may be affected by water type. In swift, choppy currents, the fish has little time to examine the fly and cannot see details very well; on flat, slow currents, the trout can examine the imitation carefully and leisurely. For this reason, bushy dries that would fail horridly on the spring creeks can be a raging success on the Madison. So, I design my flies for the most difficult angling situation that I expect to encounter: big, spooky, wild trout in smooth-flowing, crystal-clear waters. Flies that will take trout under these conditions will take trout anywhere. That is why I may also spend an hour or two trying to fool a particularly difficult fish rather than giving up and going on to an easier one. It's the difficult ones that teach us the most if we pay close attention and can refrain from becoming frustrated.

In addition to the primary and secondary triggers, there are other characteristics of the organism that should be considered. What stage of the life cycle is being imitated? How does it look from the fish's level? What presentation tactic will be necessary to ape the behavior of the natural?

When all the questions about primary and secondary triggers and other traits are answered, it's time to consider the materials and tying techniques to best suggest the food organism while still satisfying the needs imposed by the presentation method.

In the chapters on specific food organisms, I've discussed the attributes that trigger the fish's feeding response and how I've used this information to develop my approach to fly design. I've also listed most used sizes and colors of the various designs; use this information as a starting point, then refine existing designs or develop your own as you become familiar with the food organisms on the waters you fish.

Editor's note: This was chapter 1 of Gary Borger's book, Designing Trout Flies. You may purchase this and other books directly from the author at Tomorrow River Press, below.