Basically, any imitation can look "a whole lot" like what we are trying to imitate or it can look vaguely "food-like". It can give a pretty decent impression of what we are trying to imitate, or it may look almost nothing like anything that has ever been alive. Realistic and impressionistic are not so much categories as ends of a continuum from ultra-realistic to what the hell is that? At one end are the flies tied more for art than for fishing, true pieces of craftsmanship that have hours invested in them. I have seen some gorgeous work where you first think it is alive. At the other end of the spectrum, think about the materials your six year old son or daughter might grab and lash on a hook. Like life, most things fall somewhere between the extremes.
To set our compass, let us talk about a few popular flies and where they fit along the continuum. The Woolly Bugger is the classic impressionistic pattern. What does it represent? Food! I've heard people talk about or read people write about buggers imitating everything from hellgrammites and Dragonfly nymphs to minnows and crayfish. Quite simply, they are that the classic example of that overused word, "buggy". While there are not a ton of commercially available flies that are near the realistic end of the continuum, maybe the thing many of us have seen at a Wal-Mart or similar big box stores are those rubber molded nymphs.
There are certainly a few other commercially available flies along the more realistic end of the spectrum. Grasshopper patterns seem to be where we see this distinction between realistic and impressionistic is best represented in fly shops' fly bins. Patterns range from quite impressionistic - the Chernobyl family of flies and stimulators - to quite realistic, like Tomsu's Supreme Hopper. The popular Morrish Hopper seems to strike a great balance. With the cutters, it is relatively quick and easy to tie, it provides a very accurate silhouette, on a nymph hook, its density is pretty true to life, and the rubber legs give the fly some inherent movement.
Many of us that have thrown Panther Martins or similar spinners so we understand that what you are throwing does not have to look that much like food to be successful. Nobody is going to going to call a spinner a very accurate representation of a minnow. Certainly a crankbait or soft plastic looks a lot more like a minnow. But there is simply something about the spinner and that thumping vibration that turns fish on. The vibration of the spinner certainly has an interesting appeal to trout, it seems primal. Datus Proper, in his most excellent book, What the Trout Said, in his study of what trout eat and why writes, "The spinner was not food, but it provoked an aggressive reaction." I know when I have fished spinners, trout want to kill the thing! However, after the first few casts in a new spot, trout tend to stop chasing and trying to kill it. It is like its act wears thin and fish are on to the not-so-realistic looking imitation.
To stick with Datus Proper's thoughts on flies, he prioritizes behavior, then size, shape, and lastly, color. Like Vince Marinaro before him, in his study of what trout eat, In the Ring of the Rise, Proper found that trout don't see things as we do. Marinaro's thorax style has a very prominent wing, half again taller than the natural. His thoughts were that his was a trigger. Trout key in on the wing and while the fly doesn't look natural to us, it drew more strikes than flies with shorter wings in Marinaro's rather scientific study of what trout are looking for in our dry flies.
Why More Realistic Isn't Necessarily Better
Looking at Proper's list - a list you would also see pretty closely followed in Gary Borger's book, Presentation - behavior is the first characteristic listed. Does the fly I'm fishing behave similarly to natural prey items I'm trying to imitate? This was a lesson I learned on Colorado's Fryingpan River during a hatch of Red Quills, PMDs, and Western Green Drakes. I had tied a ton of flies to imitate these hatches from top to bottom. Fish were eagerly rising but not to my imitations. After a bit of observation, it was evident that trout were not eating the dead drifting mayflies but they were chasing the more active flies. My standard imitations didn't skate so well, so I tied on a fly I knew would move on the surface better when fished downstream to the trout, a CDC and Elk, my favorite caddis pattern. Until the hatch ended 20 or so minutes later, it was fish after fish, cast upon cast. My fly didn't match the shape or color but it was a decent approximation of the size of the hatching flies but more importantly, I was able to mimic their erratic behavior.
It is a rather indirect path to demonstrate that behavior matters, a lot! Typically, as you move towards the more realistic end of the spectrum, movement - an important part of behavior is sacrificed. While movement can certainly be induced by the angler, often the fly tyer can put movement in their flies by the choice of materials. CDC, marabou, soft hackles, soft dubbings, and other materials freely move in the water. Water currents or the air move them. They pulsate when our streamers are stripped and then come to a rest. Grizzly and barred patterns are thought to give the illusion of movement and hence their popularity in many flies - dry and wet.
Many of my favorite flies tend to fall more on the impressionistic end of the spectrum. Sparkle duns, CDC and Elk, woolly buggers or leeches, Hippie Stomper. I think of nymphs as thin bodied or thick bodied - they are impressionistic of different types of aquatic prey. Certainly a Perdigon type fly doesn't look much like a caddis larva or mayfly nymph but they are slim, lack bulky appendages, and are heavy so they get down quickly to where many of the fishes are looking for food. They certainly do not look all that impressive in your hand or in fly bins but they match one critical behavior - most nymphs are on or near the bottom. My favorite flies tend to be quicker and easier to tie, a plus in my book. Most of them use materials that move easily - marabou, CDC, and rubber legs and/or they're tied with barred / mottled materials. They're fishing flies. I like fishing flies.
Where do your favorite flies fall on this continuum?