Theories are just that, they cannot be proven to the point that make them fact. Theories also may not be valid 100% of the time. Over the past 45 years that I've been a fly angler & specifically a fly tier, I've developed my own ideas & theories about what works & what doesn't for the type of fishing that I do. Actually, this applies to most any fishing that I attempt, not just with flies.
I've been able to conclude only two things about what flies work & what doesn't. One, all flies will catch some fish, regardless of the design! Two, no fly works all the time! And although this is not a conclusion, there are fly patterns that will work the majority of the time, on specific waters.
When I sit down & tie flies, I usually have an idea what I want the fly to do. I attempt to use materials that impart action & movement to the fly. These would be various hairs, furs, feathers & some types of synthetics. I've always felt that a fly should have "breathing" qualities, and the materials used should be what adds this feature. I also want my flies to have a general shape & silhouette. For example, if I'm tying a baitfish pattern, intended to mimic a shad, then the fly should be generally the same shape as a shad. However, I also keep in mind that since this is a basic criteria, my flies may also represent other forms of prey.
Many years ago, I was fishing in a artificial only section of a small trout stream here in Maryland, and was having a great day fishing my Black Stone Nymph in a size 6. (See my other post about this fly pattern). I had caught & released a Brown Trout, a Rainbow, a Smallmouth & a Largemouth in this stream that day, on just that one fly. I'm fairly sure there are no large Stoneflies in that stream that my fly might be matching, but I had noticed that the grasses along the bank where full of crickets. It's possible, although I cannot verify it, that there may also be Hellgrammites in that stream, and certainly crayfish. It's not outside of possibility that this fly could be eaten by these fish as all of these prey types, since it generally has a size & shape, and even color that might be mistaken for any of them. Really doesn't matter what we think it looks like, as long as they eat it!
Many of the flies I tie, especially for bass or saltwater use, are tied in three basic colors. I use a lot of black, white & chartreuse. I also select these colors a lot when making & buying lures. They are my base selection. If I'm going somewhere to fish, I'll often start with one of these colors or a combination. They all go well together & can both compliment & contrast with one another. Plus, they represent the range of color from white at one end, to chartreuse at the mid point, to black at the other end.
I also use a wide range of colors, particularly when tying a specific pattern type, but these three have always worked well for me, under a wide range of conditions. I often combine other colors with my three base colors. For example, most any baitfish pattern will include a white belly. Then, I'll add various green, grey or sometimes brown backs. I may also add other colors, such as a black lateral line or tones of blue, purple or pink, which many baitfish exhibit. I'm not convinced that these embellishments are absolutely necessary to catch fish, but when tying it's hard to resist the temptation to be artistic!
One of my favorite baitfish patterns is a simple streamer tied with hair, either bucktail or fox fur, and is nothing more than a white belly, a thin black lateral line and an olive back. I usually add some flash too! I like flash added to most of my baitfish flies. This fly can imitate a variety of baitfish, and is often called a Baby Largemouth Bass pattern. Substitute a bronze brown color for the olive & it would be a Baby Smallmouth. Substitute grey for the back, and it becomes many other baitfish or fry imitations. Again, substitute chartreuse, and you have one of the most popular & productive attractor patterns you'll find. The use of all of these color combinations generally mimic baitfish, and the colors both compliment & contrast. In some water conditions, especially where you may encounter stained or slightly muddy conditions, these combinations allow for the fly to be more visible to the fish. Often the light or dark color will prevail, and anytime you can use a fly that is more visible for conditions, your chances for success should be improved.
Generally, I'm an advocate of big flies for bigger fish. This is not an absolute, as there will always be exceptions, but the majority of the time, I've found this to work. If you compare other forms of fishing & the size of the lures used to flies, it only makes sense to use a bigger fly. For example, when other anglers are having success catching bass on lures that are 4 inches long, why would a fly angler choose a fly that is half that size. I tie many of my bass flies on big straight shank worm hooks sold for use with plastic baits. They're a much larger hook than most hooks sold for tying bass flies, and in my opinion, stronger, often sharper & I like the wider gaps which allow a better hook up ratio. The only con to using such hooks is the price & the sizes readily available. I've quit worrying about the price, even if I'm paying 60 or 70 cents a piece for these hooks, as the overall cost of the finished fly is still well under what a fly would cost if purchased from a shop. I tie many of my flies in sizes 1/0 & 2/0, which are not always the easiest to find, since 3/0, 4/0 or 5/0 are much more popular for plastics. I browse Ebay quite a bit & also the clearance listings for many of the popular bass shops, and purchase these hooks whenever I can get a good deal.
When I first started fly fishing, my father purchased an 8 wt Cortland fiberglass outfit for me. It was a very slow action rod, and much more than was needed for the panfish I caught. But, as a bass rod is was fine. An 8 wt is a good general purpose size rod for bass fishing and I still have an 8 wt in my current arsenal of rods. However, because of the large flies I like to use, and because I often fish in some backwaters of the many tidal creeks along the Chesapeake Bay, I use even heavier rods for bass than most others might use. I have a 9 wt & a 10 wt, both well suited for tossing big flies, and even more so when fishing around the many fallen tree's or spadderdock patches found in the tidal rivers. I have no qualms about casting into the thickest of the thick, and into places others may cast weedless jigs. These are the places you'll often find the bigger fish, and the only way to get to them is go in after them. I lose some flies, but I catch some decent size fish too!
So there it is! My basic theory for fly fishing.
1) Tie with materials that provide movement.
2) Utilize the base colors of black, white & chartreuse.
3) Use big flies, tied on big, strong & sharp hooks!
4) Use heavy enough tackle for both the flies & conditions!
5) Go into the thick stuff to find the bigger fish!
If you're happy fishing small flies for bass & catching the occasional larger fish, then stick with what you're doing. I've certainly caught plenty of bass on my 6 wt, and at times that's plenty of fun to do! You won't catch as many bass doing what I do. But, if you want to catch more quality fish, then give this a try!