Flies for Trout:
I think most fly fishers already know that aquatic insects make up the vast majority of a trout's diet. We also know that there are three primary insects that comprise this diet. Generally speaking, they are the mayflies, the caddisflies and the stoneflies in that order.
We also know that there are multiple life stages for all three of these aquatic insects and that each of these life stages is important to the trout and therefore the fly fisher. To understand more about these life stages, visit the entomology pages located here...
Nymphs vs. Dry Flies: Most trout fly fishers agree that trout consume about 80% of it diet underwater in the form of nymphs, pupae and varied other morsels. However, I sometimes think there is more attention paid to the adult winged versions of the aquatic insects...what we call "dry" flies. I suspect that it is because for most fly fishers it is just more fun seeing a trout take a fly off the surface...you have a real visual connection as opposed to an unseen tightening of the fly line. Personally, as a trout guy I do just about the opposite of what the statistics suggest I should...I spend approximately 80% of my time on the water casting a dry fly. I will use a nymph or a nymph as a "dropper" only when I do not think a dry fly will not work. I will readily agree that I would have caught a lot more trout using a nymph. The excitement and anticipation of watching your dry fly disappear in a swirl is just too much for me to overcome...so I just embrace it.
Mayflies: Of all the aquatic insects, mayflies are probably most important to most trout fly fishers. This is because they are abundant in almost all trout environments and because they are always an available food source both as nymphs under the surface and as winged adults on the surface. It is widely accepted by fly fishers and fish biologists that a trout consumes approximately 80% of its diet under the surface so flies imitating nymphs are very popular.
Caddisflies: Caddisflies are often not given the respect they deserve. I am probably somewhat guilty myself. I do not know why this is. Virtually of the western trout waters I fish have numerous and consistent caddis hatches. Don't get me wrong, I do fish them a lot...mostly early and late in the day...especially late. I will typically use a #16 spent partridge caddis late, during the day I like the Goddard caddis and the x-caddis...in fast water I like the elk hair caddis at least one size larger than what I see on the water. If I am anticipating a caddis hatch, I will invariably go to some version of the LaFontaine sparkle pupae. I generally do not apply floatant as I do not care if the fly sinks. When it does sink, I let it swing through the drift and as it gets to the end of the drift it will automatically rise back to the surface thus imitating the natural movement of a caddis pupae rising to "hatch". This can be very effective.
Stoneflies: If you have never had the opportunity to fish a stonefly hatch, you have missed a truly amazing experience. It is like the fish have lost many of their natural instincts. I have always thought that fish that rarely rise to any dry fly just cannot help themselves. There is nothing delicate about they way trout attack the adult stonefly. Presentation is rarely an issue. As discussed in the entomology section, most stoneflies migrate to the banks or protruding rocks where they crawl out and undergo the final molt to become a winged adult. So in the early days of the hatch, it is most beneficial to fish relatively close to the bank. The stonefly nymph is available to the trout all year long so can be productively fished at any time.
Terrestrials: If there was a rule that you could only have one fly, I may have to consider a #14-18 black beetle as my choice. This is especially true late June through September on virtually all of the western waters I fish. I somehow think that when a trout looks up and sees a beetle, it's tiny brain says "banana-split". I cannot count the number of times I have encountered especially selective trout during a good mayfly hatch that just seem to reject everything I offer. When this happens, I will most often tie on a black beetle to change things up. The largest rainbow I ever landed on the Henry's Fork (25"-26") was taken on a #16 black beetle right in the middle of a really good PMD hatch. I had fished to this fish for 4-5 days in a row and literally tried everything in the box. The fish was tucked in a narrow channel between a large boulder and a weed bed. All I ever saw was just a nose and a white mouth. In the hundreds of PMDs I watched the fish take over those days, I never once saw him move more than 3-4 inches. When I saw that my first cast with the beetle would be at least 6-8 inches outside the small feeding window, I started to pick it up and recast. Because the fly was within 3-4 feet of the fish, I smartly just let it float by as I did not want the fish to spook.To my amazement, that trout actually turned to the side and moved almost a foot and ate the beetle in the most aggressive move I had seen that fish make in the days I had been watching and casting to this big boy. I am also a big fan of ants and grasshoppers especially on hot windy days when there is little in the way of mayfly activity.
Streamers: I readily admit to not being a big streamer guy. I do fish them under certain conditions especially on dark stormy days and often in heavier water. My favorite is either a weighted black/yellow marabou leech or some variant of a muddler minnow. As I am not a fan of heavily weighted flies, I will almost always use a sink tip line and the shortest leader I think I can get away with.
Nymphs - Approximately 80% of a trout's diet is nymphs...
Note: If you are new to tying flies for trout, remember the life cycle of almost all aquatic insects...they spend a whole year or more as nymphs and only a couple of days in the flying adult stage(s).
Hare's Ear - this is one of the more basic nymphs. It imitates mayfly
nymphs in general, it has many variations.
Article (pdf) Tying the Gold Ribbed Hare's Ear
Article by Flys and Guides.com How To Tie Gold Ribbed Hares Ear
Copper John - this is another basic nymph with many variations.
Article by John Barr (Midcurrent) Tying the Original Copper John
Stonefly nymph - here again is another basic nymph for trout. Keep in mind
that if there are no stoneflies in the water you fish, this pattern will do you
Article by Flymen Fishing Company Evolution Stonefly
Video by Phil Monahan (Orvis) How to Tie the Pat’s Rubber Legs