Tl;dr Browsing for bugs, online or in-store, can seem like an overwhelming undertaking at first glance. Flies come in what feel like an infinite number of shapes, sizes, and colors. Further, each of these iterations has names in English and Latin, and some with names that have no basis in the animal kingdom whatsoever (I’m looking at you Kelly Galloup!).

At its simplest, a fly is an imitation of a part of a fish's diet. To understand what flies you need is to understand what, where, and when fish are eating. For the purposes of this article, we will mostly be looking at the diet of a trout. This will require us to take a look at the main bugs on the menu: mayflies, stoneflies, and caddis. After doing so, we will be able to boil down everything you need to know about flies into the three main categories: nymphs, dry flies, and streamers.

The Lifecycle of A Bug

What came first: the flying, aquatic insect or the egg? For the sake of working in the same direction, we’ll start the cycle of each of the following insects as an egg.

Mayfly

After hatching from the egg, the mayfly enters its nymph stage. This is the longest stage of its life and many will spend up to the next year living on river bottoms, clinging to rocks and vegetation, or burrowed into the mud. When the conditions are right—typically in a window of a few weeks during a year—the nymph begins to emerge to the surface.

Once the mayfly has reached the surface and sheds its nymph casing, it begins the “dun” phase of its life. If it’s lucky enough to avoid detection from a trout as it awaits its wings to harden and take flight, the mayfly will then take to land and spend the next day feasting and preparing to mate.

The following day, once it has shedded it dun shell, the mayfly enters its final stage of life as a spinner. The male and female mate above the water's surface and once their procreation drives have been met, fall to their final resting place to have one final opportunity to become fish food.

Stonefly

Similar to the mayfly, upon hatching into a nymph, the stonefly will spend the majority of its life living on the river bottom. The stonefly then takes one of two paths to emergence and becoming an adult. Either to the water's surface like a mayfly, or it will crawl its way onto dry land. Once it has hatched into an adult, it will then only return to the water and the trout's menu for procreation and egg laying.

Caddis

From an anglers standpoint, the lifecycle of a caddis is fairly similar to that of a mayfly and a stonefly. But to allow you to keep up conversationally with the entomologists on the river, we'll run through the difference.

The caddis life cycle looks more like the metamorphosis of a caterpillar to a butterfly. When the caddis hatches from the egg, it spends the better part of its life on the river bottom as a larva. Once it has sufficiently gorged itself, it will pupate out of harm's way typically within the cracks of rocks. After it has undergone its metamorphosis inside the pupa, it begins to emerge and rapidly makes its way to the surface to hatch as an adult caddis. From there, similar to the stonefly, it will return to the water to lay eggs.

Midges

Midges live a life cycle not different to mayfly, stonefly, or caddis, but in most river systems they are abundant in and on the water seemingly year round. Midge has become the accepted term for any tiny fly that isn’t easily distinguished from one another with the naked eye.

Art Imitates Life(cycles)

Now that we have familiarized ourselves with the life cycles of most of a trout's diet, time to figure out what they mean to us as an angler. At this point, each life cycle of each insect has multiple fly imitations. Knowing where and when to use each can be the difference between a good day and the water, and a great one.

Nymphs

As you may have noticed, most of a bug's life is spent in the nymph/larva stage. And while shadow casting dry flies to rising trout is what every angler imagines when thinking of fly fishing, it is widely accepted that 75% of a fish's diet is consumed below the water's surface. Nymphs are always in the water and therefore, always on the menu. The most effective nymphs are those with a weighted bead head that allow them to be fished along the river bottom alongside the living nymphs.

While there are many different types of nymphs, they are mostly variations of the following aquatic fly imitations, and having some of each in your fly box will almost always guarantee success. Some may look more like mayfly nymphs, others more like stonefly nymphs, but the best part of these is that they’re ambiguous enough to cover many different species.

• Hares Ear
• Pheasant Tail
• Zebra Midge
• Prince Nymph
• Copper John

And of course, it wouldn’t feel right to not include the “junk flies” on the list. Flies that fish love to eat but don’t hatch into the beautiful, winged insects we all know and love—so they get a bad wrap.

• Eggs: Fish eat the eggs of other fish as they’re in the system
• Worms (Squirmy Worm or San Juan Worm): If fish will eat a live worm on the hook, they’ll sure eat fly that looks like one
• Mop Flies: Imitates grub or insect larva

Wet Flies and Soft Hackles

Wet flies and Soft Hackles are similar to nymphs except that they are typically unweighted. When these flies are fished on the swing, they rise through the water column and imitate a nymph making its run for the surface to hatch.

Dry Flies

Dry flies are flies that are fished on or just below the water's surface. As we can recall, there are many stages that each bug goes through once they have emerged. Having flies to match each stage is crucial for success, as each stage of the hatch can come and go at a moment's notice. Each of the following stages has a different imitation depending on the species we are trying to match, but knowing when and how to use each distinction is the key to success.

Emergers

Emergers are fished on the surface film and are used to imitate the stage where the fly is shedding its nymph skin and beginning to transition into a flying adult.

Duns

The dun stage is a fully hatched adult. It is waiting for the air to harden its wings so it can take off to safety. The fly imitation of this stage is what most people think of when imagining a fishing fly. The Adams (and Parachute Adams) dry fly is a prime example of this.

Spinners and Egg-Laying Adults

Spinners (mayflies) and egg laying adults (Caddis and Stoneflies) are the final stages of aquatic insect life. Spinners will have material splayed out on the side of the fly to imitate a spent fly's wings laid out on the water. The best examples of egg laying adults are the elk hair caddis and stimulator flies. They ride high on the water's surface and are hard to beat when fish are rising, but there is not an easily identifiable hatch.

Terrestrials

Not everything that finds its way into a trout’s gullet spends its life in and around the water. Many bugs go through their life cycle on land, only to become a clumsy adult and fall or get blown into the water to hungry trout awaiting a high-protein meal. These are usually imitated using bigger, foam-bodied dry flies and are a blast to fish in the warmer months. The main bugs you’re looking to imitate are:

• Ants
• Beetles
• Grasshoppers

Streamers

It’s true that all trout at some point in their life thrived on bugs, but big fish can’t flourish on bugs alone. These fish eat other fish, and that’s where streamers come into play. Streamers are big flies that imitate bait fish or other free swimming aquatic animals. For trout in rivers, these could be sculpin imitations. For bass, these could include minnows, crayfish, leeches, and panfish. Streamers are the ticket to big and predatory fish in any water system. They can be big and articulated, or even a stripping in a wooly bugger will almost always elicit a strike.

Put It All Together On the Water

With all this information, you now have a complete picture of a trout’s diet. Being able to identify when their food is in what stage of life and then presenting it to the fish is simply the name of the game. No matter what river in the world you’re fishing, each species of bug will act the same, and it’s all just a matter of matching the hatch. Looking up hatch charts for the area you plan on fishing will tell you: what type of bugs are in the system; and what times of year they hatch.

Year round, it is crucial to have the nymphs of the bugs that reside in the river. Matching the size is an absolute necessity as well. Whether matching the color is necessary is open for debate—so I’ll leave that up to whatever you find works. You’ll also want the full life cycle of dry flies for the lucky times you’re on the river during a hatch or a spinner fall.

If you’re looking for more advice on choosing the best fly for target species and fishing conditions, reach out to a Curated Fly Fishing Expert, like me. We love to chat bugs and any other fly-gear needs!