Fly Fishing Entomology: The Kinds of Insects, Bugs, and Flies You Need to Know

The first time you hear someone say “match the hatch,” you look at them a bit confused. Asking for clarification, you learn that's how most anglers figure out which bugs the trout are feeding on, and which flies to tie on.

Sounds easy, right? Wrong!

Entomology is the study of insects. For someone to consider themselves a trout angler, they need to have an amateur degree in entomology, to aid in their fly selections. Entomology has been around for centuries, with people using insects and other creepy crawlies to figure out poisons, antidotes, medicines, and more. But today, many of the amateur entomologists out there are fly fishing anglers.

But where do you start? How can you learn about the insects in the river if you have no understanding of the ecosystem or don’t spend every waking moment on the river watching tiny bugs grow? How can you find out what is on the trout menu? Empty lines can cause a lot of frustration, so we will give you a crash course in fly fishing entomology so that you might see in your local trout stream so that you can catch more fish.

How Trout Feed

Trout feed on insects, crustaceans, like scuds, and other invertebrates in the river. Which bugs they eat and when changes almost daily. 70% of a trout's diet is underwater nymphs, worms, and other small insects. In the early morning, trout are in deeper water, feeding on underwater nymphs and larvae. As the water heats up, so does the bug activity. When the mayflies, caddis, stoneflies, dragonflies, and other insects start to hatch from their nymph stage to their terrestrial stage, trout move into the faster riffles. These riffles have very oxygenated water which allows the trout to feed easily without expelling too much energy. As the water temp climbs past 63°, the larger terrestrial bugs begin to hatch, and the trout begin to look upwards for those easy large calorie meals. Trout feed almost exclusively upstream, waiting for bugs to get dislodged from their rocks or for hatched bugs to be drying off their wings.

Categories of Insects and Where to Find Them

I am writing this article as a broad view into the entomology of a trout’s diet. There are always better and more specific examples of bugs and their matching flies in your local fly shop. Identifying bugs as they fly through the air is a learning experience and can take some time. Below, I’ll explore the six main types of insects that trout will feed on.

1. Midges

One of the most important insects, you can find midges in most water, most of the time. Midges are small bugs that have a four-stage life cycle, similar to many of these other insects. Midges start out as an egg, develop into a larval stage, then a pupa, then onto adulthood.

Midge eggs are so small that you won't really have any luck finding patterns to match them, but for the other life cycles, you can use a multitude of different flies. Zebra Midges are the most popular midge pattern because they are fairly easy to tie but still catch A LOT of fish. Other flies, such as a Top-Secret midge, a Chocolate Thunder, or a Neon Nightmare will also work to replicate the profile of a midge in the water.

When midges do hatch to adulthood, they cluster together on the top of the water to mate with other adults. These midge clusters can be replicated by a Griffith’s gnat, a Trico pattern, or a Renegade midge pattern.

2. Mayfly

Next up is the trout’s prime meal—their favorite order at a steakhouse and one of the main bugs people associate with fly fishing. The mayfly.

Mayflies are one of the main sources of food for trout. Mayflies hatch from February to December, and they also have a four-stage life cycle. Mayfly adults lay their eggs on the top of the water like mini sailboats, and their eggs drift down into the water, sticking to the rocks on the bottom. These eggs hatch into a nymph stage for as short as two weeks or as long as a year! The nymph stage of mayflies can be replicated by a Pheasant Tail nymph or a Hare’s Ear nymph. Mayfly nymphs sometimes get pushed off the rocks and trout eat them as they drift past their faces.

If these swimmers make it to adulthood, they hatch into the subsurface stage called dun. These dun mayflies are trying to dry off their wings and hatch into adulthood where they can reproduce. Dun mayflies are sometimes mistaken for spinners. The spinner stage is the final variation of mayflies. These bugs usually get eaten by trout in a sipping motion. The adult mayfly is called a drake. The trout’s nose barely breaks the surface of the water when eating drakes. Trout gorge themselves on all stages of the mayfly. While there are many different sizes and colors of mayflies, the most popular mayfly patterns are the Parachute Adams, Patriot Adams, Blue Winged Olives (BWO), Pale Morning Duns (PMD), and the Green Drake.

3. Caddis

My personal favorite insect is the caddis fly. Caddis thrive from April to November. These unique little bugs create their own “burrows” where they metamorphosis into their adult bugs. They have four stages in their life cycle as well—egg, pupa, larva, and adult.

When caddis eggs hatch into the pupa stage, they live underneath rocks, sticks, or anything else they can stick to. Caddis grab gravel, sticks, and other small structures to build a cocoon-like housing for their larva stage to grow wings and hatch into adults. Caddis nymphs and caddis pupa can be replicated by cased caddis patterns, both in black and in green, as the nymph is a dark-headed bug with a lime green body.

As the water temperature rises in April and May, these moth-like bugs begin to crawl to the surface and fly off the water to mate. Caddis lay eggs by bouncing up and down on the water, depositing eggs each time they hit the water. Trout love to eat caddis, and do so with gusto. Caddis bounce off the water trying to dry off their wings, and the trout try to eat them as fast as possible. Adult caddis patterns can be replicated by Elk Hair Caddis, X Caddis, Moths, and even Deer Hair caddis.

4. Stonefly

One of the largest insects that trout eat is called a stonefly. Stoneflies have a few different variations, like the salmon fly. These bugs have only a three-stage life cycle, and it lasts years for some of them. Stoneflies only have eggs, nymphs, and adults. These crawlers stay under the water breathing through gills along their abdomen. They stay in their nymph stage for one to three years. When these bugs reach maturity, they climb up on top of exposed rocks, and vegetation along the river bank. As they reach their spot above the water, the adult stage of the stonefly hatches into a large flying trout meal. While stoneflies are always in the water, the nymph stage is one of the biggest meals a trout will have. Stonefly nymphs can be replicated by Pat's Rubber Legs, Copper Stonefly, and even Biot Stoneflies.

When these large bugs hatch into flying bugs, the dry fly hatch is outstanding. These bugs stay really high on the water and have very large white wings and antennas. Trout key into these larger flies during the months of May to August. The salmon fly hatch in Colorado and Wyoming, usually only lasts about a week or so in early June, but during that week, trout gorge themselves on these huge floating bugs. Stonefly dry flies are dependent on the species of stonefly in the water. The predominant stonefly pattern is the stimulator. There are many different variations of a stimulator, so try to match size more than color. Other large dry fly patterns are the Chubby Chernobyl and the Godzilla Salmonfly.

Terrestrial Bugs

As the summer days get hotter and the days get shorter towards the end of the summer, the terrestrials start being much more active. Terrestrial means “land-based” and covers the vast range of bugs that fall under that umbrella. These bugs are most active in the months of April to September. Grasshoppers of all shapes and sizes and colors, beetles, crickets, dragonflies, damselflies, ladybugs, and even bumblebees. When there are ant colonies along the river banks, trout will eat ants that fall into the water. When the sun gets hot, these bugs go in search of water, but what they don’t know is that there is a trout under the water waiting for them to land. Some notable dry fly patterns include Chubby Chernobyl, Dave’s Hopper, Charlie Boy Hopper, Amy’s Ant, Hippy Stomper, and Gorilla Dragon.

Other Aquatic Insects

• Scuds (Freshwater Shrimp)
• Annelids (worms)
• Water Boatman

To Sum It Up…

If you can, do your research before you hit the water. Look at local fly fishing reports and talk to fellow fly fishers. When I go to a new area, I will call the local fly shop in the area, and simply ask which bugs are working. If I don't have any of those bugs, I get to go into a new fly shop and meet some like-minded individuals. Having a general knowledge of insect species in the water will help you catch more and bigger fish! If you have any questions or want to find the perfect flies of your own, don't hesitate to reach out to a Fly Fishing Expert here on Curated!