A well-tied fishing fly is a tiny piece of art that is meant to mimic a fish’s natural diet. When it’s time to choose a fly, you need to think from the fish’s perspective, which means you need to think about what natural insects fishing flies mimic. To do that, it helps to know a little entomology. With a little added knowledge around fly fishing entomology, you could be on your way to catching more fish, and making fly selections a bit easier.
What Is Entomology?
Entomology is the study of insects. This often branches out into arthropods and other invertebrates, but for fly fisherman, there’s only a few families of these creatures that are important to know. It’s also important to know what stage of life the fish are feeding on. By having the entomological background presented here, you will be able to know what family of insect and what life cycle stage is currently on the water. That will help you know which flies you should use to entice the fish!
Why Is Knowing Entomology Important For Fishing?
Fish like to eat insects, and to get them biting you need to know what they’re eating. If you can choose flies with the proper look and size to mimic what the fish are feeding on, you’re more likely to catch them.
You also need to know where in the water column a particular bug is likely to be. This determines whether or not you use a dry fly, wet fly, or both, as well as how much weight is needed, tippet length, and other factors. You can read all about those differences here, but usually if a fly mimics an adult insect, it’s a dry fly. That means it sits on top of the water. Flies resembling the aquatic larvae– immature insects– are wet flies. Most of the insects featured here lay their eggs in water, which then hatch into juvenile forms that live in the water until their final molt or emergence from a pupa. Flies that resemble these insects will be wet flies and will hang below the surface of the water.
You don’t need to know every bug and insect that fish might eat– just some of the common ones and categories that are explained below.
Bug & Insect Categories
Complicating things a little more is that you need to know the life stages of the insect. Generally, these are larva, pupa or emergers, and adults. Some insects also have a spinner stage.
- Larvae are juvenile insects, which often look worm-like. These are also called nymphs, especially when grouped with emergers. “Nymphing” refers to the specific practice of using subsurface wet flies to entice fish.
- Emergers are larvae in their final molts. They often look just like smaller adults and often have an exoskeleton stuck to them. Some emergers take the form of a pupa, which means they encase themselves to undergo metamorphosis. These often can be fed on by fish mid water column, or just below the surface.
- Adults are the final life stage of the insect.
- Spinners are the dead insects that fall to the surface of the water. Not all insects have substantially different spinner forms- midges, for example, don’t have a separate spinner fly pattern.
You’ve definitely swatted a midge or two in your life. These tiny insects look like mosquitoes and swarm like mosquitoes, but fortunately for every trout fisher who’s ever met them, they don’t bite like mosquitoes (mostly). Midges are very small, and flies tied in a midge pattern will always be on smaller hooks. Midges are available year-round on the water and are a stable food source for fish.
- Larvae: Midge larvae look like little worms with segmented bodies. Unlike the adult midges, they may be red or colored due to their diet. The midge larva is an extremely common pattern to base nymphing flies on.
- Emergers: During this phase, midges take on a C-shape and are encased in an air bubble that raises them to just below surface level. This air bubble is shiny and attractive to fish.
- Adults: Adult midges look like mosquitoes. Males have feathery antennae. Most adult midges have black bodies and transparent, scaleless wings. You’ll also find midge patterns that mimic mating midges.
Popular midge patterns include the Pale Olive Midge Larva (size 16-20), The CDC Transitional Midge (size 18-20), and the Griffin’s Gnat (size 16-22).
Mayflies aren’t just found in May, although in many places that is when they emerge. These insects typically hatch in the morning and evening hours.
Mayflies are instantly recognizable because of their triangular, upright wings.
- Larvae: Mayfly larvae is highly variable. Some are oblong and swim; some have flat, wide bodies and cling to rocks in fast-moving water; some crawl on the streambed, and some burrow down into the silt. Many have three tails, a trait unique to mayflies. Mayfly larvae live year-round in the water, so you can fish with them at any time.
- Emergers: Mayfly emergers have a shuck behind their bodies as they make their way to the surface. Once they are on the surface, they leave this exoskeleton behind and fly away. The emergers are vulnerable during this stage, so fish are keen to look for them.
- Adults: Adult mayflies are often referred to as “dun” mayflies once their wings have hardened. Their wings are white and triangular.
- Spinners: Adult mayflies at the end of their life cycle have clear wings and are highly active as they try to find a mate before dying.
Common mayfly patterns include the Hare’s Ear (size 10-22), Parachute Flies (size 10-22), and Rusty Spinners (size 16-20). Callibaetis spinners (size 12-18) also deserve a mention, as they can be used year-round for dry fly fishing.
The caddis is another abundant food source for trout, and fish eat them at every stage of the life cycle. Caddis nymphs live underwater and can take up to two years to metamorph into adults.
- Larvae: Caddis larvae come in two forms, cased and uncased. Uncased caddis larvae look like grubs. They are similar to midge larvae, except they have feet where the midges do not. To protect themselves, caddis larvae collect bits of detritus such as gravel, sand, twigs, and pieces of plants. Using the silk they produce, they make themselves a hard, durable case. Unfortunately for the caddis, however, this cannot protect them from hungry trout. Fish will go for both cased and uncased caddis larva flies. Cased caddis nymphs can be recognized by the case– they’re the only insects on this list that make those!
- Emergers: For the final molt as a larva, the caddis nymph seals the end of its tube. After metamorphosis, it breaks from the case and floats to the surface.
- Adults: The adult caddis has hairy wings that form a tent over their bodies when the fly comes to rest. Caddis adults don’t have a separate visual spinner stage.
Unlike most insects, which emerge in late spring and early summer, stoneflies emerge as adults in winter and early spring. They like fast-moving streams with rocky bottoms, so flies that mimic them can be great for smaller brooks and fast moving streams. Stoneflies are larger than many other insects that fish like to eat, and often represent the bulk of the biomass in an area.
- Larvae: Stonefly nymphs have two tails and legs that have two hooks at the ends. They resemble mayfly larvae but never have three tails as many mayflies do. They live as nymphs for between one and three years, so this is another pattern you can use year-round. Emerging stoneflies don’t use gas propulsion to get to the surface of the water. Instead, they crawl onto dry land, rocks, or plants. Once out of the water, they have their final molt.
- Adults: Adult stoneflies can be found in vegetation along the creek. Like mayflies, stoneflies die immediately after mating. The females can be seen gliding across the surface of the water laying their eggs, so this is a great dry fly to use. Adult stoneflies have no distinct spinner stage.
Some common fly patterns that mimic stoneflies include the Little Black Stonefly (size 12-18), the Knuckle Dragger (size 8-10), and the Chubby Chernobyl (size 6-16).
Terrestrials are the largest and least specific category of insect. It includes anything that lives primarily on land that might end up in the water… and then in the belly of a hungry trout. When fishermen use terrestrial flies, they usually mimic grasshoppers, beetles, ants, spiders and other arthropods that might incidentally land in the water. As such, terrestrials are usually dry flies. These come in many patterns and are particularly good for windy days or along steep embankments where a bug could get swept into the water.
Scuds & Sowbugs
Scuds are little crustaceans that resemble shrimp. Sowbugs look a lot like other isopods (like pill bugs), but they can’t roll into a complete ball. Both scuds and sowbugs are bottom-dwelling, so flies that resemble them will be wet flies. Because these animals are always in the water, it can be beneficial to use scud and sowbugs whenever a lack of other food sources appears to be occurring. Flies that mimic them all mimic the adult stages, as the larvae look just like miniature adults. You can tell these crustaceans from insects by their two pairs of antennae, jointed legs, and segmented abdomens. You’ll find that these patterns can have a range of sizes.
Popular scud patterns include the Garbage Scud, the Olive Scud Nymph, and the Ray Charles Scud. Popular sowbug patterns include the Firebead Ray Charles Soft Hackle, the Killer Bug, and the Pete’s Carpet Bug. Scuds and sowbugs are fairly small, so most of these flies should be tied in the 14-22 range.
Annelids, or worms, are another invertebrate type that can be fed on by fish. Even though we think of worms as subterranean, during rainstorms many worms get washed into waterways, and other annelids are frequently in the water. Fish love them. Worms can be used throughout the year but are especially effective after rain and in spring. Leeches also fall in the annelid category and are present in many waterways. Often fished in stillwater, but can be quite effective in cold water or winter scenarios. Worms and leeches only have one recognizable life stage– juvenile worms look just like adult worms, only smaller, and all worms are going to be wet flies.
Popular patterns for worms include the San Juan Worm, the Squirmy Wormy, and the Chamois Worm. Worms are appropriately sized in the 10-14 range. You’ll often find San Juan worms larger, but may find other annelid patterns as small as size 22.
Damselflies, Dragonflies, and Water Boatmen
Our last group of insects are the damselflies, dragonflies, and water boatmen. These only have two fishable life stages, the larva, and the adult. Though these insects are not closely related, we include them together here because they don’t have mass hatch cycles like the various flies discussed above. They lay fewer eggs and survive after mating, so you don’t have massive die-offs as you do with say, mayflies. These insects are larger than many other aquatic insects, making them a prize for hungry trout. Additionally, water boatmen have massive egg-laying flights, where hundreds to thousands of them will dive-bomb the water, trying to swim to the bottom to lay their eggs.
- Larva: Dragonfly and damselfly larvae look a bit like elongated crickets. Their gill structures are at the tip of the abdomen, not up by the head, and they may have a chitinous “mask” formed by their mouthparts. Water boatmen nymphs look like small adults.
- Adult: Adults have two pairs of wings, large round eyes, and thin, long bodies. Water boatmen have oar-shaped legs and domed, shiny shells.
Common patterns for these insects include Whitlock’s Gorilla Damsel/Dragon (size 2-8), Red Shoulder Dragonfly Nymph (size 6-10), and Tim’s Water Boatman (size 12-16).
While there might be hundreds of insect species on any given waterway, knowing these general categories can tell you what the fish are eating, and where they’re feeding in the water column. You don’t have to be an expert or even know any species names– if you can remember the life cycles and their appearances, you’ve got the entomology expertise you need.
We have also included an extremely helpful video from our friends at Drifthook:
Max DesMarais is the founder of Hiking & Fishing. He has a passion for the outdoors and making outdoor education and adventure more accessible. Max is a published author for various outdoor and marketing websites. He is an experienced hiker, backpacker, fly fisherman, trail runner, and spends his free time in the outdoors. These adventures allow him to test gear, learn new skills, and experience new places so that he can educate others. You can read more about him here: hikingandfishing/about