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Lateral view of a Male Baetis (Baetidae) (Blue-Winged Olive) Mayfly Dun from Mystery Creek #43 in New York
Blue-winged Olives

Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.

27" brown trout, my largest ever. It was the sub-dominant fish in its pool. After this, I hooked the bigger one, but I couldn't land it.
Troutnut is a project started in 2003 by salmonid ecologist Jason "Troutnut" Neuswanger to help anglers and fly tyers unabashedly embrace the entomological side of the sport. Learn more about Troutnut or support the project for an enhanced experience here.

Aquatic Insects of our Trout Streams

Aquatic Insects of our Trout Streams

Troutnut.com has 7594 pictures of 1384 subjects, and a growing library of behavioral information to help us understand and better imitate the insects and other invertebrates trout eat.

The site is organized by scientific name, but you can find things through the directory of common names too. You may also find the introduction to the taxonomic classification system helpful.

The big four

The vast majority of flies we use as anglers, if they are meant to be literal imitations of anything, imitate one of these four kinds of insects. Mayflies were the first to be taken seriously, and they inspired many classic old-time dry flies. However, it's common for any of the other three to be equally or more important to trout.

Artistic view of a Male Drunella flavilinea (Ephemerellidae) (Flav) Mayfly Dun from the Cedar River in Washington

Mayflies are the most important insects for anglers to understand, because they are common on trout streams, they often hatch in frenzied bursts of activity, and their behavior varies so widely between families and sometimes even species that it's useful to know and imitate the habits of each. They are a primitive order of insects, and their elegance and delicate lives have made them popular beyond the world of trout fishing.

Lateral view of a Female Grammotaulius lorettae (Limnephilidae) (Northern Caddisfly) Caddisfly Adult from the Henry's Fork of the Snake River in Idaho

Some anglers consider caddisflies to be even more important than mayflies. On many rivers, they're right. Angler-entomologists focus less energy on them because they are slightly less prone to cause a feeding frenzy among the trout. Although that happens sometimes, they are more commonly an intermittent food source during the times when it seems like nothing's hatching. Understanding their life cycle is of paramount importance to any fly fisher, but learning their species-specific idiosyncrasies is less useful than with mayflies.

Artistic view of a Male Isoperla fulva (Perlodidae) (Yellow Sally) Stonefly Adult from Mystery Creek #295 in Washington

Though less prolific than mayflies and caddisflies, many stonefly species make up for in size what they lack in numbers. Because they emerge by crawling out of the water on land, the hatching adults are not eaten for trout, but their large nymphs are welcome food year-round. In the Western states, where stoneflies are most prolific, trout sometimes feed selectively on the egg-laying adults as they return to the water.

Lateral view of a Chironomidae (Midge) True Fly Adult from the South Fork Snoqualmie River in Washington

Midges are most popular with anglers who fish fertile, placid spring creeks and stillwaters. Their often tiny size frustrates anglers who encounter trout feeding selectively on their frequent hatches. Their many thousand species are impossible to sort out, but they all share similar stages and behavior as far as the angler is concerned.

Other invertebrates to know

All of these are eaten by trout at times, although they may not often elicit the kind of picky feeding that requires a convincing imitation. Nevertheless, as you study the bugs you find on the water yourself, it's nice to know the basic identities of the oddballs that frequently turn up.

Tipulidae (Crane Fly) True Fly Adult from Brodhead Creek in Pennsylvania

Craneflies, which look like midges only much larger, are occasionally important to trout as they lay their eggs over the water.

Dorsal view of a Gomphidae Dragonfly Nymph from the Namekagon River in Wisconsin

Dragonfly adults are very rarely vulnerable to trout because they are superb at flight, but their large, slow nymphs are welcome food. The adults and nymphs are both impressive predators upon other insects, and I have watched adult dragonflies kill large mayfly duns and tear through a cloud of midges at rate of several per second.

Argia (Coenagrionidae) Damselfly Nymph from Fall Creek in New York

Damselflies mirror the closely related dragonflies (scientifically, they are different suborders of the order Odonata) in that the adults are excellent at flight and only the nymphs are likely to fall prey to trout. Their nymphs differ in the way they look, move, and respirate, but they share the same alpha predator status in the world of aquatic insects.

Lara (Elmidae) Riffle Beetle Larva from the South Fork Snoqualmie River in Washington

Beetles are the largest order of insects, and their many species come in contact with trout in three ways. The most popular among fly fishers are the species which live on land and occasionally fall or get blown into the water. But some species live their entire lives underwater, and still others live underwater only as larvae with a wide variety of unusual shapes and habits.

Dorsal view of a Male Caecidotea (Asellidae) (Cress Bug) Arthropod Adult from the Namekagon River in Wisconsin

Sowbugs are not quite as prolific or as important as scuds (Amphipoda), but in certain waters they are more common and serve the same role.

A freshwater amphipod (scud). Tiny crustacean common to many freshwater environments

Dorsal view of a Amphipoda (Scud) Arthropod Adult from Salmon Creek in New York

Scuds are not insects but small crustaceans, sometimes called freshwater shrimp, and in some streams they are a primary food source for trout. They grow quickly and can survive in a variety of habitats, but they are most prolific and important to trout in slow, weedy spring creeks. Unlike most aquatic insects, they never "hatch" into a dry form.

Dorsal view of a Cambaridae Crayfish Adult from unknown in Wisconsin

Crayish require no introduction, but their importance to trout is often understated. They are a favorite of many large freshwater gamefish, and trophy trout are no exception.

Lateral view of a Lepidoptera (Moth) Insect Adult from the Henry's Fork of the Snake River in Idaho

Some species of moths have aquatic larvae or live near streams where the adults are commonly available for trout. They can be easily mistaken for caddisflies.

Dorsal view of a Corixidae (Water Boatman) True Bug Adult from unknown in Wisconsin
Water Boatmen

Water boatmen are in the order Hemiptera, the "true bugs," along with water scorpions, giant water bugs, and backswimmers. They are present in many trout streams but are more important as a food source in lakes and spring ponds. The nymphs and adults look similar and the adults cannot breathe water, but carry small air bubbles with them for respiration. They sometimes fly up out of the water in the early season to mate, leaving anglers witnessing a very confusing "hatch."

Male Sialis hamata (Sialidae) Alderfly Adult from the  Touchet River in Washington

Alderflies are close relatives of dobsonflies and their larvae resemble the better-known hellgrammites. Their life cycle is similar and they are never available to the trout en masse, but they are loose in the drift often enough to be of value.

Large hellgrammite (dobsonfly larva). This nearly two inch long larva from the genus Corydalus is a fearsome predator

Dorsal view of a Corydalus (Corydalidae) (Dobsonfly) Hellgrammite Larva from Paradise Creek in Pennsylvania
Dobsonflies and Hellgrammites

Dobsonflies and their predatory larvae, the hellgrammites, are among the largest trout stream insects. They are best known as a good excuse for the rigid imitationist to fish a wooly bugger. Their close relatives, the fishflies of Nigronia and Chauliodes, are more good reasons.

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