Mayflies, Caddisflies and Midges: An Angler’s Guide to Aquatic Insect Life Cycles

Almost everything hatching out of a trout stream follows one of two life plans. Learn them, and a blizzard of insects collapses into two questions: which stage are the fish eating, and how do I imitate it? This guide covers the two kinds of metamorphosis, every stage from nymph to spinner, drift, hatch triggers, and how to tell the orders apart.

Key Takeaways

• Two blueprints cover nearly everything: incomplete metamorphosis (egg → nymph → adult) in mayflies and stoneflies; complete metamorphosis (egg → larva → pupa → adult) in caddisflies and midges.
• Trout eat every stage, but the emerging insect is the easiest meal — which is why fish often refuse a high-floating dry mid-hatch.
• Mayflies alone carry two winged stages, the dun and the spinner; spinner falls are often the best dry-fly fishing of the day.
• Invertebrate drift never stops, so a deep, dead-drifted nymph produces even when nothing is visibly hatching.
• Size and silhouette usually matter more than exact colour.
• Hatches follow water temperature, photoperiod and species habit; the best chart for your river is the one you record.

Two Ways to Grow Up: Incomplete and Complete Metamorphosis

Troutnut, the entomology reference built by salmonid ecologist Jason Neuswanger, places mayflies and caddisflies first among the insects that matter to anglers, stoneflies third, with midges arguably challenging that position.

Incomplete metamorphosis: mayflies and stoneflies

Eggs hatch into nymphs that already resemble small wingless adults. They moult repeatedly, growing external wing pads, and the final moult produces a winged adult. There is no pupa. Mayfly nymphs are shaped by habitat: swimmers such as Baetis are streamlined, clingers lie flattened against stones, crawlers move slowly over weed and gravel, and burrowers such as Ephemera live in silt. Stonefly nymphs are flatter, with two tails.

Because these nymphs grow up in the open rather than sealed in a pupal case, trout can eat them all year. That fact is the foundation of nymph fishing.

Complete metamorphosis: caddisflies and midges

These hatch into larvae that look nothing like the adult, then pass through a pupa in which the body is reorganised. Caddis larvae vary enormously: some build portable cases from sand or plant fragments, some spin fixed retreats and catch food in silk nets, and some, like Rhyacophila, are free-roaming predators. The pupa — the stage anglers most often overlook — may swim or crawl to the surface, or the adult may emerge at the surface and run for shore. Midge larvae are slender, worm-like and often red with haemoglobin, hence the name bloodworm; the pupa hangs beneath the surface film.

The blueprint tells you what is present and when. Know a species’ nymphal habitat and you know where to drift your fly; know how it emerges and you know which stage the trout are keyed on. Troutnut makes the point directly: an angler who understands one species’ life cycle has the upper hand when it hatches, while stoneflies and midges reward that fine-tuning less.

What Trout Eat at Each Stage

Nymph and larva: the underwater year

This is the bulk of the diet by volume on most streams, because it exists for months rather than minutes. Presentation matters more than pattern: a nymph drifting at the fish’s level, near current speed, reads as food.

Emerger and pupa: the vulnerable transition

A mayfly nymph rises, splits its shuck and struggles to pull its wings free — half in and half out of the water, often stuck in the film and unable to escape. MidCurrent calls emergers an easier meal for exactly this reason, and notes that many caddisflies escape quickly, so the ones that stall become a fast snack. Emerger and cripple patterns imitate that half-born insect; some sit low in the film, where a partly submerged fly throws a bold silhouette against the reflective underside of the surface.

Dun and adult: wings on the surface

Clear of its shuck and riding the surface, the insect becomes a dry-fly target. Mayfly duns drift with wings upright like little sails; caddis adults sit tent-winged and often skitter. This is the stage most anglers picture when they hear “hatch” — and often not the stage the fish are eating.

Spinner and ovipositing adult: the forgotten fall

Mayflies are unusual in that the winged insect leaving the river is not yet a true adult. As Tom Rosenbauer of Orvis explains, the dun flies to streamside vegetation, moults once more into maturity, then returns a day or two later to mate, lay eggs and die. The egg-layer — the spinner, or imago to scientists — falls spent onto the water with wings outstretched and nearly invisible, because they lie flat.

Duns and spinners of the same species look unrelated: duns are slow, clumsy fliers with fat bodies and translucent wings like waxed paper, while spinners are fast and agile, with skinny bodies, long tails and wings like cling film, often a different colour. Spinner falls arrive in the evening, in low light, with fish rising in a confident regular rhythm. Caddisflies have an equivalent event in egg-laying females dipping at dusk; for midges it is the adult and the pupa.

Behavioural Drift: The Food That Never Stops

Invertebrate drift is the downstream transport of aquatic insects, and it is why nymphing works when nothing is on the wing. Some drift is accidental; much is behavioural, as insects leave the bottom — often under cover of darkness, when the risk from visually hunting fish is lower — to relocate, feed or find better habitat. Drift has a daily and a seasonal rhythm, and it never reaches zero.

That changes the question from “is anything hatching?” to “where are the fish looking?”. Trout hold in lies where current concentrates drifting food and take it without showing themselves — which is why a deep nymph at current speed catches fish in flat water at noon in July. It is also why the dead drift is the foundation of tight-line nymphing: as Troutbitten makes clear, the goal is a drift with no unnatural drag, because drag is what turns a nymph into something a trout will refuse.

Hatch Timing and Triggers

Two long clocks govern the season and a third decides the hour. Water temperature sets the pace of development: each species has its own window, and cumulative warmth over preceding days matters more than a single reading. Photoperiod — day length — is the more dependable cue, which is why some hatches arrive within about a week of the same date each year whatever the spring did. Light and weather then decide when the flies show: some mayflies emerge in the morning, others at midday or evening, while caddis hatches are tied to dusk and low light. Midges emerge almost year-round, which is why they carry so many winter tailwater fisheries.

The last variable is the species itself: how long an insect spends in the film, and whether adults return to the water immediately or the next day. These habits differ between close relatives, which is where a good local hatch chart earns its keep. Nymphs and pupae also become restless before emergence — Orvis puts that window anywhere from a couple of days to an hour ahead of the hatch — so expect drift to rise first.

Telling the Orders Apart and Matching the Stage

You need silhouette, wings, resting posture, antennae and tails — not a microscope. Orvis News notes that Latin names mainly prevent regional confusion, and that matching the natural to its imitation is what catches fish.

In practice: a mayfly dun rides upright with long tails and no visible antennae; a caddis sits tent-winged with long antennae and no tails; a midge is tiny, slender and tailless; a stonefly folds four wings flat.

Treat the next table as starting logic, not a rulebook. Hook sizes are typical ranges, and when in doubt change size before changing pattern. Orvis News offers a useful calibration: most aquatic insects are grey, cream, brown or olive, and one pattern in each colour across a size run from roughly 10 down to 24 will match almost any hatch.

How to Observe and Record a Hatch: A Step-by-Step Procedure

The gap between guessing and matching is a routine run on arrival.

1. Stop fishing and watch for ten minutes. Stand well back so you do not spook the fish, and pick one steadily rising trout.
2. Read the rise form. A gentle dimple with no bubbles suggests emergers or spent spinners in the film; bubbles usually mean adults off the surface. A splashy rise with an adult flying away is a caddis emergence — Orvis News advises fishing a caddis pupa just below the surface.
3. Look into the air with the sun behind you. Hovering or yo-yoing insects over a riffle hint at a spinner fall.
4. Search the surface film. Floating shucks, empty pupal cases and spent spinners show which stages are on the water, not merely in the air.
5. Catch a natural. Orvis News is blunt: flies look larger in the air and colour deceives, so net one and lay it beside your fly box lid.
6. Match in order — size, silhouette, then colour. Hatch Magazine’s account of a hard-fought PMD hatch on the Missouri makes the point: with every dun and spinner refused, a change of size and silhouette produced the eat. If a fish refuses a good drift, go one size down or up before digging for a new fly.
7. Turn over rocks at the stream edge. Orvis News notes that insects due to emerge cluster near the edge and their wing cases darken beforehand.
8. Choose the stage the fish are using, not the one you prefer. If rises are subtle, start with an emerger or soft hackle; MidCurrent suggests starting 30 to 60 minutes before the expected hatch.
9. Write it down. Record date, river, water temperature, weather, hatch start and finish, rise form, the stage taken and the fly that worked — a season of this builds a hatch chart for your own water.

Common Mistakes

• Assuming no visible hatch means no feeding. Drift continues day and night; fish a nymph deep first.
• Getting colour right and size wrong. Size and silhouette dominate; colour is the last variable, not the first.
• Fishing a dry when the rises have no bubbles. Those dimples are fish on emergers and spinners a centimetre down.
• Going home before the spinner fall. The last hour of light is often the best dry-fly window of the day.
• Ignoring caddisflies because they are not mayflies. A splashy rise with an escaping adult is a caddis cue, and midges may be the only thing hatching on a tailwater or in winter.
• Being fooled by a masking hatch. When a large obvious insect hatches alongside a smaller abundant one, fish may be on the smaller fly.
• Never recording anything. Without notes you re-learn the same river every season.

Sources & Further Reading

• Troutnut — aquatic insect entomology reference — Reference for mayflies, caddisflies, stoneflies and midges.
• Troutnut — Mayfly Genus Baetis (Blue-Winged Olives) — The swimmer mayflies behind many dependable hatches.
• Troutnut — Caddisfly Genus Hydropsyche (Spotted Sedges) — The net-spinning caddis genus, all stages.
• Troutnut — Midge Genus Chironomus — A widespread midge genus and its bloodworm larvae.
• Orvis Learning Center — The Hatch You Might be Missing (Tom Rosenbauer) — The dun-to-spinner moult, and spinner-fall tactics.
• Orvis Learning Center — Hatches video lesson — Instructional lesson on reading hatches.
• Orvis News — How to Match the Hatch — Rise forms, bubbles, masking hatches, and fly selection.
• MidCurrent — When Should You Use an Emerger Instead of a Dry Fly? — Why trout prefer the transitional stage, and how to rig for it.
• MidCurrent — What’s the Best Way to Fish After a Hatch is Over? — Fishing on after the hatch.
• Hatch Magazine — The PMD — A prolific mayfly hatch, and why size beats colour.
• Troutbitten — Can I Dead Drift the Nymphs Without Contact? — Tight-line nymphing and the drag-free drift.
• US EPA — Indicators: Benthic Macroinvertebrates — Bottom-dwelling macroinvertebrates as stream-health indicators.