First checkCount the tails, then look for gills along the sides of the abdomen. Three tails plus side gills is about as close to a clean mayfly call as field observation gets.
Mayflies are the only insects that grow a full set of working wings and then moult again. The winged insect that leaves the water is not the finished animal — it is a dun, a subimago, dull-winged and slow. A day or so later it sheds one more time into a spinner: clear-winged, longer-tailed, built only to mate and die. Two winged forms of the same animal, hours apart, sitting on the same water.
That matters in the field because the two forms behave nothing alike. Duns ride the film and drift, because they cannot fly well yet. Spinners fall to the surface already spent, wings flat out to the sides. If you are looking at a wing in the film and cannot decide what stage you have, the tails and the wing clarity will usually tell you before anything else does.
Underwater the nymph is the long-running form — weeks to a year depending on species. Look for plate-like or feathery gills ranked down both sides of the abdomen, and two or three long tail filaments. Those gills are the strongest single mayfly signal in hand.
The tell
What settles it, and what settles against it.
Read off this record's own diagnostic weights, heaviest first. A structure near the top of the left column does most of the work on its own; one near the bottom only leans.
Argues for Mayfly
Plate-like or feather-like abdominal gills visible+4
Three tail filaments clearly visible+3
Wings held upright (sailboat posture)+3
Winged insect at or near the surface+2
Two tail filaments clearly visible+1
Aquatic insect crawling on rock or debris+1
Argues against it
Fish-shaped forage silhouette (minnow / shiner / fry) — not an insect-5
Fallen terrestrial insect (hopper, ant, beetle, etc.) — not an aquatic emergence-4
Moving as a school or tight bait cluster, not as a single insect-4
Portable case or case-like structure present-4
One pair of long, flattened, oar-like hind legs used to row-4
Swims on its back, belly upwards-4
Mouthparts form a pointed beak rather than chewing jaws-4
Fleshy lobes or a disc at the blunt rear end-4
Head withdrawn into the front of the body, hard to see-4
A single tail filament pointing straight back, and no paired hooks-4
Hopper / grasshopper silhouette — hind legs long, body blocky, wings folded along back-3
Pinched waist (ant / flying ant) with elbowed antennae-3
Hard wing covers (elytra) meeting in a straight line down the back-3
Extendable grasping mouthparts (labium / mask) on an aquatic nymph-3
No long tail filaments visible-3
Gills at the thorax or leg bases-3
Wings held tent-like or inverted-V over body-3
No hardened head capsule and no legs — soft and segmented throughout-3
Swollen rear end gripping a rock in current, body standing up and waving downstream-3
Air bubble or silvery film carried under water (diving beetle / water beetle)-2
Three leaf-like caudal lamellae at the tail (damselfly nymph) rather than round filaments-2
Hardened head with softer worm-like abdomen-2
Terminal prolegs or hooks at the rear-2
Wings held flat and parallel along the body-2
Two long antennae-1
A structure at −4 or below is treated as a hard exclusion by the comparison tool rather than as a penalty: if you saw it, this group is ruled out and no amount of matching elsewhere brings it back. Run a comparison →
Life stages
What it looks like, stage by stage.
Mayfly — Nymphplate
Nymph
Size in hand3–25 mm (most trout-stream species 6–18 mm)
What you see
Two or three long tail filaments, held apart rather than fused
Plate-like or feather-like gills ranked along both sides of the abdomen
Dark wing pads swelling on the thorax as the nymph matures
A single claw on each leg
Where it turns up
Subsurface — clinging to the downstream face of cobble, burrowing in silt, or swimming in slack water, depending on species.
How it moves
Swimmers pulse the abdomen in an up-and-down undulation. Clingers barely move at all and are easiest to find by turning a rock.
Cues this stage offers the comparison
Three tail filaments clearly visible
Two tail filaments clearly visible
Plate-like or feather-like abdominal gills visible
Aquatic insect crawling on rock or debris
Mayfly — Dun (subimago)plate
Dun (subimago)
Size in hand4–20 mm body
What you see
Wings held upright over the back, sailboat posture
Wings opaque, dull, often grey or olive — not clear
Long tails still visible behind the body
Frequently a trailing shuck still attached
Where it turns up
In the surface film and immediately above it, drifting downstream before the wings dry enough to fly.
How it moves
Rides passively. A dun that has just emerged cannot fly properly and will drift a long way before it lifts off.
Cues this stage offers the comparison
Wings held upright (sailboat posture)
Winged insect at or near the surface
Three tail filaments clearly visible
Two tail filaments clearly visible
Mayfly — Spinner (imago)plate
Spinner (imago)
Size in hand4–20 mm body, tails often longer than the body
What you see
Wings glassy and clear, not opaque
Tails noticeably longer than on the dun
Body slimmer and often brighter or more translucent
Spent spinners lie with wings flat out to both sides, flush in the film
Where it turns up
Above the water in mating flights, then flush in the surface film once spent — often in the slack seams and eddies where they collect.
How it moves
Strong, deliberate flight while mating; motionless once spent. A spent spinner does not drift like a dun — it lies flat and almost disappears.
Cues this stage offers the comparison
Winged insect at or near the surface
Wings held flat and parallel along the body
Three tail filaments clearly visible
Two tail filaments clearly visible
Confused with
Telling it from what it isn't.
MayflyvsStonefly
Look at the gills. Mayfly gills are plate-like or feathery and run down the sides of the abdomen; stonefly gills are thread-like tufts bunched at the thorax and leg bases. Two tails alone will not separate them.
Mayfly nymphs have visible tail filaments and no case. A chunky larva with legs bunched near a hardened head, terminal hooks at the rear, and no true tails is caddis.
Midge larvae have no segmented true legs and no long tails — only prolegs. If you can see three pairs of jointed legs and long cerci, it is not a midge.
Where in the column it mattersNear the bottom. Nymphs live down; duns and spinners are a surface event. The same animal is a bottom insect for most of its life and a film insect for a day.
Read from the record's own position notes: subsurface (nymph); surface film (emergence/dun); spent on surface (spinner).
Reference windows
When this group is typically available.
Typical published ranges only. Elevation, latitude, tailwater release and a given year's weather move all of these, often by weeks. This is never a statement about today.
5 reference windows
Available year-roundnymph
JFMAMJJASOND
Nymphs are present continuously. Drift density rises in the hours before an emergence window.
All day, with a pre-emergence increase in drift
Spring emergencesubimago
JFMAMJJASOND
The classic cool-season window. Overcast, damp days hold duns on the water longer because the wings dry more slowly.
48–58 °F · Late morning to mid-afternoon
Summer emergencesubimago
JFMAMJJASOND
As water warms, emergence pushes to the cool ends of the day and off the middle entirely.
55–65 °F · Early morning and again at dusk
Autumn emergencesubimago
JFMAMJJASOND
Autumn windows narrow and centre on the warmest hours. Typically smaller-bodied than spring.
45–58 °F · Midday
Spinner fallspinner
JFMAMJJASOND
Follows an emergence by roughly a day. Spent spinners lie flush and are easy to miss even when they are the only thing on the water.
No reviewed dossier names this group. That is an absence in the dossier layer rather than a finding about the animal — four dossiers exist and only one of them names any forage group at all.
The pattern layer ties this group in Dry, Nymph, Emerger. That is a correspondence between an imitation and an animal, worked out when the patterns were written. Whether a fish is on it today is a question this page cannot reach.
Pheasant Tail Nymph
Nymph
Hook12–22
A slender swimming or clinging mayfly nymph with a dark wing case.
Dead-drifted near the bottom; the copper rib and thorax bulge read as gills and wing pads.
Gold-Ribbed Hare's Ear
Nymph
Hook10–18
A generalised scruffy nymph — mayfly first, but deliberately unspecific.
Dead-drifted. The picked-out guard hairs suggest legs and gills without committing to a species.
Parachute Adams
Dry
Hook12–22
A dun sitting in the film with upright wings — the most non-committal mayfly dry there is.
Dead-drifted on the surface. The parachute post sits the body in the film rather than on it.
Comparadun
Dry
Hook14–22
A dun flush in the film with a deer-hair wing standing in for a drying wing.
Dead-drifted in flat water where a hackled fly's footprint is too heavy.
5 of them. The limits are part of what was reviewed rather than a disclaimer bolted on afterwards. An entry that hid them would be claiming more than it checked.
Written by the reviewer
Declared by the reviewerA mayfly result means the observation is broadly consistent with Ephemeroptera, not that this exact species is hatching here now.
Declared by the reviewerMayflies vary considerably; the most useful field result combines life stage, silhouette, tails, gills or wings, size, position, and movement.
Declared by the reviewerThis guide does not claim a current emergence, distribution, or fish feeding focus.
Declared by the reviewerThis preview does not claim a current emergence, distribution, or fish feeding focus.
Read off the record
Nobody typed these. They are fields that are empty, said out loud rather than left for you to notice from a section that simply is not there.
What eats itNo reviewed species dossier names this group. Some dossiers describe forage in class terms — small forage, insects, bait — and those are shown below, but a class description is not a reviewed link to this group and none is claimed.
Sources
Everything above rests on 2 published pages.
Attribution here is to the whole record, not to individual sentences — the carrier attaches a source set to a record and does not say which page carried which line. Naming one per claim would mean inventing a mapping nobody reviewed.
A plate shows shape. It cannot show a gill beating, a mask firing, or an animal reversing into sand — so where motion is the field mark, the entry points at motion. These are demonstrations, selected by hand and by publisher. Nothing here is a source: it changes no score, no window and no reviewed claim.
György Kriska. The dun-to-spinner moult itself — the one transition in this guide that no still image can carry. Silent footage; the title is bilingual.
University or extension · external video · title read back from the publisher 2026-09-09
Carry it on
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Which fish in this kind of water are documented eating this class of forage, and how they behave when they are.
What this forage class asks of the setup — the weight it wants, and where the chain gives first.
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