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, cloudy-winged and dull-bodied. Within hours to a couple of days it sheds one more time into a spinner: clear-winged, smooth and shiny, built only to mate and die. Two winged forms of the same animal, hours apart, sitting on the same water.NOAA-GLERL-WATER-LIFE-FIELD-GUIDE, NCSU-ENT425-EPHEMEROPTERA, MDC-MAYFLIES, MAINE-DEP-MAYFLYDispute
That matters in the field because the two forms look different. The dun is a short-lived stage usually found at the water surface; the spinner is the form that joins the mating flight. If you are looking at a wing in the film and cannot decide what stage you have, the wing will usually tell you: cloudy on a dun, clear on a spinner.NOAA-GLERL-WATER-LIFE-FIELD-GUIDE, NCSU-ENT425-EPHEMEROPTERADispute
Underwater the nymph is the long-running form — from about four weeks to four years 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.NCSU-ENT425-EPHEMEROPTERA, NWS-ARX-MAYFLY-TRACKING, MAINE-DEP-MAYFLYDispute
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Head withdrawn into the front of the body, hard to see-4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 →
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.
3 reference windows
Nymphs presentnymph
JFMAMJJASOND
Mayflies spend most of their lives as nymphs, from about four weeks to four years depending on species, so nymphs are in the water in every month.
Burrowing Hexagenia mayflies on the Upper Mississippi generally emerge in late June to early July, in the evening; the biggest emergences show up on weather radar. No agency or university source consulted dates an order-level spring emergence, so no spring bar is drawn — that absence is not a claim that mayflies do not emerge in spring.
Observed: Hexagenia on the Upper Mississippi River, Davenport, Iowa to St. Paul, Minnesota.
Some mayflies are autumn emergers. A coastal Oregon study that included the mayfly Paraleptophlebia bicornuta followed emergence from late July into early December.
Observed: Coastal Oregon streams; one species, Paraleptophlebia bicornuta, in experimental troughs.
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.
6 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.Dispute
Declared by the reviewerTwo tails do not make a stonefly. Some Baetidae and Epeorus nymphs carry two filaments; three tails plus side gills is the clean mayfly call, and two tails plus side gills is still a mayfly.MACROINVERTEBRATES-ORG-BAETIS, MACROINVERTEBRATES-ORG-EPEORUSDispute
Declared by the reviewerMayflies vary considerably; the most useful field result combines life stage, silhouette, tails, gills or wings, size, position, and movement.Dispute
Declared by the reviewerThis guide does not claim a current emergence, distribution, or fish feeding focus.Dispute
Declared by the reviewerThis preview does not claim a current emergence, distribution, or fish feeding focus.Dispute
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 12 published pages.
These are every page behind this entry. Where a reviewer traced a line above to the pages that carry it, that line names them itself, with where the evidence comes from when it is narrower than the whole entry. A line with no names beside it rests on this list as a whole: nobody has yet checked which page carried it, and naming one would be a mapping nobody made.
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
Take the group to the next step.
These send the class of animal and the water this record is documented in. They do not send a sighting, because this page does not have one — you have not told it what you saw.
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.
Keep the group on the day's record, so a later entry can be compared against what you were looking at.
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