Side by side · same standard

What actually separates these two.

The comparison scores every group at once. This page is for the moment you already have a pair of names in mind — mayfly or stonefly, scud or sowbug, shad or alewife — and want the structures that keep them apart, in one look. Nothing here is a current hatch.

Group AGroup B

The differences, and nothing else

Best separating check: Leaf-like caudal lamellae (damselfly nymph) Close look It separates Mayfly from Damselfly / Dragonfly cleanly, and every pair of groups still in play differs on it (look closely with it still).

What to look at	Mayfly	Damselfly / Dragonfly
Grasping labium / mask (Odonata nymph)decisiveIn hand	No	Yes
Leaf-like caudal lamellae (damselfly nymph)decisiveClose look	Not diagnostic	Yes
Abdominal gills (plate or feather)decisiveClose look	Yes	Not diagnostic
Tail filamentsdecisiveClose look	Three tail filaments clearly visible	Varies
How the wings restAt a glance	Wings held upright (sailboat)	Varies
Reported size	2–40 mm	6–89 mm
Water it is recorded in	river, stream, lake margin	river, stream, lake margin, spring
Life stages covered	nymph, subimago, adult	nymph, adult
How it moves	swimming undulation; crawling on substrate	climbing vegetation; jet propulsion (dragonfly nymph)
Reference seasonality	Summer emergence: Jun, Jul · Autumn emergence: Sep, Oct, Nov, Dec	Adult hunting: May, Jun, Jul, Aug, Sep

Rows come from the reviewed diagnostic weights the comparison scores with. “Decisive” means one answer puts them at least four points apart — the carrier's own rule for a structure that separates a pair.

The two entries, side by side

Ephemeroptera

Mayfly

This one grows wings, and then does it over.

First check

Count 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.

What it is

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.

Stages

Nymph (2–32 mm) · Dun (subimago) (Under 4 to 40 mm body; most families 4–20 mm) · Spinner (imago) (Under 4 to 40 mm body; most families 4–20 mm)

Water

river, stream, lake-margin

Separates from Damselfly / Dragonfly

A grasping labium / mask, or leaf-like caudal lamellae, is Odonata. Mayfly cerci are round filaments, not gill plates.

Typical windows

Nymphs present (nymph) · Summer emergence (subimago) · Autumn emergence (subimago)

Patterns (correspondence)

Pheasant Tail Nymph · Gold-Ribbed Hare's Ear · Parachute Adams · Comparadun · RS2 · Spent-Wing Spinner

Limits

A mayfly result means the observation is broadly consistent with Ephemeroptera, not that this exact species is hatching here now.

Full Mayfly entry →

Odonata

Damselfly / Dragonfly

The mask is the tell. The 'tails' on a damsel nymph are gills.

First check

Look at the mouth. If you can see a grasping mask, you are in Odonata.

What it is

Odonata nymphs are predators. The extendable labium — the mask — is the strongest field separator from mayfly and stonefly.

Stages

Nymph (6–68 mm; usually 13–68 mm) · Adult (body 25–89 mm; damselflies to about 64 mm)

Water

river, stream, lake-margin, spring

Separates from Mayfly

Mayfly cerci are round filaments; damsel lamellae are leaf-like gills. The labium mask settles Odonata.

Typical windows

Nymphs present (nymph) · Adult hunting (adult)

Patterns (correspondence)

Damsel Nymph · Dragonfly Nymph

Limits

Broad order only. Not a family or species call.

Full Damselfly / Dragonfly entry →