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Hatch MatchHook the Horizon · field intelligence

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.

The differences, and nothing else

Best separating check: How the wings rest At a glance It separates Stonefly from Damselfly / Dragonfly cleanly, and every pair of groups still in play differs on it and you can check it at a glance.

What to look atStoneflyDamselfly / Dragonfly
Grasping labium / mask (Odonata nymph)decisiveIn handNoYes
Leaf-like caudal lamellae (damselfly nymph)decisiveClose lookNot diagnosticYes
Gills at the thorax or leg basesdecisiveHand lensYesNot diagnostic
How the wings restdecisiveAt a glanceWings held flat and parallel along the bodyVaries
Tail filamentsdecisiveClose lookTwo tail filaments clearly visibleVaries
Two long antennaedecisiveClose lookYesNot diagnostic
Crawling on rock or debrisWhere it wasYesNot diagnostic
Reported size5–75 mm6–89 mm
Water it is recorded inriver, stream, springriver, stream, lake margin, spring
How it movescrawling on rock or debris; weak flight; adults often stay near the groundclimbing vegetation; jet propulsion (dragonfly nymph)
Reference seasonalityPre-emergence migration: Apr, May, Jun, Jul · Adult activity: May, Jun, JulAdult hunting: May, Jun, Jul, Aug, Sep

The same for both: life stages.

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

Specimen plate: stonefly nymph in dorsal view with two tail filaments, two long antennae, and thread-like gills at the thorax rather than along the abdomen.

Plecoptera

Stonefly

Two tails, two antennae, and a poor opinion of swimming.

First check

Two tails AND two long antennae, with any gills at the thorax and leg bases rather than along the abdomen.

What it is

Most stoneflies want cold, fast, well-oxygenated water, and they say so by being where it is. The nymph is a crawler: it creeps under and among stones rather than moving about in open water.

Stages

Nymph (5–70 mm (many western species large)) · Adult (6–75 mm; most 13–50 mm)

Water

river, stream, spring

Separates from Damselfly / Dragonfly

Stonefly gills tuft at the thorax. Odonata nymphs carry a grasping labium / mask.

Typical windows

Nymphs present (nymph) · Pre-emergence migration (nymph) · Adult activity (adult) · Small winter species (adult)

Patterns (correspondence)

Rubber-Legs Stonefly Nymph · Twenty-Incher · Stimulator · Little Black Stonefly

Limits

Habitat and emergence timing vary considerably by species.

Full Stonefly 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 Stonefly

Stonefly gills tuft at the thorax. Odonata nymphs carry a mask.

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 →