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 restAt a glance It separates Damselfly / Dragonfly from Stonefly cleanly, and every pair of groups still in play differs on it and you can check it at a glance.
What to look at
Damselfly / Dragonfly
Stonefly
Grasping labium / mask (Odonata nymph)decisiveIn hand
crawling on rock or debris; weak flight; adults often stay near the ground
Reference seasonality
Adult hunting: May, Jun, Jul, Aug, Sep
Pre-emergence migration: Apr, May, Jun, Jul · Adult activity: May, Jun, Jul
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
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.
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.
