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: Two long antennaeClose look It separates Scud / Amphipod from Stonefly cleanly, and every pair of groups still in play differs on it (look closely with it still).
What to look at
Scud / Amphipod
Stonefly
Two long antennaedecisiveClose look
Not diagnostic
Yes
Crawling on rock or debrisWhere it was
Not diagnostic
Yes
Reported size
5–20 mm
5–75 mm
Water it is recorded in
river, stream, lake margin, spring
river, stream, spring
Life stages covered
adult
nymph, adult
How it moves
swims on its side (sideswimmer); rapid darting
crawling on rock or debris; weak flight; adults often stay near the ground
Reference seasonality
No family-level window
Pre-emergence migration: Apr, May, Jun, Jul · Adult activity: May, Jun, Jul
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
Amphipoda · Crustacea — not Insecta
Scud / Amphipod
Not an insect, not a hatch, and the most common false positive there is.
First check
Watch it swim. On its side means scud, and you can stop there.
What it is
This entry exists to stop a wrong call, not to make a right one. Scuds are amphipod crustaceans. They are not insects, they do not hatch, and they never emerge — so a scud can never be the answer to what is coming off the water.
Stages
Adult amphipod (5–20 mm (most under 13 mm))
Water
river, stream, lake-margin, spring
Separates from Stonefly
Three pairs of jointed legs versus seven. Count them and the question closes.
Typical windows
No family-level window in this layer.
Patterns (correspondence)
Scud
Limits
Scuds are crustaceans, not insects. This entry exists solely to stop a scud being called a mayfly, caddis, or stonefly nymph.
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 Scud / Amphipod
If it is flattened side-to-side and swims on its side, it is a scud — a crustacean — not a stonefly nymph.
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.
