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: Fish-shaped forage silhouetteAt a glance It separates Scud / Amphipod from Baitfish / forage silhouette cleanly, and every pair of groups still in play differs on it and you can check it at a glance.
What to look at
Scud / Amphipod
Baitfish / forage silhouette
Fish-shaped forage silhouettedecisiveAt a glance
No
Yes
School or bait cluster, not a single insectdecisiveWatch it
No
Yes
Reported sizedecisive
5–20 mm
38–230 mm
Water it is recorded in
river, stream, lake margin, spring
river, stream, lake margin, lake open, reservoir, spring
How it moves
swims on its side (sideswimmer); rapid darting
flashing school; coordinated turns
Reference seasonality
No family-level window
Forage present: All year
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
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 Baitfish / forage silhouette
A scud swims on its side with many pairs of legs. A forage fish has fins and a tail fin.
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.
