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 Baitfish / forage silhouette from Scud / Amphipod cleanly, and every pair of groups still in play differs on it and you can check it at a glance.
What to look at
Baitfish / forage silhouette
Scud / Amphipod
Fish-shaped forage silhouettedecisiveAt a glance
Yes
No
School or bait cluster, not a single insectdecisiveWatch it
Yes
No
Reported sizedecisive
38–230 mm
5–20 mm
Water it is recorded in
river, stream, lake margin, lake open, reservoir, spring
river, stream, lake margin, spring
How it moves
flashing school; coordinated turns
swims on its side (sideswimmer); rapid darting
Reference seasonality
Forage present: All year
No family-level window
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
Osteichthyes — not Insecta
Baitfish / forage silhouette
If it has fins and a school, stop calling it a bug.
First check
Count fins. If it schools, you are done naming insects.
What it is
This entry exists so a forage observation is not forced into an insect order. Minnows, shiners, fry and shad are not a hatch.
Stages
Forage fish (juvenile or adult) (About 38–230 mm across common baitfish species, fathead minnow to golden shiner)
Water
river, stream, lake-margin, spring
Separates from Scud / Amphipod
A scud has seven pairs of legs and no fins. A forage fish has fins.
Typical windows
Forage present (adult)
Patterns (correspondence)
Clouser Minnow · Deceiver
Limits
Silhouette class only. Many genera exist; do not name a minnow from this card.
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.
