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: Which way the body is flattenedClose look It separates Aquatic sowbug from Scud / Amphipod cleanly, and every pair of groups still in play differs on it (look closely with it still).
What to look at
Aquatic sowbug
Scud / Amphipod
Which way the body is flatteneddecisiveClose look
Body flattened top to bottom — it lies flat
Body flattened side to side
Two long antennaeClose look
Yes
Not diagnostic
Crawling on rock or debrisWhere it was
Yes
Not diagnostic
How it moves
slow deliberate crawl over rock and wood; crawls flat along the bottom, back uppermost, rather than on its side
swims on its side (sideswimmer); rapid darting
The same for both: size, water, life stages, seasonality.
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
Isopoda (Asellidae)
Aquatic sowbug
It lies flat. A scud falls on its side.
First check
Put it in a shallow tray of water. If it crawls flat, back-up, it is a sowbug. If it swims or lies on its side, it is a scud.
What it is
Sowbugs and scuds are the same size, live in the same water, and both have seven pairs of walking legs. Almost everything written about telling them apart is a list of characters you would need a lens for.
Stages
Every stage — they all look the same (5–20 mm)
Water
spring, stream, river, lake-margin
Separates from Scud / Amphipod
The flattening axis, and it is the whole tell: a sowbug is squashed top to bottom and lies flat, a scud is squashed side to side and lies on its flank. If the animal is also curved from head to tail and swims on its side, it is a scud.
Typical windows
No family-level window in this layer.
Patterns (correspondence)
Sowbug
Limits
Family level only. This does not separate Caecidotea from Asellus, and it does not tell a surface animal from a groundwater one.
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 Aquatic sowbug
The reciprocal of this record, and the confusion this entry now most often causes. A scud is flattened side to side and lies over on its flank; a sowbug is flattened top to bottom and lies flat, back uppermost, and crawls instead of flicking.
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.
