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: Pincers on the front legsAt a glance It separates Crayfish 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
Crayfish
Scud / Amphipod
Pincers on the front legsdecisiveAt a glance
Yes
No
Which way the body is flatteneddecisiveClose look
Varies
Body flattened side to side
Tail fan; flicks backwards to escapedecisiveWatch it
Yes
No
Body see-through, backbone or gut visibledecisiveAt a glance
No
Not diagnostic
Two long antennaeClose look
Yes
Not diagnostic
Crawling on rock or debrisWhere it was
Yes
Not diagnostic
Toothed blade projecting between the eyesClose look
Not diagnostic
No
Reported sizedecisive
25–150 mm
5–20 mm
Water it is recorded in
stream, river, lake margin, reservoir, spring
river, stream, lake margin, spring
Life stages covered
young-of-year, adult
adult
How it moves
walking slowly forward on the legs; flicking rapidly backwards when threatened by curling the tail fan under the body
swims on its side (sideswimmer); rapid darting
The same for both: 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
Decapoda (Cambaridae, Astacidae)
Crayfish
Claws in front, tail fan behind, bottom underneath.
First check
Are there pincers much larger than the walking legs? Then it is a crayfish.
What it is
Nothing else you are likely to meet in fresh water carries a pair of pincers that size. The claws settle the identification on their own; everything after that is about which crayfish and what condition it is in.
Stages
Adult (25–150 mm, excluding antennae and legs)
Water
stream, river, lake-margin, reservoir, spring
Separates from Scud / Amphipod
A body flattened side to side with no enlarged pincers is a scud. A crayfish has claws, and its tail is flattened top to bottom.
Typical windows
No family-level window in this layer.
Patterns (correspondence)
Near Nuff Crayfish
Limits
Sources disagree on basic counts — four pairs of walking legs plus chelipeds or five pairs with the first enlarged, and six or seven abdominal segments. Neither is offered as a field character.
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 Crayfish
This entry does not name Crayfish as a look-alike. That is a gap in the prose, not proof they cannot be confused.
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.
