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 Aquatic sowbug from Crayfish cleanly, and every pair of groups still in play differs on it and you can check it at a glance.
What to look at
Aquatic sowbug
Crayfish
Pincers on the front legsdecisiveAt a glance
No
Yes
Tail fan; flicks backwards to escapedecisiveWatch it
No
Yes
Which way the body is flatteneddecisiveClose look
Body flattened top to bottom — it lies flat
Varies
Body see-through, backbone or gut visibleAt a glance
Not diagnostic
No
Toothed blade projecting between the eyesClose look
No
Not diagnostic
Reported sizedecisive
5–20 mm
25–150 mm
Water it is recorded in
spring, stream, river, lake margin
stream, river, lake margin, reservoir, spring
Life stages covered
adult
young-of-year, adult
How it moves
slow deliberate crawl over rock and wood; crawls flat along the bottom, back uppermost, rather than on its side
walking slowly forward on the legs; flicking rapidly backwards when threatened by curling the tail fan under the body
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
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 Crayfish
Pincers. A crayfish has large claws, far bigger than its walking legs, and a tail fan it flicks backwards with. A sowbug's front legs are small, at most claw-like.
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.
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 Aquatic sowbug
This entry does not name Aquatic sowbug 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)
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.