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: Crawling on rock or debrisWhere it was It separates Mole crab (sand flea) 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
Mole crab (sand flea)
Scud / Amphipod
A shell with no claws or pincers at alldecisiveClose look
Yes
No
Backs into wet sand, rear end firstdecisiveWatch it
Yes
No
A pair of feathery antennae fanned outdecisiveClose look
Yes
No
Which way the body is flatteneddecisiveClose look
Varies
Body flattened side to side
Body see-through, backbone or gut visibledecisiveAt a glance
No
Not diagnostic
Crawling on rock or debrisdecisiveWhere it was
No
Not diagnostic
Reported size
5–51 mm
5–20 mm
Water it is recorded indecisive
surf
river, stream, lake margin, spring
Life stages covered
young-of-year, adult
adult
How it moves
burrows rear-first, tail and abdomen going in before the head; pops up out of the sand, rides a wave, and burrows again
swims on its side (sideswimmer); rapid darting
Reference seasonality
Swash zone, ice-free season: Apr, May, Jun, Jul, Aug, Sep, Oct
No family-level window
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 (Hippidae)
Mole crab (sand flea)
It cannot pinch you. It goes into the sand backwards. It rides waves on purpose.
First check
Look for claws. There are none on the front legs and nothing that can pinch. Add an egg-shaped smooth shell and a pair of feathery antennae and you are holding a mole crab.
What it is
The animal most of the surf coast calls a sand flea is not a flea and barely reads as a crab. It is an egg-shaped, smooth thing with no claws you can see, that lives inside the swash — not beside it, inside it, in the strip of wet sand where the waves run up and drain back. Florida's extension service puts the useful fact plainly: they are clawless and cannot bite or pinch. Whatever it is that just came up in the wash with your foot, if it has no claws on its front legs, this is it.
Stages
Young (5–13 mm) · Full-grown (13–51 mm, and the sources disagree about that — see the limitations)
Water
surf
Separates from Scud / Amphipod
The name collision is real: the other animal called a sand flea is an amphipod, which is flattened side to side, swims on its side, and lives on the bottom rather than in it.
Typical windows
Swash zone, ice-free season (adult)
Patterns (correspondence)
None mapped yet.
Limits
Which Emerita is the sand flea differs by source and by place — two Florida sources name two different species.
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 Mole crab (sand flea)
This entry does not name Mole crab (sand flea) 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.
