Not an insect, not a hatch, and the most common false positive there is.
First checkWatch it swim. On its side means scud, and you can stop there.
Disambiguation entry. This group is here to stop a wrong call, not to make a right one. It is not an insect and it never hatches.
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.
They earn their place here because in a tray of stream samples a scud reads, at a glance, like a small pale nymph. The separators are decisive once you look: the body is compressed side to side rather than top to bottom, there are seven pairs of walking legs rather than three, there are no wing pads and no wing buds ever, and there are no insect tail filaments.
The behaviour is the fastest tell of all. A scud swims on its side. Nothing in the insect groups here does that.
The tell
What settles it, and what settles against it.
Read off this record's own diagnostic weights, heaviest first. A structure near the top of the left column does most of the work on its own; one near the bottom only leans.
Argues for Scud / Amphipod
Body flattened side to side, often lying or swimming on its side+5
No long tail filaments visible+3
Aquatic insect crawling on rock or debris+1
Body translucent — light passes through it, and the backbone or gut shows+1
Argues against it
Fish-shaped forage silhouette (minnow / shiner / fry) — not an insect-5
Three tail filaments clearly visible-5
Two tail filaments clearly visible-5
Body flattened top to bottom — it lies flat with its back uppermost-5
Fallen terrestrial insect (hopper, ant, beetle, etc.) — not an aquatic emergence-4
Moving as a school or tight bait cluster, not as a single insect-4
Portable case or case-like structure present-4
Wings held upright (sailboat posture)-4
Pincers or claws on the front legs-4
Hopper / grasshopper silhouette — hind legs long, body blocky, wings folded along back-3
Pinched waist (ant / flying ant) with elbowed antennae-3
Hard wing covers (elytra) meeting in a straight line down the back-3
Extendable grasping mouthparts (labium / mask) on an aquatic nymph-3
Winged insect at or near the surface-3
Plate-like or feather-like abdominal gills visible-3
Wings held tent-like or inverted-V over body-3
Wings held flat and parallel along the body-3
Fan-shaped tail; escapes by flicking rapidly backwards-3
A shell with no claws or pincers on it at all-3
Backs into wet sand rear end first-3
A pair of feathery antennae fanned out into the water-3
Back legs flattened into paddles-3
A toothed blade projecting forward between the eyes-3
Air bubble or silvery film carried under water (diving beetle / water beetle)-2
Three leaf-like caudal lamellae at the tail (damselfly nymph) rather than round filaments-2
Hardened head with softer worm-like abdomen-2
Terminal prolegs or hooks at the rear-2
Gills at the thorax or leg bases-2
Two long antennae-2
No hardened head capsule and no legs — soft and segmented throughout-2
A structure at −4 or below is treated as a hard exclusion by the comparison tool rather than as a penalty: if you saw it, this group is ruled out and no amount of matching elsewhere brings it back. Run a comparison →
Life stages
What it looks like, stage by stage.
Scud / Amphipod — Adult amphipodplate
Adult amphipod
Size in hand3–20 mm (most 5–15 mm)
What you see
Body compressed side to side — taller than wide seen from above
Seven pairs of walking legs; the first pairs often claw-like
No wings, no wing pads, no insect-style cerci at any point in life
Distinct black eyes on many forms
Where it turns up
Among vegetation, in leaf packs, and on substrate — particularly in spring creeks, tailwaters and weedy margins.
How it moves
Swims on its side in rapid darting bursts, then crawls among plants. The sideways swim is characteristic.
Cues this stage offers the comparison
No long tail filaments visible
scud side swim
Confused with
Telling it from what it isn't.
Scud / AmphipodvsAquatic 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.
These groups name Scud / Amphipod in their own separators. If you arrived holding one of them and were not sure, this is the sentence that pointed you over.
Crayfishsaid
A body flattened side to side with no enlarged pincers is a scud. Crayfish is flattened top to bottom and has claws.
Under about 13 mm, flattened side to side, swimming on its side and living on rather than in the bottom, is an amphipod. Compression alone does not separate them — size, transparency, the clawed legs and the toothed rostrum do.
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.
Where in the column it mattersNear the bottom. Tied to cover. Present year-round with no emergence, so it never concentrates into a window the way an insect does.
Read from the record's own position notes: among vegetation, leaf packs, or substrate; swimming in open water near cover.
What eats it
Which fish the dossiers actually name.
No reviewed dossier names this group. That is an absence in the dossier layer rather than a finding about the animal — four dossiers exist and only one of them names any forage group at all.
The pattern layer ties this group in Nymph. That is a correspondence between an imitation and an animal, worked out when the patterns were written. Whether a fish is on it today is a question this page cannot reach.
Scud
Nymph
Hook12–18
A freshwater amphipod — a crustacean, not an insect and not a hatch.
Dead-drifted or twitched through weed and leaf pack. Tied curved, with a shellback over the top.
7 of them. The limits are part of what was reviewed rather than a disclaimer bolted on afterwards. An entry that hid them would be claiming more than it checked.
Written by the reviewer
Declared by the reviewerScuds are crustaceans, not insects. This entry exists solely to stop a scud being called a mayfly, caddis, or stonefly nymph.
Declared by the reviewerNever treat a scud result as a biological hatch cue or as an insect identification.
Declared by the reviewerBroad Amphipoda only; many genera exist and habitat preferences vary.
Declared by the reviewerScuds are crustaceans, not insects. This record exists solely to reduce false positive calls of mayfly, caddis, or stonefly nymphs when the observer sees a laterally flattened, side-swimming form.
Declared by the reviewerNever treat a scud result as a biological 'hatch' cue for trout or as an insect identification.
Read off the record
Nobody typed these. They are fields that are empty, said out loud rather than left for you to notice from a section that simply is not there.
SeasonalityNo reference window is published for this group, so this entry says nothing about when it is available. That is an absence in the calendar layer, not a claim that the animal has no season.
What eats itNo reviewed species dossier names this group. Some dossiers describe forage in class terms — small forage, insects, bait — and those are shown below, but a class description is not a reviewed link to this group and none is claimed.
Sources
Everything above rests on 2 published pages.
Attribution here is to the whole record, not to individual sentences — the carrier attaches a source set to a record and does not say which page carried which line. Naming one per claim would mean inventing a mapping nobody reviewed.
A plate shows shape. It cannot show a gill beating, a mask firing, or an animal reversing into sand — so where motion is the field mark, the entry points at motion. These are demonstrations, selected by hand and by publisher. Nothing here is a source: it changes no score, no window and no reviewed claim.
MPR News. Live amphipods in a sample tray. The side-to-side scudding motion is what tells a scud from a sowbug before you ever look at which way it is squashed.
Public agency or non-profit · external video · title read back from the publisher 2026-09-09
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