Bowling pin, gripping the rock, waving downstream.
First checkIs the back end swollen, and is it holding on to the stone with the body standing up in the current?
Turn a rock over in fast water and this is the thing that is standing up off it. A black fly larva anchors by its swollen back end to a pad of its own silk, so the body stands out into the current and bends downstream, and they do it in colonies — a whole stone of them, all leaning the same way.
The shape says the same thing as the posture: narrow at the head, conspicuously swollen at the rear. A midge larva is a worm of even thickness that is not attached to anything, and a cased caddis is inside a tube it built.
The pair of brush-like fans at the head is the classic character and the least reliable one in the field: it is visible on a larger larva and not on a small one. Use the shape and the posture.
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 Black fly larva
Swollen rear end gripping a rock in current, body standing up and waving downstream+7
No long tail filaments visible+3
Hardened head with softer worm-like abdomen+2
Terminal prolegs or hooks at the rear+2
Aquatic insect crawling on rock or debris+2
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
Moving as a school or tight bait cluster, not as a single insect-4
No hardened head capsule and no legs — soft and segmented throughout-4
Head withdrawn into the front of the body, hard to see-4
A single tail filament pointing straight back, and no paired hooks-4
Plate-like or feather-like abdominal gills visible-4
Gills at the thorax or leg bases-4
Extendable grasping mouthparts (labium / mask) on an aquatic nymph-4
Three leaf-like caudal lamellae at the tail (damselfly nymph) rather than round filaments-4
Hard wing covers (elytra) meeting in a straight line down the back-4
Fallen terrestrial insect (hopper, ant, beetle, etc.) — not an aquatic emergence-4
Hopper / grasshopper silhouette — hind legs long, body blocky, wings folded along back-4
Pinched waist (ant / flying ant) with elbowed antennae-4
One pair of long, flattened, oar-like hind legs used to row-4
Swims on its back, belly upwards-4
Mouthparts form a pointed beak rather than chewing jaws-4
Portable case or case-like structure present-3
Air bubble or silvery film carried under water (diving beetle / water beetle)-3
Wings held upright (sailboat posture)-3
Wings held tent-like or inverted-V over body-3
Wings held flat and parallel along the body-3
Two long antennae-3
Fleshy lobes or a disc at the blunt rear end-2
Winged insect at or near the surface-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.
Black fly larva — Larvadrawn
Larva
Size in hand5–15 mm
What you see
A swollen rear end gripping the rock; the body stands up and waves downstream
Dense colonies on one stone, all bending the same way
A hardened rounded head, with brush-like fans on larger larvae
No jointed thoracic legs — one proleg at the front
Where it turns up
Flowing water only: rock faces, wood and trailing weed where the current is fast, including pond spillways.
How it moves
Anchored and filtering rather than swimming. It repositions by an inchworm loop, and relocates by paying out a silk thread and drifting on it — which is a real way for one to end up in front of a fish.
Cues this stage offers the comparison
Swollen rear end gripping a rock in current, body standing up and waving downstream
No long tail filaments visible
Hardened head with softer worm-like abdomen
no reviewed plateNo drawing yet. The pupa sits in a silk pocket on the same stone as the larvae and is worth a plate it has not been given.Black fly larva — Pupa — and this one is under watergap
Pupa — and this one is under water
Size in hand4–8 mm
What you see
A silk pocket attached to the same substrate, open downstream
Filament gills projecting from the front of the pocket
Where it turns up
On the same stones as the larvae, in the same current.
How it moves
Fixed. The adult leaves it and rises to the surface.
no reviewed plateNo drawing yet. The adult is small, dark and humpbacked, and leaves the water quickly; no plate has been authored for it.Black fly larva — Adultgap
Adult
Size in hand2–5 mm
What you see
Small, dark, humpbacked
Leaves the water and is gone quickly
Where it turns up
At and above the surface over the riffle it emerged from.
How it moves
Rises through the water and flies.
Confused with
Telling it from what it isn't.
Black fly larvavsMidge (non-biting)
A midge larva is a worm of even thickness, free-living or in a soft tube, and it is not holding on to anything. A black fly larva has a swollen rear gripping the stone.
Where in the column it mattersNear the bottom. Anchored to the rock, not drifting — except that it relocates by paying out silk and riding it, which is a real and documented way for one to arrive in front of a fish.
Read from the record's own position notes: attached rear-first to rock, wood and trailing weed in current; in dense colonies on the same stone.
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.
Nothing in the pattern layer is mapped to this group, so no presentation family follows from it here.
Not known
What this entry cannot tell you.
10 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 reviewerFamily level. Genera differ in seasonality, so this does not date the observation.
Declared by the reviewerIt does not give the instar, and instar matters here: 5 mm and 15 mm are a threefold difference in imitation size.
Declared by the reviewerThe family spans a broad tolerance range. Finding these tells you the water is flowing over stable substrate, and very little about its quality.
Declared by the reviewerThe published larval size range disagrees between two sources. The wider figure is the one used here, and the disagreement is recorded in the carrier rather than averaged away.
Declared by the reviewerA group-level call is the family. It does not give the genus, and genera differ in seasonality, so this record does not date the observation.
Declared by the reviewerThe family spans a broad tolerance range, so finding these tells you the water is flowing over stable substrate and very little about its quality.
Sources disagree[object Object]
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.
PatternsNo fly pattern in the pattern layer is mapped to this group. Whether anything imitates it is not answered here.
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 4 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.
Leaf Pack Network. Anchored by the swollen rear while the body sweeps in the flow. Posture plus motion is the whole signature of this larva.
Research institute · external video · title read back from the publisher 2026-09-09
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Which fish in this kind of water are documented eating this class of forage, and how they behave when they are.
What this forage class asks of the setup — the weight it wants, and where the chain gives first.
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