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: Swollen rear gripping a rock, body waving in the currentWhere it was It separates Black fly larva from Cranefly larvae cleanly, and every pair of groups still in play differs on it and you can check it at a glance.
What to look at
Black fly larva
Cranefly larvae
Head withdrawn into the front of the bodydecisiveClose look
No
Yes
Swollen rear gripping a rock, body waving in the currentdecisiveWhere it was
Yes
No
Fleshy lobes or a disc at the blunt reardecisiveClose look
Not diagnostic
Yes
Hardened head with softer worm-like abdomendecisiveClose look
Yes
No
Terminal prolegs or hooks at the reardecisiveClose look
Yes
No
Gills at the thorax or leg basesdecisiveHand lens
Not diagnostic
No
Winged insect at or near surfaceWhere it was
Not diagnostic
No
No hard head capsule and no legsClose look
No
Not diagnostic
Reported size
1.2–15 mm
3–60 mm
Water it is recorded in
stream, river
stream, river, lake margin, spring, reservoir
Life stages covered
larva, pupa, adult
larva
How it moves
anchored and filtering rather than swimming; relocates by paying out a silk thread and drifting on it
classified as a crawler and burrower; no source consulted describes how it actually moves
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
Diptera (Simuliidae)
Black fly larva
Bowling pin, gripping the rock, waving downstream.
First check
Is the back end swollen, and is it hooked on to the stone with its head held downstream?
What it is
Turn a rock over in fast water and this may be what is attached to it. A black fly larva anchors by its swollen back end to a pad of its own silk, with its head held downstream to filter the current.
Stages
Larva (5–15 mm) · Pupa — and this one is under water (2–3 mm) · Adult (About 1–5 mm (NC State 1.2–3 mm; Pennsylvania DEP 2–5 mm))
Water
stream, river
Separates from Cranefly larvae
Both are legless grubs. A cranefly larva is plump and even, with a rear disc ringed by fleshy lobes, and is not anchored in current.
Typical windows
No family-level window in this layer.
Patterns (correspondence)
None mapped yet.
Limits
Family level. Species differ in seasonality, so this does not date the observation.
A grub with nothing to walk on and a head it keeps to itself.
First check
Look for legs. None at all, on a plump segmented grub, means a fly larva. Then look at the blunt back end for a disc ringed with lobes.
What it is
A cranefly larva is a plump, blunt, segmented grub, tan to grey to greenish, with a dark line of gut showing through the skin. It has no legs — and that is not a small observation, because one state assessment guide puts the whole order behind it: flies are the only aquatic insect whose larvae lack fully developed legs. If you can see three pairs of jointed legs, you are not holding one of these.
Stages
Larva (3–60 mm across the consulted sources, which disagree substantially)
Water
stream, river, lake-margin, spring, reservoir
Separates from Black fly larva
This entry does not name Black fly larva 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)
None mapped yet.
Limits
Sources disagree about the head: Missouri says the larvae have a definite head; WVDEP says it is often reduced and barely visible, and a university key notes the thorax often has to be cut to see it. This entry follows the second.