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: Winged insect at or near surfaceWhere it was It separates Cranefly larvae from Aquatic beetles cleanly, and every pair of groups still in play differs on it and you can check it at a glance.
What to look at
Cranefly larvae
Aquatic beetles
Head withdrawn into the front of the bodydecisiveClose look
Yes
No
Fleshy lobes or a disc at the blunt reardecisiveClose look
Yes
No
Hard wing covers (elytra) in a dorsal linedecisiveClose look
No
Yes
Air bubble / silvery film underwaterdecisiveClose look
No
Yes
Hardened head with softer worm-like abdomendecisiveClose look
No
Not diagnostic
How the wings restdecisiveAt a glance
None of these
Varies
Winged insect at or near surfacedecisiveWhere it was
No
Not diagnostic
One pair of long flattened oar-like legsdecisiveClose look
No
Not diagnostic
Reported size
3–60 mm
3–76 mm
Water it is recorded in
stream, river, lake margin, spring, reservoir
river, stream, lake margin, spring
Life stages covered
larva
larva, adult
How it moves
classified as a crawler and burrower; no source consulted describes how it actually moves
rowing with hind legs; carrying a silvery air bubble or film
Reference seasonality
No family-level window
Diving / whirligig adults: Apr, May, Jun, Jul, Aug, Sep, Oct · Present under ice: Dec, Jan, Feb
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 (Tipulidae)
Cranefly larvae
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 Aquatic beetles
Three pairs of segmented legs rules out every fly larva.
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.
Hard wing covers in a dorsal line, or an air bubble under water. If you see a case, look at caddis instead.
What it is
Water beetles are easy to overcall as caddis or terrestrials. The adult tell is elytra meeting in a straight line, often with a visible air bubble when diving.
Stages
Larva (3–76 mm; predaceous diving beetle larvae to about 51 mm) · Adult (3–38 mm)
Water
river, stream, lake-margin, spring
Separates from Cranefly larvae
This entry does not name Cranefly larvae as a look-alike. That is a gap in the prose, not proof they cannot be confused.
Typical windows
Diving / whirligig adults (adult) · Present under ice (adult)
Patterns (correspondence)
Soft-hackle Beetle
Limits
Broad Coleoptera only. Diving beetles, whirligigs and scavengers differ.