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: No hard head capsule and no legsClose look It separates Cranefly larvae from Aquatic worms cleanly, and every pair of groups still in play differs on it (look closely with it still).
What to look at
Cranefly larvae
Aquatic worms
No hard head capsule and no legsdecisiveClose look
Not diagnostic
Yes
Reported size
3–60 mm
1–150 mm
Water it is recorded in
stream, river, lake margin, spring, reservoir
lake margin, river, stream, reservoir
Life stages covered
larva
adult
How it moves
classified as a crawler and burrower; no source consulted describes how it actually moves
stretching and pulling like an earthworm; waving in the water while anchored in the sediment
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 (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 worms
No head, thorax or abdomen distinction at all, with bristle bundles along the whole body, is an aquatic worm. A cranefly larva has a head capsule, even a withdrawn one, and a distinct lobed tail disc.
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.
Segmented, round, no legs, no hard head — then look for bristle bundles and check both ends for suckers.
What it is
A soft segmented tube, round in cross section, with no legs and no hardened head. Bundles of fine bristles run along the segments and are the character that separates it from a leech.
Stages
Adult (1–150 mm across the group, most 1–30 mm; tubificids to about 25 mm)
Water
lake-margin, river, stream, reservoir
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
No family-level window in this layer.
Patterns (correspondence)
None mapped yet.
Limits
The regional guide gives 1–30 mm for most aquatic worms and up to 150 mm for the largest; treat the upper figure as the extreme, not the norm.