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: A sucker at each endClose look It separates Leech from Aquatic worms cleanly, and every pair of groups still in play differs on it (look closely with it still).
What to look at
Leech
Aquatic worms
A sucker at each enddecisiveClose look
Yes
No
Bristle bundles along the segmentsdecisiveHand lens
No
Yes
Reported size
5–500 mm
1–150 mm
How it moves
swimming; looping along like an inchworm using both suckers
stretching and pulling like an earthworm; waving in the water while anchored in the sediment
The same for both: water, life stages, 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
Hirudinea
Leech
A sucker at each end, the front one often small.
First check
Look for a sucker at each end — the front one can be small. Suckers mean a leech.
What it is
One character does most of the job: a sucker at each end of the body, though the front one can be small. An aquatic worm has none.
Stages
Adult (5–500 mm — it stretches, so treat any measurement as approximate)
Water
lake-margin, river, stream, reservoir
Separates from Aquatic worms
Bristle bundles along the segments and no suckers is an aquatic worm. Suckers and no bristles is a leech. The pair is reciprocal, but the bristles are often too fine to see, so check the suckers first.
Typical windows
No family-level window in this layer.
Patterns (correspondence)
Bunny Leech
Limits
No source consulted states whether leeches are nocturnal or day-active beyond the observation that they usually avoid light, and none addressed seasonal activity.
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 Leech
A sucker at each end is a leech. Bristles and no suckers is a worm.
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.