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: ShellAt a glance It partly separates 0 of the 1 pairs still in play and you can check it at a glance.
What to look at
Freshwater snail
Leech
Reported size
2–70 mm
5–500 mm
Water it is recorded in
spring, stream, river, lake margin, lake open, reservoir
lake margin, river, stream, reservoir
How it moves
glides slowly on a muscular foot
swimming; looping along like an inchworm using both suckers
The same for both: 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
Gastropoda
Freshwater snail
One shell, usually coiled, and a foot out of it.
First check
Is it a single shell — coiled, or a low cone in a limpet — with a foot rather than a head with legs coming out of it?
What it is
A single shell with a soft foot gliding out of it is one of the few things in this guide that is hard to confuse once you have it in your hand. The shell is usually coiled; a freshwater limpet's is a low cone.
Stages
Every stage — juveniles are the same animal, smaller (2–70 mm (most under 20 mm))
Water
spring, stream, river, lake-margin, lake-open, reservoir
Separates from Leech
A leech has no shell at all and carries a sucker at each end.
Typical windows
No family-level window in this layer.
Patterns (correspondence)
None mapped yet.
Limits
'Snail' and no finer. This guide does not attempt the gilled/lunged split, because the character that decides it, the operculum, is hard to see on a small shell.
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 Freshwater snail
This entry does not name Freshwater snail 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)
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.