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: Crawling on rock or debrisWhere it was It partly separates 1 of the 1 pairs still in play and you can check it at a glance.
What to look at
Freshwater snail
Caddisfly
Crawling on rock or debrisWhere it was
Yes
Not diagnostic
Reported size
2–70 mm
3–40 mm
Water it is recorded in
spring, stream, river, lake margin, lake open, reservoir
river, stream, lake margin
Life stages covered
adult
larva, pupa, adult
How it moves
glides slowly on a muscular foot
crawling with case; swimming pupa
Reference seasonality
No family-level window
Emergence: Sep, Oct, Nov
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 Caddisfly
Both can look like a lump of grit on a stone. A caddis case has a jointed head and legs coming out of the front; a snail has a soft foot.
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.
Not every caddis builds a house, and the ones that don't are the ones you miss.
First check
Ignore the case. Look for a hard dark head against a soft pale abdomen, and hooks at the back end.
What it is
Caddis is the group most often identified by a thing that is only sometimes there. The portable case — grains of sand, cut leaf, twig fragments, spun into a tube the larva carries — is the classic image, and when you find one the call is easy. But a large share of caddis larvae never build a portable case at all. Free-living forms hunt loose among the rocks; net-spinners build a fixed retreat with a catch-net beside it and leave the retreat behind when disturbed.
Stages
Larva (3–40 mm (often 13–25 mm with case)) · Pupa (No source consulted gives a pupal size) · Adult (4–38 mm)
Water
river, stream, lake-margin
Separates from Freshwater snail
At arm's length on a stone, a grit-built caddis case and a snail shell look the same. Pick it up: a caddis case has a jointed head and legs coming out of the front, a snail has a soft foot that grips your finger.
Typical windows
Larvae present (larva) · Emergence (pupa)
Patterns (correspondence)
Elk Hair Caddis · X-Caddis · Sparkle Pupa · Green Rock Worm · Peeking Caddis
Limits
Case-building behavior varies and absence of a visible case does not rule out caddis.
