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 separates Clam (small freshwater bivalve) from Caddisfly cleanly, and every pair of groups still in play differs on it and you can check it at a glance.
What to look at
Clam (small freshwater bivalve)
Caddisfly
Crawling on rock or debrisdecisiveWhere it was
No
Not diagnostic
Reported size
2–20 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
ploughs slowly through sediment on a muscular foot; pulls its two shells shut with strong muscles
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
Bivalvia
Clam (small freshwater bivalve)
Two halves and a hinge. That is as far as this goes.
First check
Two halves with a hinge along the top edge, or one coiled shell?
What it is
Two hinged shell halves that close on the animal. In the hand it is quick: you can see the seam and feel the hinge, and a live one pulls its shell shut. The one two-shelled lookalike, the clam shrimp, has jointed legs and feathery antennae.
Stages
Every stage — young are miniature adults (2–20 mm for fingernail and pea clams, most under 13 mm)
Water
spring, stream, river, lake-margin, lake-open, reservoir
Separates from Caddisfly
A caddis case is built of glued grit and has a jointed head and legs coming out of it. A clam is a shell the animal grew, with nothing coming out but a foot.
Typical windows
No family-level window in this layer.
Patterns (correspondence)
None mapped yet.
Limits
Honest at 'small bivalve' and no finer. A young Asian clam can be mistaken for a fingernail clam; raised ridges and a thick shell point to the Asian clam, but this guide does not make that call.
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 Clam (small freshwater bivalve)
This entry does not name Clam (small freshwater bivalve) as a look-alike. That is a gap in the prose, not proof they cannot be confused.
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.
