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: Portable case or case-like structureAt a glance It separates Aquatic beetles 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
Aquatic beetles
Caddisfly
Hard wing covers (elytra) in a dorsal linedecisiveClose look
Yes
No
Portable case or case-like structuredecisiveAt a glance
No
Yes
Air bubble / silvery film underwaterdecisiveClose look
Yes
Not diagnostic
How the wings restdecisiveAt a glance
Varies
Wings tent-like / inverted-V
Terminal prolegs or hooks at the reardecisiveClose look
Not diagnostic
Yes
One pair of long flattened oar-like legsdecisiveClose look
Not diagnostic
No
Hardened head with softer worm-like abdomenClose look
Not diagnostic
Yes
Fallen terrestrial (hopper, ant, beetle)At a glance
Not diagnostic
No
Reported size
3–76 mm
3–40 mm
Water it is recorded in
river, stream, lake margin, spring
river, stream, lake margin
Life stages covered
larva, adult
larva, pupa, adult
How it moves
rowing with hind legs; carrying a silvery air bubble or film
crawling with case; swimming pupa
Reference seasonality
Diving / whirligig adults: Apr, May, Jun, Jul, Aug, Sep, Oct · Present under ice: Dec, Jan, Feb
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
Coleoptera
Aquatic beetles
Hard covers, a silver bubble, and no case.
First check
Hard wing covers in a dorsal line, or an air bubble under water. If you see a case, look at caddis instead.
What it is
Water beetles are easy to overcall as caddis or terrestrials. The adult tell is elytra meeting in a straight line, often with a visible air bubble when diving.
Stages
Larva (3–76 mm; predaceous diving beetle larvae to about 51 mm) · Adult (3–38 mm)
Water
river, stream, lake-margin, spring
Separates from Caddisfly
A portable case is caddis. Elytra plus an air bubble is a beetle.
Typical windows
Diving / whirligig adults (adult) · Present under ice (adult)
Patterns (correspondence)
Soft-hackle Beetle
Limits
Broad Coleoptera only. Diving beetles, whirligigs and scavengers differ.
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 Aquatic beetles
Hard elytra meeting in a line, or an air bubble on a diving adult, is a beetle. A portable case or terminal hooks is caddis.
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.
