Side by side · same standard

What actually separates these two.

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.

Group AGroup B

The differences, and nothing else

Best separating check: How the wings rest At a glance It separates Aquatic beetles from Damselfly / Dragonfly 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	Damselfly / Dragonfly
Grasping labium / mask (Odonata nymph)decisiveIn hand	No	Yes
Hard wing covers (elytra) in a dorsal linedecisiveClose look	Yes	No
Leaf-like caudal lamellae (damselfly nymph)decisiveClose look	No	Yes
Air bubble / silvery film underwaterdecisiveClose look	Yes	No
Tail filamentsdecisiveClose look	No long tail filaments visible	Varies
One pair of long flattened oar-like legsdecisiveClose look	Not diagnostic	No
Swims on its back, belly upwardsWatch it	Not diagnostic	No
Reported size	3–76 mm	6–89 mm
Life stages covered	larva, adult	nymph, adult
How it moves	rowing with hind legs; carrying a silvery air bubble or film	climbing vegetation; jet propulsion (dragonfly nymph)
Reference seasonality	Diving / whirligig adults: Apr, May, Jun, Jul, Aug, Sep, Oct · Present under ice: Dec, Jan, Feb	Adult hunting: May, Jun, Jul, Aug, Sep

The same for both: water.

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 Damselfly / Dragonfly

A labium mask is Odonata, not 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.

Full Aquatic beetles entry →

Odonata

Damselfly / Dragonfly

The mask is the tell. The 'tails' on a damsel nymph are gills.

First check

Look at the mouth. If you can see a grasping mask, you are in Odonata.

What it is

Odonata nymphs are predators. The extendable labium — the mask — is the strongest field separator from mayfly and stonefly.

Stages

Nymph (6–68 mm; usually 13–68 mm) · Adult (body 25–89 mm; damselflies to about 64 mm)

Water

river, stream, lake-margin, spring

Separates from Aquatic beetles

This entry does not name Aquatic beetles as a look-alike. That is a gap in the prose, not proof they cannot be confused.

Typical windows

Nymphs present (nymph) · Adult hunting (adult)

Patterns (correspondence)

Damsel Nymph · Dragonfly Nymph

Limits

Broad order only. Not a family or species call.

Full Damselfly / Dragonfly entry →