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: Swims on its back, belly upwardsWatch it It separates Backswimmers from Aquatic beetles cleanly, and every pair of groups still in play differs on it (watch how it moves).
What to look at
Backswimmers
Aquatic beetles
Mouthparts are a pointed beak, not jawsdecisiveIn hand
Yes
No
Hard wing covers (elytra) in a dorsal linedecisiveClose look
No
Yes
Swims on its back, belly upwardsdecisiveWatch it
Yes
Not diagnostic
One pair of long flattened oar-like legsdecisiveClose look
Yes
Not diagnostic
Abdominal gills (plate or feather)decisiveClose look
No
Not diagnostic
Hardened head with softer worm-like abdomendecisiveClose look
No
Not diagnostic
No hard head capsule and no legsdecisiveClose look
Not diagnostic
No
Fallen terrestrial (hopper, ant, beetle)At a glance
No
Not diagnostic
Reported size
10–29 mm
3–76 mm
Water it is recorded in
lake margin, reservoir, stream, river, spring
river, stream, lake margin, spring
Life stages covered
nymph, adult
larva, adult
How it moves
rows with long oar-like hind legs, belly upwards; rests tilted head-down at the surface with the tip of the abdomen through the film
rowing with hind legs; carrying a silvery air bubble or film
Reference seasonality
Overwinter in the water: Nov, Dec, Jan, Feb, Mar
Diving / whirligig adults: Apr, May, Jun, Jul, Aug, Sep, Oct · Present under ice: Dec, Jan, Feb
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
Hemiptera (Notonectidae)
Backswimmers
Upside down, keeled like a hull, and it does bite.
First check
Which way up? Belly upwards, on a body keeled down the back, is a backswimmer. Then check the mouth is a beak, not jaws — that is what makes it a bug and not a beetle.
What it is
A backswimmer swims on its back. Two independent sources state it plainly — belly-up, ventral side facing upwards — which makes it the settled half of a pair that is otherwise easy to muddle. Its back, which is underneath while it swims, is keeled like the bottom of a boat, and carries a dark V-shaped mark rather than the ruled crosslines a water boatman has.
Stages
Nymph (Not given by the sources; a smaller, wingless version of the adult) · Adult (10–29 mm, usually under 13; the sources bracket it three different ways)
Water
lake-margin, reservoir, stream, river, spring
Separates from Aquatic beetles
Head-down at the surface settles nothing — a diving beetle does exactly the same. Look at the mouth: a beak is a bug, chewing jaws are a beetle. Hard wing covers meeting in a straight seam are a beetle too.
Typical windows
Overwinter in the water (adult)
Patterns (correspondence)
None mapped yet.
Limits
The most detailed account consulted covers a Eurasian species; its air-film duration, temperature behaviour and one-brood-a-year seasonality are not offered here as North American facts.
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 Backswimmers
This entry does not name Backswimmers as a look-alike. That is a gap in the prose, not proof they cannot be confused.
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.