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: Hard wing covers (elytra) in a dorsal lineClose look It separates Water boatmen from Aquatic beetles cleanly, and every pair of groups still in play differs on it (look closely with it still).
What to look at
Water boatmen
Aquatic beetles
Mouthparts are a pointed beak, not jawsdecisiveIn hand
Yes
No
Hard wing covers (elytra) in a dorsal linedecisiveClose look
No
Yes
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
Swims on its back, belly upwardsdecisiveWatch it
No
Not diagnostic
Fallen terrestrial (hopper, ant, beetle)At a glance
No
Not diagnostic
Reported size
4–29 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 fringed with fine hairs; swims the right way up, back uppermost
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 (Corixidae)
Water boatmen
Right way up, ruled across the back, and one of the few aquatic bugs said not to bite.
First check
Which way up is it swimming? Back uppermost, with dark parallel crosslines across a flat back, is a water boatman.
What it is
A water boatman is a flattened oval bug that rows with one pair of long oar-like hind legs and swims the right way up. That last clause is the whole entry, because its near-twin does the opposite, and this is the pair anglers get backwards more reliably than any other in still water.
Stages
Nymph (Not given by the sources; a smaller, wingless version of the adult) · Adult (Usually under 13 mm; the sources bracket it three different ways, one of them running to 29 mm)
Water
lake-margin, reservoir, stream, river, spring
Separates from Aquatic beetles
Hard wing covers meeting in a straight seam down the back, threadlike antennae you can actually see, and chewing mouthparts, is a beetle.
Typical windows
Overwinter in the water (adult)
Patterns (correspondence)
None mapped yet.
Limits
Size is stated three incompatible ways across three sources.
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 Water boatmen
This entry does not name Water boatmen 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.