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 Water boatmen from Backswimmers cleanly, and every pair of groups still in play differs on it (watch how it moves).
What to look at
Water boatmen
Backswimmers
Swims on its back, belly upwardsdecisiveWatch it
No
Yes
Reported size
4–29 mm
10–29 mm
How it moves
rows with long oar-like hind legs fringed with fine hairs; swims the right way up, back uppermost
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
The same for both: water, life stages, seasonality.
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 Backswimmers
Belly-up swimming and a keeled back with no crosslines is a backswimmer. A boatman swims back-uppermost on a flat ruled back — and, per Missouri, does not bite.
Typical windows
Overwinter in the water (adult)
Patterns (correspondence)
None mapped yet.
Limits
Size is stated three incompatible ways across three sources.
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 Water boatmen
Right-way-up swimming, a flat back ruled by dark parallel crosslines, and scoop-shaped front feet is a boatman. Missouri says it also does not bite, and this one does.
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.