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: Hopper / grasshopper silhouetteAt a glance It separates Aquatic beetles from Terrestrials (ants, beetles, hoppers) cleanly, and every pair of groups still in play differs on it and you can check it at a glance.
Air bubble / silvery film underwaterdecisiveClose look
Yes
No
Hopper / grasshopper silhouettedecisiveAt a glance
No
Yes
Fallen terrestrial (hopper, ant, beetle)decisiveAt a glance
Not diagnostic
Yes
Abdominal gills (plate or feather)decisiveClose look
Not diagnostic
No
Mouthparts are a pointed beak, not jawsdecisiveIn hand
No
Not diagnostic
Hardened head with softer worm-like abdomendecisiveClose look
Not diagnostic
No
One pair of long flattened oar-like legsClose look
Not diagnostic
No
Reported size
3–76 mm
2–76 mm
Water it is recorded in
river, stream, lake margin, spring
river, stream, lake margin
Life stages covered
larva, adult
adult
How it moves
rowing with hind legs; carrying a silvery air bubble or film
struggling or kicking on the surface; hopping on bank vegetation
Reference seasonality
Diving / whirligig adults: Apr, May, Jun, Jul, Aug, Sep, Oct · Present under ice: Dec, Jan, Feb
Bank falls: Jun, Jul, Aug, Sep
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 Terrestrials (ants, beetles, hoppers)
One swimming under water with a bubble is aquatic Coleoptera. A beetle on the film is not automatically a terrestrial: whirligigs live there.
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.
Did it fall from the bank, or did it come up from the water? If you cannot tell, do not name it a terrestrial.
What it is
Terrestrial invertebrates are an important part of stream fish diets, especially in summer, and none of them lived in the water. Hoppers, ants and beetles arrive from the bank. The observation class is 'fallen terrestrial', not a hatch.
Stages
Fallen adult (2–76 mm: ants to about 25 mm, grasshoppers mostly 13–25 mm and up to 76 mm. No source here gives beetle sizes.)
Water
river, stream, lake-margin
Separates from Aquatic beetles
If it is swimming under water with an air bubble, it is the aquatic beetle group, not a fallen land beetle.
Typical windows
Bank falls (adult)
Patterns (correspondence)
Dave's Hopper · Fur Ant · Foam Beetle
Limits
Broad observation class only — not a species ID and not a hatch.