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: Two long antennaeClose look It separates Aquatic sowbug from Mayfly cleanly, and every pair of groups still in play differs on it (look closely with it still).
What to look at
Aquatic sowbug
Mayfly
Two long antennaedecisiveClose look
Yes
Not diagnostic
Crawling on rock or debrisWhere it was
Yes
Not diagnostic
Reported size
5–20 mm
2–40 mm
Water it is recorded in
spring, stream, river, lake margin
river, stream, lake margin
Life stages covered
adult
nymph, subimago, adult
How it moves
slow deliberate crawl over rock and wood; crawls flat along the bottom, back uppermost, rather than on its side
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
Isopoda (Asellidae)
Aquatic sowbug
It lies flat. A scud falls on its side.
First check
Put it in a shallow tray of water. If it crawls flat, back-up, it is a sowbug. If it swims or lies on its side, it is a scud.
What it is
Sowbugs and scuds are the same size, live in the same water, and both have seven pairs of walking legs. Almost everything written about telling them apart is a list of characters you would need a lens for.
Stages
Every stage — they all look the same (5–20 mm)
Water
spring, stream, river, lake-margin
Separates from Mayfly
Count the legs. Seven pairs is a crustacean; three pairs plus tail filaments and gills is a mayfly nymph.
Typical windows
No family-level window in this layer.
Patterns (correspondence)
Sowbug
Limits
Family level only. This does not separate Caecidotea from Asellus, and it does not tell a surface animal from a groundwater one.
Count the tails, then look for gills along the sides of the abdomen. Three tails plus side gills is about as close to a clean mayfly call as field observation gets.
What it is
Mayflies are the only insects that grow a full set of working wings and then moult again. The winged insect that leaves the water is not the finished animal — it is a dun, a subimago, cloudy-winged and dull-bodied. Within hours to a couple of days it sheds one more time into a spinner: clear-winged, smooth and shiny, built only to mate and die. Two winged forms of the same animal, hours apart, sitting on the same water.
Stages
Nymph (2–32 mm) · Dun (subimago) (Under 4 to 40 mm body; most families 4–20 mm) · Spinner (imago) (Under 4 to 40 mm body; most families 4–20 mm)
Water
river, stream, lake-margin
Separates from Aquatic sowbug
This entry does not name Aquatic sowbug as a look-alike. That is a gap in the prose, not proof they cannot be confused.
