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: How the wings restAt a glance It separates Mayfly from Stonefly cleanly, and every pair of groups still in play differs on it and you can check it at a glance.
What to look at
Mayfly
Stonefly
Gills at the thorax or leg basesdecisiveHand lens
No
Yes
How the wings restdecisiveAt a glance
Wings held upright (sailboat)
Wings held flat and parallel along the body
Abdominal gills (plate or feather)decisiveClose look
Yes
Not diagnostic
Tail filamentsdecisiveClose look
Three tail filaments clearly visible
Two tail filaments clearly visible
Two long antennaedecisiveClose look
Not diagnostic
Yes
Crawling on rock or debrisWhere it was
Not diagnostic
Yes
Reported size
2–40 mm
5–75 mm
Water it is recorded in
river, stream, lake margin
river, stream, spring
Life stages covered
nymph, subimago, adult
nymph, adult
How it moves
swimming undulation; crawling on substrate
crawling on rock or debris; weak flight; adults often stay near the ground
Pre-emergence migration: Apr, May, Jun, Jul · Adult activity: May, Jun, Jul
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
Ephemeroptera
Mayfly
This one grows wings, and then does it over.
First check
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 Stonefly
Look at the gills. Mayfly gills are plate-like or feathery and run down the sides of the abdomen; stonefly gills are thread-like tufts bunched at the thorax and leg bases. Two tails alone will not separate them.
Two tails, two antennae, and a poor opinion of swimming.
First check
Two tails AND two long antennae, with any gills at the thorax and leg bases rather than along the abdomen.
What it is
Most stoneflies want cold, fast, well-oxygenated water, and they say so by being where it is. The nymph is a crawler: it creeps under and among stones rather than moving about in open water.
Stages
Nymph (5–70 mm (many western species large)) · Adult (6–75 mm; most 13–50 mm)
Water
river, stream, spring
Separates from Mayfly
Gill position is the honest separator. Stonefly gills tuft at the thorax and leg bases; mayfly gills are plate-like along the sides of the abdomen. Two tails alone proves nothing.
Typical windows
Nymphs present (nymph) · Pre-emergence migration (nymph) · Adult activity (adult) · Small winter species (adult)
Patterns (correspondence)
Rubber-Legs Stonefly Nymph · Twenty-Incher · Stimulator · Little Black Stonefly
Limits
Habitat and emergence timing vary considerably by species.
