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 Midge (non-biting) 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
Midge (non-biting)
Abdominal gills (plate or feather)decisiveClose look
Yes
No
Tail filamentsdecisiveClose look
Three tail filaments clearly visible
No long tail filaments visible
How the wings restdecisiveAt a glance
Wings held upright (sailboat)
Varies
Reported size
2–40 mm
2–20 mm
Water it is recorded in
river, stream, lake margin
river, stream, lake margin, lake open, spring
Life stages covered
nymph, subimago, adult
larva, pupa, adult
How it moves
swimming undulation; crawling on substrate
wriggling or undulating; anchored by posterior prolegs
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 Midge (non-biting)
Midge larvae have no segmented true legs and no long tails — only prolegs. If you can see three pairs of jointed legs and long cerci, it is not a midge.
Small, everywhere, and the reason a dead-looking river isn't.
First check
No true legs and no long tails. If you see jointed legs or filaments, look somewhere else in the guide.
What it is
Chironomidae is enormous — more than 7,300 described species worldwide, found in every kind of water, including salt water. It is also the group most often written off, because everything about it is small.
Stages
Larva (2–20 mm (New Mexico State gives up to 12.5 mm)) · Pupa (No source consulted gives a pupal size) · Adult (2–12 mm)
Water
river, stream, lake-margin, spring
Separates from Mayfly
Mayfly nymphs have three pairs of jointed legs, long tails, and abdominal gills. Midge larvae have none of those — only prolegs.
