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: Winged insect at or near surfaceWhere it was It separates Alderfly larva from Mayfly cleanly, and every pair of groups still in play differs on it and you can check it at a glance.
What to look at
Alderfly larva
Mayfly
Tail filamentsdecisiveClose look
One tail filament, pointing straight back
Three tail filaments clearly visible
How the wings restdecisiveAt a glance
Varies
Wings held upright (sailboat)
Abdominal gills (plate or feather)decisiveClose look
Not diagnostic
Yes
Winged insect at or near surfacedecisiveWhere it was
No
Yes
Hardened head with softer worm-like abdomendecisiveClose look
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
Megaloptera (Sialidae)
Alderfly larva
One filament, straight back. A hellgrammite has two hooks.
First check
Look at the back end. One filament, or two hooks?
What it is
This is the confusion this set was most likely to get wrong, and it is decided at the back end. An alderfly larva finishes in a single tail filament pointing straight back. A hellgrammite finishes in a pair of hooked, leg-like grapples.
Stages
Larva — the only stage in the water (Up to about 25 mm) · Adult (Usually no more than 25 mm)
Water
stream, river, lake-margin, lake-open
Separates from Mayfly
Three tails and plate gills down the abdomen sides is a mayfly. One filament and pointed lateral filaments with no tufts is an alderfly.
Typical windows
Larvae leave the water (larva)
Patterns (correspondence)
None mapped yet.
Limits
The pupa is terrestrial. Alderflies crawl out of the water at night and pupate ashore, so nothing hatches off the surface — this is year-round subsurface forage, not a hatch.
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 Alderfly larva
This entry does not name Alderfly larva as a look-alike. That is a gap in the prose, not proof they cannot be confused.
