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 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
Alderfly larva
Stonefly
Tail filamentsdecisiveClose look
One tail filament, pointing straight back
Two tail filaments clearly visible
How the wings restdecisiveAt a glance
Varies
Wings held flat and parallel along the body
Two long antennaedecisiveClose look
Not diagnostic
Yes
Gills at the thorax or leg basesdecisiveHand lens
Not diagnostic
Yes
Hardened head with softer worm-like abdomendecisiveClose look
Yes
Not diagnostic
Winged insect at or near surfacedecisiveWhere it was
crawls and burrows in silt and detritus; undulates the abdomen when swimming, but is not an open-water animal
crawling on rock or debris; weak flight; adults often stay near the ground
Reference seasonality
Larvae leave the water: Apr, May, Jun
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
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 Stonefly
A stonefly nymph has two tails and wing pads. An alderfly has one filament and no wing pads at all.
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.
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 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.
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.
