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: Terminal prolegs or hooks at the rearClose look It separates Alderfly larva from Hellgrammite cleanly, and every pair of groups still in play differs on it (look closely with it still).
What to look at
Alderfly larva
Hellgrammite
Tail filamentsdecisiveClose look
One tail filament, pointing straight back
Varies
Terminal prolegs or hooks at the reardecisiveClose look
crawls and burrows in silt and detritus; undulates the abdomen when swimming, but is not an open-water animal
strong crawling on rock; mostly crawls, but can swim by undulating the body
Reference seasonality
Larvae leave the water: Apr, May, Jun
Largest larvae leave the water: May, Jun
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 Hellgrammite
The back end. One straight filament is an alderfly; a pair of hooked prolegs, plus cottony gill tufts along the abdomen, is a hellgrammite; hooks without the tufts is a fishfly.
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.
Big, armoured, and genuinely able to pinch you back.
First check
Size first. If it is under 25 mm you are almost certainly not looking at a mature hellgrammite. Then check for the row of lateral filaments down both sides.
What it is
The hellgrammite is the aquatic larva of a dobsonfly, and it is the largest thing most people will turn over a rock and find. Mature larvae run about 50–90 mm and reach about 100 mm. Nothing else in cold flowing water looks quite like it once you have seen one.
Stages
Larva (25–100 mm (mature larvae mostly 50–90 mm))
Water
river, stream
Separates from Alderfly larva
The back end. A hellgrammite finishes in a pair of hooked prolegs and carries eight pairs of lateral filaments, most with cottony gill tufts at the base; an alderfly finishes in a single filament pointing straight back, with seven pairs and no tufts. A larva with the hooks but no tufts is a fishfly. The alderfly is also smaller, paler and usually in soft silt rather than fast cobble.
Typical windows
Larvae present (larva) · Largest larvae leave the water (larva)
Patterns (correspondence)
Hellgrammite / Woolly Bugger
Limits
Hellgrammite refers specifically to the aquatic larva of dobsonflies (Corydalidae); related alderfly larvae (Sialidae) are smaller and differ in filament arrangement.
