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: Crawling on rock or debrisWhere it was It separates Midge (non-biting) from Hellgrammite cleanly, and every pair of groups still in play differs on it and you can check it at a glance.
What to look at
Midge (non-biting)
Hellgrammite
Abdominal gills (plate or feather)decisiveClose look
No
Not diagnostic
Crawling on rock or debrisdecisiveWhere it was
Not diagnostic
Yes
Tail filamentsClose look
No long tail filaments visible
Varies
Reported sizedecisive
2–20 mm
25–100 mm
Water it is recorded in
river, stream, lake margin, lake open, spring
river, stream
Life stages covered
larva, pupa, adult
larva
How it moves
wriggling or undulating; anchored by posterior prolegs
strong crawling on rock; mostly crawls, but can swim by undulating the body
Reference seasonality
Emergence: Jan, Feb, Mar, Nov, Dec
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
Diptera · Chironomidae
Midge (non-biting)
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 Hellgrammite
Size and filaments. A large robust larva with lateral abdominal filaments and powerful mandibles is a hellgrammite; midges are small and smooth-sided.
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 Midge (non-biting)
Scale. A midge larva is small, legless and smooth-sided; a hellgrammite is large, legged, and fringed.
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.
