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: Portable case or case-like structureAt a glance It separates Hellgrammite from Caddisfly cleanly, and every pair of groups still in play differs on it and you can check it at a glance.
What to look at
Hellgrammite
Caddisfly
Portable case or case-like structuredecisiveAt a glance
No
Yes
How the wings restdecisiveAt a glance
Varies
Wings tent-like / inverted-V
Crawling on rock or debrisWhere it was
Yes
Not diagnostic
Hardened head with softer worm-like abdomenClose look
Not diagnostic
Yes
Reported size
25–100 mm
3–40 mm
Water it is recorded in
river, stream
river, stream, lake margin
Life stages covered
larva
larva, pupa, adult
How it moves
strong crawling on rock; mostly crawls, but can swim by undulating the body
crawling with case; swimming pupa
Reference seasonality
Largest larvae leave the water: May, Jun
Emergence: Sep, Oct, Nov
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 · Corydalidae
Hellgrammite
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 Caddisfly
No case, and far larger than a typical caddis larva. The mandibles alone usually settle it.
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.
Not every caddis builds a house, and the ones that don't are the ones you miss.
First check
Ignore the case. Look for a hard dark head against a soft pale abdomen, and hooks at the back end.
What it is
Caddis is the group most often identified by a thing that is only sometimes there. The portable case — grains of sand, cut leaf, twig fragments, spun into a tube the larva carries — is the classic image, and when you find one the call is easy. But a large share of caddis larvae never build a portable case at all. Free-living forms hunt loose among the rocks; net-spinners build a fixed retreat with a catch-net beside it and leave the retreat behind when disturbed.
Stages
Larva (3–40 mm (often 13–25 mm with case)) · Pupa (No source consulted gives a pupal size) · Adult (4–38 mm)
Water
river, stream, lake-margin
Separates from Hellgrammite
A large robust larva with prominent lateral filaments down the abdomen and strong visible mandibles is a hellgrammite, not a caddis.
Typical windows
Larvae present (larva) · Emergence (pupa)
Patterns (correspondence)
Elk Hair Caddis · X-Caddis · Sparkle Pupa · Green Rock Worm · Peeking Caddis
Limits
Case-building behavior varies and absence of a visible case does not rule out caddis.
