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: Two long antennaeClose look It separates Crayfish from Hellgrammite cleanly, and every pair of groups still in play differs on it (look closely with it still).
What to look at
Crayfish
Hellgrammite
Two long antennaedecisiveClose look
Yes
Not diagnostic
Reported size
25–150 mm
25–100 mm
Water it is recorded in
stream, river, lake margin, reservoir, spring
river, stream
Life stages covered
young-of-year, adult
larva
How it moves
walking slowly forward on the legs; flicking rapidly backwards when threatened by curling the tail fan under the body
strong crawling on rock; mostly crawls, but can swim by undulating the body
Reference seasonality
No family-level window
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
Decapoda (Cambaridae, Astacidae)
Crayfish
Claws in front, tail fan behind, bottom underneath.
First check
Are there pincers much larger than the walking legs? Then it is a crayfish.
What it is
Nothing else you are likely to meet in fresh water carries a pair of pincers that size. The claws settle the identification on their own; everything after that is about which crayfish and what condition it is in.
Stages
Adult (25–150 mm, excluding antennae and legs)
Water
stream, river, lake-margin, reservoir, spring
Separates from Hellgrammite
A hellgrammite has no pincers of that size and no tail fan; it has lateral filaments and terminal hooks instead.
Typical windows
No family-level window in this layer.
Patterns (correspondence)
Near Nuff Crayfish
Limits
Sources disagree on basic counts — four pairs of walking legs plus chelipeds or five pairs with the first enlarged, and six or seven abdominal segments. Neither is offered as a field character.
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 Crayfish
This entry does not name Crayfish as a look-alike. That is a gap in the prose, not proof they cannot be confused.
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.
