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: Leaf-like caudal lamellae (damselfly nymph)Close look It separates Damselfly / Dragonfly from Hellgrammite cleanly, and every pair of groups still in play differs on it (look closely with it still).
What to look at
Damselfly / Dragonfly
Hellgrammite
Grasping labium / mask (Odonata nymph)decisiveIn hand
strong crawling on rock; mostly crawls, but can swim by undulating the body
Reference seasonality
Adult hunting: May, Jun, Jul, Aug, Sep
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
Odonata
Damselfly / Dragonfly
The mask is the tell. The 'tails' on a damsel nymph are gills.
First check
Look at the mouth. If you can see a grasping mask, you are in Odonata.
What it is
Odonata nymphs are predators. The extendable labium — the mask — is the strongest field separator from mayfly and stonefly.
Stages
Nymph (6–68 mm; usually 13–68 mm) · Adult (body 25–89 mm; damselflies to about 64 mm)
Water
river, stream, lake-margin, spring
Separates from Hellgrammite
Hellgrammites have lateral abdominal filaments and heavy mandibles, not a labium mask.
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 Damselfly / Dragonfly
Hellgrammites have lateral abdominal filaments and heavy mandibles, not a labium mask.
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.
