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 Stonefly 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
Stonefly
Caddisfly
Portable case or case-like structuredecisiveAt a glance
No
Yes
How the wings restdecisiveAt a glance
Wings held flat and parallel along the body
Wings tent-like / inverted-V
Gills at the thorax or leg basesdecisiveHand lens
Yes
Not diagnostic
Hardened head with softer worm-like abdomendecisiveClose look
Not diagnostic
Yes
Terminal prolegs or hooks at the reardecisiveClose look
Not diagnostic
Yes
Tail filamentsdecisiveClose look
Two tail filaments clearly visible
Varies
Two long antennaedecisiveClose look
Yes
Not diagnostic
Crawling on rock or debrisWhere it was
Yes
Not diagnostic
Reported size
5–75 mm
3–40 mm
Water it is recorded in
river, stream, spring
river, stream, lake margin
Life stages covered
nymph, adult
larva, pupa, adult
How it moves
crawling on rock or debris; weak flight; adults often stay near the ground
crawling with case; swimming pupa
Reference seasonality
Pre-emergence migration: Apr, May, Jun, Jul · Adult activity: May, Jun, Jul
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
Plecoptera
Stonefly
Two tails, two antennae, and a poor opinion of swimming.
First check
Two tails AND two long antennae, with any gills at the thorax and leg bases rather than along the abdomen.
What it is
Most stoneflies want cold, fast, well-oxygenated water, and they say so by being where it is. The nymph is a crawler: it creeps under and among stones rather than moving about in open water.
Stages
Nymph (5–70 mm (many western species large)) · Adult (6–75 mm; most 13–50 mm)
Water
river, stream, spring
Separates from Caddisfly
Caddis larvae carry a case or a hard head with terminal hooks and have no long tails or antennae. Caddis adults tent their wings; stonefly adults lie them flat.
Typical windows
Nymphs present (nymph) · Pre-emergence migration (nymph) · Adult activity (adult) · Small winter species (adult)
Patterns (correspondence)
Rubber-Legs Stonefly Nymph · Twenty-Incher · Stimulator · Little Black Stonefly
Limits
Habitat and emergence timing vary considerably by species.
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 Stonefly
Stonefly adults hold their wings flat and parallel along the body, not tented. Stonefly nymphs have two long tails and two long antennae; caddis larvae have neither.
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.
