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 Caddisfly from Stonefly cleanly, and every pair of groups still in play differs on it and you can check it at a glance.
What to look at
Caddisfly
Stonefly
Portable case or case-like structuredecisiveAt a glance
Yes
No
How the wings restdecisiveAt a glance
Wings tent-like / inverted-V
Wings held flat and parallel along the body
Gills at the thorax or leg basesdecisiveHand lens
Not diagnostic
Yes
Hardened head with softer worm-like abdomendecisiveClose look
Yes
Not diagnostic
Terminal prolegs or hooks at the reardecisiveClose look
Yes
Not diagnostic
Tail filamentsdecisiveClose look
Varies
Two tail filaments clearly visible
Two long antennaedecisiveClose look
Not diagnostic
Yes
Crawling on rock or debrisWhere it was
Not diagnostic
Yes
Reported size
3–40 mm
5–75 mm
Water it is recorded in
river, stream, lake margin
river, stream, spring
Life stages covered
larva, pupa, adult
nymph, adult
How it moves
crawling with case; swimming pupa
crawling on rock or debris; weak flight; adults often stay near the ground
Reference seasonality
Emergence: Sep, Oct, Nov
Pre-emergence migration: Apr, May, Jun, Jul · Adult activity: May, Jun, Jul
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
Trichoptera
Caddisfly
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.
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.
