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 Midge (non-biting) 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
Midge (non-biting)
Caddisfly
Portable case or case-like structuredecisiveAt a glance
No
Yes
How the wings restdecisiveAt a glance
Varies
Wings tent-like / inverted-V
Abdominal gills (plate or feather)Close look
No
Not diagnostic
Hardened head with softer worm-like abdomenClose look
Not diagnostic
Yes
Tail filamentsClose look
No long tail filaments visible
Varies
Terminal prolegs or hooks at the rearClose look
Not diagnostic
Yes
Reported size
2–20 mm
3–40 mm
Water it is recorded in
river, stream, lake margin, lake open, spring
river, stream, lake margin
How it moves
wriggling or undulating; anchored by posterior prolegs
crawling with case; swimming pupa
Reference seasonality
Emergence: Jan, Feb, Mar, Nov, Dec
Emergence: Sep, Oct, Nov
The same for both: life stages.
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
Diptera · Chironomidae
Midge (non-biting)
Small, everywhere, and the reason a dead-looking river isn't.
First check
No true legs and no long tails. If you see jointed legs or filaments, look somewhere else in the guide.
What it is
Chironomidae is enormous — more than 7,300 described species worldwide, found in every kind of water, including salt water. It is also the group most often written off, because everything about it is small.
Stages
Larva (2–20 mm (New Mexico State gives up to 12.5 mm)) · Pupa (No source consulted gives a pupal size) · Adult (2–12 mm)
Water
river, stream, lake-margin, spring
Separates from Caddisfly
A classic architectural portable case of sand or cut leaf is caddis. Most midge tubes are simple fixed shelters; portable midge cases exist but are far less common.
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 Midge (non-biting)
This entry does not name Midge (non-biting) as a look-alike. That is a gap in the prose, not proof they cannot be confused.
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.
