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: Swollen rear gripping a rock, body waving in the currentWhere it was It separates Black fly larva 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
Black fly larva
Caddisfly
Swollen rear gripping a rock, body waving in the currentdecisiveWhere it was
Yes
No
Portable case or case-like structuredecisiveAt a glance
No
Yes
How the wings restdecisiveAt a glance
None of these
Wings tent-like / inverted-V
Abdominal gills (plate or feather)decisiveClose look
No
Not diagnostic
Tail filamentsdecisiveClose look
No long tail filaments visible
Varies
Two long antennaeClose look
No
Not diagnostic
Fleshy lobes or a disc at the blunt rearClose look
anchored and filtering rather than swimming; relocates by paying out a silk thread and drifting on it
crawling with case; swimming pupa
Reference seasonality
No family-level window
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 (Simuliidae)
Black fly larva
Bowling pin, gripping the rock, waving downstream.
First check
Is the back end swollen, and is it hooked on to the stone with its head held downstream?
What it is
Turn a rock over in fast water and this may be what is attached to it. A black fly larva anchors by its swollen back end to a pad of its own silk, with its head held downstream to filter the current.
Stages
Larva (5–15 mm) · Pupa — and this one is under water (2–3 mm) · Adult (About 1–5 mm (NC State 1.2–3 mm; Pennsylvania DEP 2–5 mm))
Water
stream, river
Separates from Caddisfly
A caddis is inside a case it built, or behind a fixed net. A black fly larva is attached directly to the rock by its own back end.
Typical windows
No family-level window in this layer.
Patterns (correspondence)
None mapped yet.
Limits
Family level. Species differ in seasonality, so this does not date the observation.
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 Black fly larva
This entry does not name Black fly larva 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.
