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 Midge (non-biting) from Black fly larva 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)
Black fly larva
Swollen rear gripping a rock, body waving in the currentdecisiveWhere it was
wriggling or undulating; anchored by posterior prolegs
anchored and filtering rather than swimming; relocates by paying out a silk thread and drifting on it
Reference seasonality
Emergence: Jan, Feb, Mar, Nov, Dec
No family-level window
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 Black fly larva
A midge larva is a worm of even thickness, free-living or in a soft tube, gripping at most with small hooked prolegs. A black fly larva has a swollen rear end hooked into a silk pad on the rock, and faces downstream.
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 Midge (non-biting)
A midge larva is a worm of even thickness, free-living or in a soft tube, gripping at most with small hooked prolegs. A black fly larva has a swollen rear hooked into a silk pad on the stone.
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.
