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: No hard head capsule and no legsClose look It separates Leech from Midge (non-biting) cleanly, and every pair of groups still in play differs on it (look closely with it still).
What to look at
Leech
Midge (non-biting)
No hard head capsule and no legsdecisiveClose look
Yes
No
Reported size
5–500 mm
2–20 mm
Water it is recorded in
lake margin, river, stream, reservoir
river, stream, lake margin, lake open, spring
Life stages covered
adult
larva, pupa, adult
How it moves
swimming; looping along like an inchworm using both suckers
wriggling or undulating; anchored by posterior prolegs
Reference seasonality
No family-level window
Emergence: Jan, Feb, Mar, Nov, Dec
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
Hirudinea
Leech
A sucker at each end, the front one often small.
First check
Look for a sucker at each end — the front one can be small. Suckers mean a leech.
What it is
One character does most of the job: a sucker at each end of the body, though the front one can be small. An aquatic worm has none.
Stages
Adult (5–500 mm — it stretches, so treat any measurement as approximate)
Water
lake-margin, river, stream, reservoir
Separates from Midge (non-biting)
A hard head capsule and prolegs at each end is a midge larva, which is an insect. A leech has neither.
Typical windows
No family-level window in this layer.
Patterns (correspondence)
Bunny Leech
Limits
No source consulted states whether leeches are nocturnal or day-active beyond the observation that they usually avoid light, and none addressed seasonal activity.
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 Leech
This entry does not name Leech as a look-alike. That is a gap in the prose, not proof they cannot be confused.
