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: Hopper / grasshopper silhouetteAt a glance It separates Terrestrials (ants, beetles, hoppers) from Mayfly cleanly, and every pair of groups still in play differs on it and you can check it at a glance.
What to look at
Terrestrials (ants, beetles, hoppers)
Mayfly
Abdominal gills (plate or feather)decisiveClose look
No
Yes
Fallen terrestrial (hopper, ant, beetle)decisiveAt a glance
Yes
No
Hopper / grasshopper silhouettedecisiveAt a glance
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
Various — not an aquatic hatch order
Terrestrials (ants, beetles, hoppers)
It did not hatch here. It fell.
First check
Did it fall from the bank, or did it come up from the water? If you cannot tell, do not name it a terrestrial.
What it is
Terrestrial invertebrates are an important part of stream fish diets, especially in summer, and none of them lived in the water. Hoppers, ants and beetles arrive from the bank. The observation class is 'fallen terrestrial', not a hatch.
Stages
Fallen adult (2–76 mm: ants to about 25 mm, grasshoppers mostly 13–25 mm and up to 76 mm. No source here gives beetle sizes.)
Water
river, stream, lake-margin
Separates from Mayfly
Gills and long tails rule out a terrestrial immediately.
Typical windows
Bank falls (adult)
Patterns (correspondence)
Dave's Hopper · Fur Ant · Foam Beetle
Limits
Broad observation class only — not a species ID and not a hatch.
Count the tails, then look for gills along the sides of the abdomen. Three tails plus side gills is about as close to a clean mayfly call as field observation gets.
What it is
Mayflies are the only insects that grow a full set of working wings and then moult again. The winged insect that leaves the water is not the finished animal — it is a dun, a subimago, cloudy-winged and dull-bodied. Within hours to a couple of days it sheds one more time into a spinner: clear-winged, smooth and shiny, built only to mate and die. Two winged forms of the same animal, hours apart, sitting on the same water.
Stages
Nymph (2–32 mm) · Dun (subimago) (Under 4 to 40 mm body; most families 4–20 mm) · Spinner (imago) (Under 4 to 40 mm body; most families 4–20 mm)
Water
river, stream, lake-margin
Separates from Terrestrials (ants, beetles, hoppers)
This entry does not name Terrestrials (ants, beetles, hoppers) as a look-alike. That is a gap in the prose, not proof they cannot be confused.
