You’re crouched on a wet rock with something in your palm and about four seconds before it decides to leave. That’s the window. What you manage to notice in those four seconds is the whole basis of any comparison you run afterward, so record what was there and leave the name blank until the evidence earns one.
One thing up front. The Hatch Match app and these guides don’t currently agree on every cue, and that’s being fixed at the app end. Where the two split, go with what’s written here and with what you actually saw, and let the group stay unresolved.
Seven checks, name last
- Position: Under a rock, on the bottom, swimming in the water column, stuck in the surface film, riding on top, flying above the water, or sitting on the bank?
- Life stage: Aquatic nymph or larva, pupa, emerging insect, or winged adult? If you can’t tell, write “unknown.” Unknown is data.
- Tail filaments: None, two, or three — and only if you can genuinely see them. Don’t count tails off a blurred photo or a bug that won’t hold still.
- Wing posture: Upright, laid flat along the body, held tent-like over it, spent flat on the water, or not visible at all.
- Case or shuck: A portable case, a fixed retreat, a trailing shuck, or an empty skin stuck to rock or weed. No visible case doesn’t rule out caddisflies.
- Relative size: Hold it against a fingernail, a hook, or anything else already in view. Millimeters are a bonus. Tiny, small, medium and large still get the job done.
- Movement: Crawling, swimming, drifting, rising through the film, skating, clustering, or not moving at all.
Three groups you can split streamside
Mayfly nymphs commonly show two or three tails and plate- or feather-like gills along the abdomen. Winged mayflies keep the long tails. The dun, or subimago, comes first and doesn’t last long; the spinner, or imago, is the final stage.
Stonefly nymphs generally show two tails, long antennae, and developing wing pads. Adults hold their wings flat and parallel down the body instead of upright.
Caddisfly larvae often have a hardened head and thorax, a softer abdomen, and hooks at the back end. Some build portable cases, some build fixed retreats, and some are free-living. Adults carry two pairs of hairy wings, held downsloping or tent-like at rest.
If tails and wing pads still read as noise, that’s normal — nobody picks them out on the first ten bugs. Record size and movement only, and ignore the rest for a while. Those two alone will change what you tie on, and the finer cues turn up on their own once you’ve handled enough of them.
What the observation cannot establish
Getting the broad group right doesn’t mean a hatch is on, doesn’t mean fish are eating it, doesn’t mean the insect occurs across the region, and doesn’t mean any particular fly or lure will work. Use it to decide what to look at next, and to hold onto the profile, size, position and movement that were actually in front of you.
Field handling
Disturb as little as you can. If you lift a rock, put it back gently the way it came out, same side up. Don’t collect anything where access rules, protected-species rules, research permits or site rules say not to.
Reviewed sources
- NOAA Great Lakes Environmental Research Laboratory — Aquatic Insect Field Guide
- U.S. EPA — Macroinvertebrate Identification Tip Sheet
- U.S. EPA — Benthic Macroinvertebrate Indicators
Source check: July 29, 2026. This page supports broad field observation, not species-level identification.
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