Flies by hatch, stage by stage.
The fly box answers "what kind of fly is this?" - this page answers the question you actually have on the water: "bugs are coming off, which fly do I tie on?" Each hatch below runs from the subsurface stages trout eat most to the surface stage everyone casts first. For the bugs themselves - lifecycles and a 12-month hatch calendar - see the match-the-hatch entomology guide.
🦋 Mayfly hatches Nymph deep, emerge in the film, dun on top, spinner at dusk - one insect, four windows.
Below & in the film
Duns on the surface
Spinner falls
🐛 Caddis hatches Trout eat the sprinting pupa far more than the adult - fish the ascent first.
Pupa & emergence
Adults on top
🪨 Stonefly hatches No mid-column stage - nymphs crawl to the bank to hatch, so fish deep or at the edges.
Nymphs along the bottom
Adults at the banks
🦟 Midge hatches Tiny, year-round, and the whole menu on tailwaters all winter.
Larva & pupa
Adults & clusters
🦗 Terrestrial season Not a hatch but a season - land insects falling in from June to first frost.
Hoppers & crickets
When fish are "rising" but refusing your dry, they are usually eating the stage just below it - the emerger, the pupa, the spinner flush in the film. Step one stage earlier in the lifecycle before you change patterns. Role-based lineup instead? See the fly box. No hatch at all? That is what the entomology guide and attractor patterns are for.