The thing about fly fishing in general, and fly fishing for trout specifically, is that you are forced — if you really want to understand the world in which you wade, and if you really want to catch fish — to pay attention to what they eat. Trout eat a lot of bugs and, at certain times of day and evening, they eat whatever bug makes itself available. They eat aquatic insects in their various stages of development — nymph, or larval stage, pupa and winged adult. (Pupae is where metamorphosis occurs between the larval and adult stages.)
Trout prefer to eat their prey at the nymphal stage, when a caddis fly or mayfly is still crawling or swimming around the river-bottom rocks. They’ll eat a fly at the emerger stage in the surface of a stream and, of course, they’ll go pretty much nuts and rise to the surface when there’s a full hatch of a certain fly. That last phenomenon, mayflies having just hatched and struggling on the surface to take flight, presents the optimum experience for the fly angler. It’s when we present dry imitations of the natural fly, hoping to get a rise out of a trout.
So we pay a lot of attention to what emerges from a stream. These photographs show the shucked exoskeletons of various river creatures, the large stonefly (Acroneuria) most prominent. They are a significant part of the trout diet. Stoneflies crawl on rocks, break out of their nymphal cast and emerge as winged adults, and they do this at night, when only the trout are watching.