Instead of grabbing a fly and hoping that you’re close, get some inside information by seining a stream before you fish it.
First wade out to where fish typically hold. Firmly grasp a small hand seine downstream of your feet on the creekbottom and turn over a dozen rocks or so. You’re not working a backhoe, just disturbing the bed a bit. Bring up the net and slip on the reading glasses. Also check the surface flow in the current below if fish are actively feeding around you. You should pick up hatching insects, as well as any terrestrials that have the fish turned on.
Know what’s in your net: You don’t have to be an entomologist to figure out what to do with what you seine. Learn a few insect forms and you’re well on your way to matching the bugs.
Mayfly nymphs come in many forms depending on the particular species: There are crawling, swimming, and burrowing nymphs. Try to match the general size, color, and profile of the insect.
These often large aquatic insects can’t swim, so they must crawl from stream bottoms to dry land or overhanging vegetation to emerge. Trout gorge on them. Match color and size.
Caddisflies have two aquatic life stages. The larva lives in a tiny tube made of twigs and sand grains. It then seals itself into a case to pupate and grow legs and wing pads before emerging.