This is not a full-fledged entomology guide or discussion. There are a lot of books, videos and websites available to provide more information. Our intent is provide enough information so that the casual or novice fisherman has a better opportunity to catch more fish.
what insects are in the river
The simplest approach is to turn over a few rocks and see what’s clinging to the rock. This approach requires no additional equipment.
While easy, it can provide a limited and often misleading view because the nymphs most available to the fish are often the active ones.
A more effective approach is to hold a net and catch anything floating on the surface of the water or hold the net underwater and kick up the substrate upstream of the net.
The current will wash any dislodged insects into the net. Deciding what insets are most abundant is simply a case at looking in the net.
In either case, the best habitat to collect from is usually moderately fast riffles, 6” to 2’ deep, with a gravel to cobble bottoms. These riffles produce the majority of insects drifting in the current.
To get a good idea of the diversity, select 4-6 locations to collect samples.
Identifying insects while you’re on the stream as members of the correct scientific order - mayfly, caddis, stonefly or midge - is probably is far as most people need to go.
identifying bugs based on flight patterns
One of the first things you can do when you see bugs flying around a stream is to look at their flight pattern. The flight patterns of the three major orders of aquatic insects–mayflies, caddisflies, and stoneflies–are distinctive. While each of these invertebrate families might be made up of hundreds or even thousands of individual species, these flight patterns will hold true to all within a given family. This allows an angler to quickly narrow down a dry fly to tie on from hundreds of patterns to just a few.
The dominant movement of the mayfly in motion is an even, steep to gently rolling, up and down flight. Adults move back and forth across the water in a wave-like pattern. Think of bobbing your head up and down.
When you see caddis flying, think of second graders with unrestricted access to chocolate and Mountain Dew. They spiral and bounce, cork-screw and dive in a sporadic flight pattern.
The B-52 bombers of the trout stream. Stoneflies are fairly weak fliers and are conspicuous by their size. They don’t waste energy dancing or twirling like other insect, but push straight forward with their two pairs of long wings.