The single most useful fact in fly selection is that a trout’s diet is dominated by what is easy to catch in quantity, not by what is most visible to you. Most of it is underwater and most of it is small.

The main groups

• Aquatic insects — mayflies, caddisflies, stoneflies and midges. Each has a lifecycle: nymph (underwater), emerger (at the surface), adult (on the surface and in the air). Trout eat them at every stage, and they eat nymphs far more often than adults.
• Terrestrials — ants, beetles, grasshoppers, caterpillars. Seasonally important, especially on windy days when they blow onto the water.
• Crustaceans and worms — scuds, shrimp, aquatic worms. Often the bulk of the diet in spring creeks and tailwaters.
• Fish and eggs — larger trout eat smaller fish; eggs matter during spawning runs.

What this means on the water

1. Match the size first. Size and profile beat exact colour most days.
2. Fish the depth the food is at. A perfect mayfly dun skated over a fish that is eating nymphs at the bottom will be ignored.
3. Look for the odd one out. If you see three caddis and one mayfly, fish the mayfly — the fish see caddis all day.
4. Learn four or five patterns properly. A hare’s ear nymph, a pheasant tail, an elk hair caddis, a parachute Adams and a Woolly Bugger cover most days on most rivers. Our five essential trout flies are those, with the materials list for each.

The exam-adjacent version

If you are preparing for a teaching task, the examiner is not testing your Latin. They are testing whether you can explain why a fly is the right choice for the water in front of you: what stage the insect is at, how deep the fish is feeding, and how the drift will be affected.