Entomology - fly fishing for trout and grayling
- understanding insects

Fishing instructor, fly fishing tuition and fishing trips based in Scotland. Salmon and trout fishing advice, flies and articles.

Fly fishing entomology - the basis for most trout flies.

Fly fishing for trout and grayling requires a knowledge of entomology - the study of insects that form a large part of the fishes diet. This introduction covers the four main types of insects of interest to trout and anglers. I have sketched them at various life stages, nymphs, larvae, creeper, pupae, buzzer, spinner, caddis fly, stone fly, mayfly, and midge. There are several excellent books available that provide detailed information.


Ephemeroptera - Mayflies
Tricoptera - Caddis flies
Diptera - Gnats, midges etc.
Plecoptera - Stoneflies


Right is a mayfly dun.
Left is an adult mayfly type spinner
Adult mayflies lay eggs in the water, attached by threads or adhesive and some species crawl underwater to do this. Nymphs feed on plant material and have many growth stages (instars). Two basic forms exist - still or slow water types and fast water types. Duns dry on the water surface. Duns then leave to shelter and moult into adults called spinners.
Right is a fast water species of mayfly nymph.
midges and olive nymph
caddis Most do it within 24 hr.. They then swarm to breed and females lay their eggs by crawling into water (e.g. Baetis), flying upstream and dropping eggs onto the water or dipping their abdomen to loose them (e.g. Ephemerella) or they land on the water to lay eggs. All species die after mating and laying.
caddis pupa Adult Caddis fly

Caddis flies are very important to river anglers in all their life stages. Eggs are laid either on the water, by the insect crawling underwater and attaching them or they are laid on waterside vegetation. Most of the larvae make protective cases from silk and other materials. They feed on plant and animal matter and pupate for a time before emerging. The pupa swim to the surface to hatch. Adults resemble moths in flight and usually hatch in the evenings.

Left is a Caddis pupae.

Diptera covers thousands of two winged flies. Reed Smuts, Gnats and Midges are most important to the fly fisherman. Eggs are laid in either on or by crawling into the water. Larvae crawl like worms - some of them are called (bloodworms) they then pupate and rise to the surface where the trout feast on the hatching pupa or "buzzers". Emerging adults and spent or egg laying adults are also eaten by the fish.

Right hand side top to bottom is an adult midge, buzzer and larvae.
Below is a Caddis larvae in its case.
caddis larva Stoneflies lay their eggs in the water and the nymphs crawl around eating plant material and insects. The creeper take three years to develop and it then crawls ashore to become an adult. Adult females lay eggs back into the water.
Below is a creeper and right is an adult Stonefly
stonefly creeper
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Copyright 2007 Alastair Gowans AAPGAI and FFF Master and THCI, APGAI. All rights reserved.