Aquatic Insects

Certain aquatic insects make up the major portion of a fish diet. The four main insect orders that emerge from our streams and lakes are mayflies, caddisflies, midges, and stoneflies.

Types of Flies

To be an effective hatch-matcher you must be able to quickly differentiate between adult mayflies, caddisflies, midges, stoneflies and all the immature stages of each. Many hatches and most spinner falls do not last very long, so you don't have time to try four or five patterns. Instead, you'll need to choose the right fly at the right time; only then will you be able to hook your share of fish. It's easier than you think. Just remember to match the hatch.


The mayfly is the most important order of trout-stream insects. All mayflies have two large, upright wings; two or three tails; and most have two very small hind wings. Mayflies look like little sailboats floating in the current and are the only trout-stream insects with upright wings. The life cycle is: egg, nymph, dun, spinner.


Caddisfiles are also very important insects, and in some locations they are even more numerous than mayflies. Caddisflies can easily be distinguished by their four wings of nearly equal length, which are covered with tiny hairs and, when at rest, are carried in an inverted V or tent over the back. Caddisflies are usually medium to small in size (#14 to #24) and have no tails. There are more than 1,000 known species on this continent.


Midges have only two short wings (shorter than the body), which lie flat along the top of the body, usually slightly to the side in a V, and they have no tails. Most midges are small, size 22 to 28 or smaller. The life cycle is egg, larva, pupa, adult. At hatch time the pupa ascends to the surface where it drifts for a time; the winged insect then emerges and flies away.

During the hatch, a pupa or stillborn artificial midge is usually effective; a hackled adult type can be used later during the emergence or at the egg-laying flight.


This small order of flies is of very little importance in slow waters, yet in turbulent, rocky streams, such as the Madison and the Big Hole in Montana, stoneflies provide the largest flies and the most spectacular fishing of the season. In certain Oregon streams, stoneflies are the second most important fish food. Stoneflies vary in size from very large to very small (#2 to #20). Adult stoneflies have four long wings, which are hard, shiny, heavily veined, and held flat over the back when at rest.

Other Food Items

Trout and other freshwater gamefish feed on many other food items besides the four major orders. Although these other orders are normally of lesser importance, when they are numerous, fish will feed on them selectively, so a few representative imitations should be carried for the other aquatic, semi-aquatic or terrestrial forms, such as dragonflies and damselflies, grasshoppers and crickets, leafhoppers, true bugs, spongilla flies, Dobsonflies, fish flies, alderflies, aquatic moths, beetles, true flies, and aquatic wasps.

Courtesy of Fly Fisherman Magazine