HATCH — a magic word. What is hatching? Many fly fishermen travel hundreds of miles to fish a stream or lake when a particular insect is emerging. The appearance of a major mayfly, stonefly, or caddisfly becomes an event, even a ritual, that anglers in an area wait for each season.

The change from larva or nymph to adult (with an intermediate pupal stage for caddisflies) marks a period of abundance. It is almost as if nature deems that the bulk of a particular insect population is suddenly expendable. The insects that have been so well camouflaged expose themselves completely to the fish. Caution is possibly unnecessary at this point in the insect life cycle because, with the large number of eggs produced by each female, most of the immature survivors are not needed to replenish the stream. The emerging insects gather at specific levels in the current and create an immense food supply. For many species, forty to seventy percent of the fish predation for the entire life cycle occurs just prior to or during hatching.

A trout in a stream can gain as much as fifty percent of its yearly growth during two months of spring or early summer. The heavy hatches of these months provide an abundance of easy prey. The fish settle into a daily schedule and feed actively only when they can forage with the greatest efficiency. Often they lie semidormant except during the emergence periods. When they do begin to work a hatch the trout concentrate on the specific areas in the stream where the insects appear.

Is there any doubt why the emergence of a major insect is so important to fly fishermen? The activity is going to control not only what the fish feed on, but also where, when, and how they feed. Nearly every healthy trout stream contains insect species that reach high population densities and create these opportunities.

RESISTANCE BY ANGLERS to the entomology of fly-fishing is baffling for two reasons: one, because entomology is so vital to angling success; and two, because it is so easy. It is the way for even a beginner to understand the movements and preferences of the fish.

Entomology and fly-fishing for trout are inseparably linked. Certain flies simulate food types other than insects, such as minnows, crustaceans, leeches, or land creatures, but insects are still the major food for the average-size trout. Too many fly fishermen choose to ignore entomology completely because they misunderstand the value of it. Perhaps they see the more complicated manifestations of angling entomology, such as the exact-imitation theories, and do not see the more basic functions of entomology.

As a fly-fishing friend once stated, "Entomology will let me know if the thorax of my fly should be reddish brown instead of olive brown, or if the hook on my fly should be a number twenty-eight instead of a number twenty-six, or if the wings on my fly should be forty-five degrees up instead of seventy-five degrees up; none of which I want to know." Such minor refinements of imitation, of course, can be critical during periods of selective feeding, but situations when the trout are that fussy are uncommon — less than ten percent of the feeding time in most streams — and even then the presentation of the fly is as important as the choice of fly.

The major benefit derived from even a basic study of entomology is not the ability to solve minor problems of imitation. The important advantage a fly fisherman gains with that knowledge is the ability to predict trout feeding patterns — and consequently the ability to suit his techniques to those patterns.

A sad fact of modern fly-fishing is that so much of the lore is geared to one insect, mayflies, that the typical angler has difficulty adapting his methods to the feeding that occurs during a caddisfly hatch. He is conditioned to fish his flies to simulate the typical habits of a mayfly, not a caddisfly.

The information that has been written about caddisflies, however, is not particularly accurate either. The typical angling description of an emerging pupa usually paints a picture of a super insect: "The pupa rips free of the cocoon and rises like a rocket through the water, popping through the surface and flying off immediately." Pity the poor trout trying to capture such an energetic creature. And pity the poor fisherman trying to imitate such insect behavior. This account of the emergence, fortunately, is an exaggeration.

The speed with which caddisflies, the swimming type, ascend to the surface varies with the species, but it is doubtful if any of them rise like a rocket and shoot out into the air. A few accounts by entomologists describe the struggles the emerging insect goes through. For example, Dr. Cornelius Betten, in The Caddis Flies, or Trichoptera, of New York State, in a section written in approximately 1915, states about a common Spotted Sedge (Hydropsyche sp.): "I did not find the larvae but observed the pupae transforming on the surface of the water alongside of the government breakwater. . ." and in another section, "The pupae were caught as they were coming up for emergence alongside the government breakwater, but these specimens had doubtless been carried some distance by the swift current since they left the rocks."

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Gary LaFontaine, a guide, instructor and lecturer, was author of The Dry Fly: New Angles, Trout Flies: Proven Patterns, All About Flies and many other influential fly fishing books and magazine articles. This article is excerpted from his seminal 1981 book Caddisflies (The Lyons Press, 344 pages). Copyright © 2005 The Lyons Press and MidCurrent.