Our Man In Canada
December 25th, 2000

The Absolute Beginner
A Fly Fisher's Basic Entomology, Part One

Chris Marshall

By Piscator

We've had a number of enquiries from novice fly fishers about the entomology of fly tying. This is understandable, as the complexity of species and the various metamorphoses they manifest can be confusing. A one-page column is no place deal with such complexity, but beneath the complexity there lies a relatively simple basic pattern which is easy to explain and understand. Moreover, once it's understood, it provides two clear conceptual frameworks on which to hang the profusion of details which come later.

The two frameworks are:

    1. The kinds of flies. These are what biologists call "orders." There are many of these, but only a few are of interest to the fly fisher. In fact, the vast majority of artificials are tied to imitate just three orders: mayflies, caddis flies, and stoneflies. There are others, such as midges, which are also important, but tend to be more restricted in terms of habitat and geography. All are winged insects which are hatched and spend their larval stages under water.

    2. The stages of development. These are the larva (or nymph), the pupa (or chrysalis), and the winged adult. There are differences in how the three orders go through these stages, but they're easy to understand.

(Order Ephemeroptera)

By far the majority of artificial fly patterns are mayfly imitations. With a few exceptions these correspond to the three stages and one transitional stage of development.


Mayflies lay their eggs on the water and the nymphs (larvae) develop on the bottom. Most spend a year there before hatching, although a few of the larger species spend two. Nymphs are generally split into four categories: swimmers, which dart about on the bottom in the faster water; clingers, which cling to rocks (usually the underside) in fast water; crawlers, which crawl around in the stuff on the bottom; and burrowers, which burrow in soft sandy or muddy bottoms in slower or still water.

All have distinctly different body shapes suited to their habits and habitat. Swimmers are slim and streamlined; clingers are flattened with strong, thick legs; crawlers are plump and cylindrical with thinner legs; burrowers are long and slim with feathery gills.

Artificial flies tied to imitate nymphs conform to these body shapes, and are designed to be fished on a dead drift beneath the surface, usually close to the bottom.


Mayflies do not have a pupal stage. The dun (adult) hatches directly from the nymph. In some species, the nymphs simply swim to the surface and split open, allowing the dun to crawl out. Because this happens while the creature is floating down the stream, they're prime targets for feeding fish. Many of the legendary mayfly hatches are of this kind, such as Hendicksons and Blue Winged Olives.

Other species crawl out of the water on to rocks or emergent vegetation and hatch there. Some fly away without touching the water, but many get caught on the water and drift downstream.

Imitating these downstream drifting flies with an artificial is the essence of classic dry fly fishing. This is where "matching-the-hatch" started. Artificial flies tied to imitate duns are designed to float either right on the surface or just in the surface film. Most have stiff hackles to aid in floating.


As they float downstream, the newly hatched duns dry their wings in preparation for their first flight. The time this takes varies according to the dryness of the air, the heat of the sun, and the degree of wind - just like the wash hanging out on a clothesline. Those which are fortunate enough not to be eaten by a fish, take to air and head for the vegetation along the banks of the stream. Here they have to run another gauntlet - birds this time. One of the indicators that a hatch is going on, even though the flies might not be immediately visible, is the activity of swallows, waxwings, kingbirds and other fly-eaters working above the stream.

In the face of so many predators, it's a wonder that any flies manage to make it to the safety of the streamside foliage, but thousands do. Here, they rest for the day\emdash sometimes two. And, as they rest, they go through a final metamorphosis: the skin splits, and a very different fly emerges. Unlike the sombre coloured dun, this second stage adult has a hard, almost metallic sheen to its body, and its wings (in most cases) rather than grey or brown, are crystal clear.

This stage of the mayfly is called a "spinner." The name is very old, and derives from the behaviour of the fly over the water. Once the metamorphosis is complete, the spinners return to the air above the stream, where clouds of them dance or "spin" together. This is their mating ritual. Gradually they will drop closer to the water, and you'll see males and females coupling in the air. When this is done both males and females drop to the water, dying. This what we call a "spinner fall." Again, the fish are provided with a feast, but enough fertilized eggs from dying females sink to the streambed to ensure survival of the next generation.

Winter 2000 issue

Spinner imitations, like duns, are designed to float, but unlike duns, which have upright wings, they have horizontal wings, which lie flat on the water. Like duns, they are usually fished dead drift with either an upstream or an across stream cast.

Part II

In our next issue, we'll publish the second part of this basic entomology - just in time for the opening of the resident trout season. This will cover the other orders of flies, as well as some variations on the basic mayfly patterns, such as emergers, crippled duns, and drowned spinners. ~ Piscator

If you just can't wait until the next issue of the Canadian Fly Fisher, you will find more on the specific insects in the Not Quite Entomology section - or Fly Fishing 101.

We thank the Canadian Fly Fisher for re-print permission!

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