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How To Match the Hatch Tactics

Understanding Mayflies

by Paul Weamer   |  May 16th, 2016 0
Understanding Mayflies

The giant Green Drake is one of North America’s best-known mayflies, though it is only found east of the Mississippi River. It is one of dozens of species that are collectively important to both trout and fly fishers across the United States and Canada. Photo | Paul Weamer

Make nymphs, duns, and spinners part of your vocabulary—and your tactics

Throughout most of human history, fishing was simply a matter of survival, just a way to get protein into our diet. For thousands of years, various peoples around the globe used natural toxins to poison fish. They built weirs to force fish into collection points where they could be netted or speared.

Somewhere during our shared history, humans noticed fish feeding on the surface of lakes and streams, eating insects, bringing the fish almost within reach. These hunters devised a way to catch the fish by mimicking the flies they were eating—and fly fishing was born. The first written account of fly fishing doesn’t mention the exact bugs the anglers were imitating, but it does suggest that they were fishing for trout. And those trout were probably eating mayflies.

Common Names

Mayflies belong to the order Ephemeroptera, meaning “short lived.” Most mayfly lives are very short; some species die within hours of their emergence from the water. There are more than 600 mayfly species in North America. Generally speaking, mayflies from the Mississippi River east to the Atlantic Ocean are of similar types, and different from those living in the Rocky Mountain West—but there is some overlap.

The mayflies we call Blue-winged Olives live throughout North America. And the small, closely related, yellowish bugs called Sulphurs in the East and Pale Morning Duns in the West are important in most U.S. trout streams. Tiny Tricos, found swarming over riffles most mornings throughout the summer, are just as important in Montana as they are in Maine. And Brown Drakes (Ephemera simulans) are all across the country, from sea to shining sea.

The term “Brown Drake” is the common name of a particular insect, a name given to a mayfly species by anglers. The italicized words beside it—Ephemera simulans—is the scientific or Latin name (genus: Ephemera, species: simulans). These two possible monikers create one of the most debatable points in the fly-fishing world: should we use common or scientific names when discussing aquatic insect hatches?

Those who favor using common names argue that the scientific names are always changing as aquatic entomologists (who study insects that live in water) define and develop new relationships for mayfly species, most often now with DNA testing. Those who favor using scientific names quickly note that there is no standard for common names, so someone in western Pennsylvania may use a name that means something completely different to an angler in eastern Pennsylvania. And both of these arguments are valid.

Many anglers who do not know the scientific names are fond of saying that trout don’t speak Latin. Of course they are right. But it won’t hurt you to know both the Latin and common names for mayflies. Fly fishing is a sport where the more you know, the more successful you usually are at catching fish.

Circle of Life

Mayflies have a three-stage life cycle, transforming from egg to adult. They lack a fourth stage, also known as the pupa or resting stage, which would give them a complete life cycle like other insects such as butterflies.

Mayfly adults swim or crawl to the stream bottom to deliver their eggs, or they drop or dip them from above the stream. The time it takes the eggs to hatch is dependent upon the species, but the average is one to three months. Some mayfly species spend an entire winter as eggs.

Once the egg hatches it becomes a larva, though nearly all fly fishers call it a nymph. Nymphs live in the stream bottom either around rocks and vegetation or buried in the sediment. Anglers divide them into four descriptive families based upon where they live and how they move: swimmers, clingers, crawlers, and burrowers.

Understanding Mayflies

Mayfly spinners are easy to identify because their wings are more transparent than the opaque wings of a mayfly dun. Use Rusty Spinners with wings made from either hackle fibers or synthetic yarn. Photo | Paul Weamer

Nymphs live in the water for anywhere from a couple months to a couple years, again according to the species. Generally, those mayflies that spend the longest amount of time as nymphs are also the largest. But regardless of the species, all mayflies spend the majority of their lives as nymphs, many of them living 51 weeks subsurface. Others spend all but a couple hours underwater, breathing air for only the briefest moments of their lives.

At approximately the same time each year, the mayflies within each species emerge from their nymphal bodies, transforming into winged, air-breathing duns (also called subimagos). Males usually begin the emergence, and are quickly followed by females. Some species emerge on the stream bottom and swim to the surface, some crawl onto dry rocks or vegetation to emerge, and yet others emerge at or near the water’s surface. Water temperature is the most important factor in determining when hatches will occur. And though a specific hatch date may deviate as much as a month from one year to the next, the order in which the various mayfly species emerge always remains the same.

Most winged duns fly to streamside trees or vegetation to molt into spinners (also called imagos), though not every species completes its cycle in this manner. After a brief period, usually one to three days, the sexually mature spinner breaks out of the dun’s body. Mayflies are the only insects in the world that molt again after attaining a winged form.

The spinners gather over riffles, most often (but not always) in the evening just before dark. They mate, and the males usually fall to the water in a near death state with outstretched wings that anglers call “spent.” The females then deposit their fertilized eggs into the water, and they too usually fall spent. This creates a great feeding opportunity for the fish as hundreds or thousands of spent spinners lie nearly motionless on the surface.

Understanding Mayflies

Flies such as the Adams and the Parachute Adams are good general imitators of almost any species of mayfly dun. Photo | Paul Weamer

Identifying Mayflies 

So how do you know which mayfly species live in the streams you fish? If you live near a local fly shop, they can help. Trout Unlimited chapters, the Internet, and knowledgeable local anglers can also be great resources. There has been more information written about fly fishing than all other sports combined, and a great deal of that information is about mayflies, so books and magazines are a great aid.

If you capture a mayfly outside of the water in its dun or spinner stage, you’ll need to make a few observations about its body to help you identify it. How many tails does it have? All mayflies have either two or three tails. Are the tails barred or a solid color? All mayflies have six legs, but you should still look closely at them. Is the front pair of legs the same color as the two back pairs? Are there any markings on the legs?

Be wary of broken and missing tails and legs. Mayflies are very fragile. They do not have mouths once they attain their winged life stages, so they cannot feed after they hatch from the water. Their life is nearly over, and it is very easy for them to lose a leg or tail when you touch them. I’ve witnessed a missing mayfly tail cause a great deal of confusion for new fly anglers.

What about mayfly wings? Mayfly duns and spinners have two, large, upright wings and most, but not all, species have two smaller wings at their base called hind wings. Are the wings a solid color or are they mottled with several colors? Do the wings have prominent, dark veins running through them or are the veins more subtle? Are the wings clear? Most mayfly spinners have translucent wings, but dun wings are opaque.

It seems that body color would be a good species indicator but it really isn’t. Body color even among the same mayfly species in the same stream can be quite variable, depending upon the nymphs’ diet and even small genetic differences.

Aquatic entomologists place little emphasis on body color when attempting to identify a mayfly species. They collect virgin, male, mayfly spinners, and dissect them to clearly see their penes (mayflies have two). They then count the spines on the penes and compare them to photos in books to identify each species. There is no way to be sure of the species from a female spinner, and you may not be able to tell from a nonvirgin male. No kidding! I couldn’t make this stuff up.

You can often differentiate males and females by their eyes. Males will usually have much larger eyes than females. Males also have claspers beneath their tails that are used to hold the female while they fly and mate.

In a few rare circumstances you won’t be able to tell the difference between males and females because there isn’t one. There are parthenogenetic mayflies, not male or female but both, each member capable of reproducing individually.

Understanding Mayflies

Trout feed on mayfly nymphs below the surface all season. Good imitations include standard versions of the Pheasant Tail and Hare’s Ear nymphs (right). Use beadhead flashback versions for deep, fast, or off-color water. Photo | Paul Weamer

The Art of Imitation

Once you identify the mayflies in your stream, you’ll want to imitate them with flies. While wild trout in small, infertile streams are fairly easy to catch with almost any fly pattern, their brethren living in famous, catch-and-release streams may not be so easy. These “educated” fish sometimes key on specific stages of the mayfly life cycle, and it can be very important to imitate them.

Many anglers who prefer to fish flies beneath the surface (called nymphing) enjoy mentioning that trout spend 90 percent of their time feeding subsurface on nymphs.

But the motion caused by large numbers of mayflies emerging from nymphs into duns often gets the attention of the trout, and focuses their feeding closer to the surface. Anglers imitate this stage with fly patterns called emergers. Emerger flies often float on the surface but also hang into the water, giving the illusion of a transforming insect.

Sometimes, after emerging from nymphs, mayflies spend extended periods riding a stream’s surface as they fill their wings with body fluid and attempt flight. This phenomenon is often exacerbated by cold air temperatures or cloudy, rainy days. This is another precarious stage for mayflies that makes an easy meal for trout.

How you imitate these various mayfly life stages with artificial flies has been the subject matter for hundreds of books and magazine articles. That amount of material can leave new fly fishers overwhelmed and confused when it comes to choosing which fly to tie to the end of their line. But one of the great things about fly fishing is that you don’t have to know everything (no one actually does) to catch a few fish.

Understanding Mayflies

“Spent” mayfly spinners float at the surface for a time, but eventually they sink. Sunk spinners, fished after the surface feeding declines, can pick up additional trout. Photo | John Barr

Perhaps you’re not interested in purchasing or tying great numbers of exacting flies to imitate every stage of every mayfly hatch, and that’s perfectly fine. General nymph patterns like Pheasant Tails and Hare’s Ears, in sizes 10 to 22, tied with and without beadheads will do an excellent job of imitating most mayfly nymphs.

Parachute-style dry flies, tied in a couple colors—olive, yellow, gray—in sizes 10 to 22, do a good job of imitating most mayfly duns. Parachutes work very well when fishing to rising trout in flatwater pools. And those same flies tied with a dark brown material for their tails imitate emergers.

You can also use Catskill- or Wulff-style flies in the same colors for fishing rough water and riffles. These styles stand high on their hackles (chicken feather fibers) on the surface, keeping their bodies dry.

For spinners, rusty and cream colored imitations in sizes 10 to 22 are all you really need. Have some tied with a buoyant material like Antron for their wings to help you see them during low-light conditions and some with hackle fiber wings to look more realistic to the fish during daylight hours.

Modern fly fishing is an amazingly diverse activity where anglers can chase the ocean’s sailfish and sharks with large synthetic streamers and heavy fly rods, or hook warmwater species like bass and muskies with colorful deer-hair flies.

But at its core, fly fishing will always be a sport that was invented to catch trout with bits of feathers and fur lashed onto a small hook. And that’s pretty amazing too.

Paul Weamer is Fly Fisherman’s Mid-Atlantic field editor and the author of Fly-Fishing Guide to the Upper Delaware River (Stackpole Books, second edition 2011) and coauthor with Charlie Meck of Pocketguide to Pennsylvania Hatches (Headwater Books, 2009).

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