The Mayfly Life Cycle

There’s nothing more exciting than reflecting on the life cycle of a mayfly. Well, actually there is. It’s catching trout—and lots of them. If you want to catch more trout, it’s helpful (though not necessarily exciting) to think about the life cycle of a mayfly. It will help you know what you’re trying to imitate.

1. The Nymph stage

A mayfly spends all but one or two days of its life underwater as a nymph. It’s no wonder, then, that 85% of a trout’s diet comes from beneath the surface. It’s why fishing nymph patterns is almost always a sure bet. Nymphs move about the stream as they feed and molt and then drift into the current or dart from place to place.

If you want to get technical, there are four categories of Mayfly nymphs. Dave Hughes, in his Pocketguide to Western Hatches, classifies them as swimmers, crawlers, clingers, or burrowers. You could vary your strategy if you’re trying to imitate a certain kind of mayfly. However, most of the time, the tried-and-true dead drift method will work. Standard patterns include Hare’s Ear or Pheasant Tail. A Prince Nymph works fine, too, even though it was originally designed as a Stonefly nymph.

2. The Emerger stage

In this brief stage, the child becomes an adult when the skin splits along the back of the nymph and the winged dun escapes. This happens as the emerger rises to the surface and sheds its skin underwater. Some nymphs, however, crawl to the edge of the river where they shed their skin on the rocks or grass. This explains why you often see empty “casings” on rocks near a river’s edge.

It’s often a good idea to trail your dry fly with an emerger pattern, which you fish just under the film. Sometimes you’ll even see “rising” trout which don’t seem to be feeding on the surface flies. If so, definitely switch to an emerger pattern.

3. The Dun stage

Now the fly has become a young adult. The dun stage is a favorite for fly fishers, and many standard patterns—such as the Parachute Adams, the Comparadun, and attractors like a Royal Wulff—imitate this stage. Mayfly duns ride the surface until their upright wings are dry and hardened for flight. This ride can last for ten to twenty feet.

Fortunately for fly fishers, most mayflies hatch (technically “emerge”) during daylight hours. Prime time is 11 a.m. to 2 p.m., depending on wind and water temperature. Overcast, cool days are ideal, especially for Baetis flies and Blue-Winged Olives (BWOs).

4. The Spinner stage

This is the fully formed adult stage in which mayflies are ready to mate. As Dave Hughes says, “Mating takes place in the air, another bit of incomparable grace.” At this point, the females are spent and fall to the water. This creates a “spinner fall” — another opportunity for the trout to roil the surface as they feed. Anglers who see mayflies with flat wings like an airplane – rather than with wings sticking up – should switch to a spinner or “spent wing” pattern.

On some days, you might be able to catch a trout on a pattern that imitates any of these stages. But other days, trout are more selective and zone in on a particular stage. Switching to a pattern that reflects a different stage in a mayfly’s life cycle might trigger some superb fishing.

3 Replies to “The Mayfly Life Cycle”

  1. There are sub imago and imago stages, I believe. Sub imago is the first moly and the mayfly is able to fly. A second molt occurs and the adult imago emerges (dun).
    are there differences in color?
    As fly fisherman do we even need to care?

    1. Totally agree. No. I never hear fly fishing guides talk at this level of geek. Yes, there may be a handful who have a deep interest in the nuances of entomology, but my guess is if we did an analysis of the ratio of knowledge of entomology to the # of fish caught by that person, at some point, there would be a vectoring downward due to the law of diminishing returns.

      Great comment!


  2. I listen to your talk on Bud Lilly and like the way it presented. Looking to more. Always had an interest in fly fishing and want to learn more, Thank You

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