Mayfly - Stages of an Insect | Fly Fishing Insider Podcast

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Mayfly - Stages of an Insect

I’m certainly a faux entomologist, or ecologist, but I’m a fly-fishing enthusiast, and, I can appreciate the intricate dance between aquatic insects and the world of fly fishing. Understanding these life stages is not just crucial for science but also for the angler looking to "match the hatch" and select the perfect fly. Here's a detailed and memorable summary of the four life stages of many aquatic insects, especially mayflies, which are often of interest to fly fishers:

When you are done with this, go back and read the OSSCP article about selecting the perfect fly!

Swarm of Mayflies

Mayfly Nymph:

Description: The nymph is the juvenile, aquatic stage of the insect. It spends the majority of its life underwater, living amongst the substrate like rocks, vegetation, or silt.

Appearance: Mayfly nymphs look like little armored aliens. They have a grasshopper-looking head followed by a slightly humped back and a butt section that resembles the later half of a miniature snake that is followed but slim antenna-like tales, sometimes in the amount of two or three.  Many have gills for underwater breathing. Their legs protrude from the sides of their upper bodies around the shoulder and stop before the longer butt section.

Mayfly Nymphs

Behavior: Nymphs often crawl or swim around in search of organic matter to feed on. Some may be more active swimmers, while others might be more sedentary. Crawlers are often called clingers because they cling to the substrate and crawl around.  Pick up a few rocks, and you’ll likely see a few examples. 

Fly Fishing Implication: Nymphs are a staple in the trout's diet, so nymph flies are popular among anglers. They are typically weighted to sink below the surface and mimic the behavior of real nymphs.

My Favorite Mayfly Nymph Pattern - Flashback Pheasant Tail - I know…it’s basic.  The pheasant tail is so good for just about anything. I change the size so it matches up with the silhouette.  However, I think the flashback is for me.  I have fewer decisions to make, and I’m confident in it.

Flashback Pheasant Tail Mayfly Nymph

Mayfly Emerger:

Description: This is the transitional stage between the nymph and the dun. The insect rises from the bottom, often in a bubble of air trapped under its casing, to the surface and sheds its nymphal case. The bubble accelerates the rise, and while rising, the dun starts willing out of the front of its case.  While the nymph is partially in the case and the wings are barely protruding, the fly is ready to emerge from its concealed casing and into an adult or dun.

Appearance: It looks like a nymph transitioning into the adult form, with developed wings and evidence starting to unveil themselves.

Fresh Emerger Mayfly

Behavior: Emergers struggle to break free from their nymphal shuck and often drift just below or in the surface film. Fishing the skim can be deadly but often hard to see.  Tying off the bend can make this a little easier.  

Fly Fishing Implication: Trout often target emergers since they're vulnerable during this stage. Anglers use emerger flies that sit in or just below the water's surface to mimic this stage. Tricks like ginking your leader but not your fly or the opposite.  Fishing fluorocarbon and ginking your fly can help.  Find what works for you. 

My Favorite Emerger Pattern - CDC Emerge, and a close second is a Foam RS2 - both are deadly and can be fished well in the film.  I found the trick with the CDC emerger is to not gink the CDC at all.  When fishing, at first, it will float well.  After the CDC gets wet, the fly will sink.  Sometimes, that is the perfect fly, but if you want it dry again, then use this trick!


CDC Emerger MayflyRS2 Foam Emerger Mayfly

How to dry CDC



Mayfly with its discarded schuck.

Mayfly with its discarded shuck.

Dun (also called the Subimago):

Description: The first winged stage of the insect post-emergence. It's a short-lived stage where they fly to nearby vegetation to rest and mature into spinners. Sometimes, as they start to emerge, the outer casing, often called the shuck, is not completely shed. The emerging nymph pushes its head and wings out of the top front of the case, and just after getting the wings out, the shuck stays attached, and you get the infamous and deadly “Cripple!”  Correct, not politically correct, but well-defined. With a hanging shuck, the mayfly is slightly disabled and becomes an easy-to-identify and easy-to-chase target for trout. 

Appearance: Delicate, translucent wings and often lighter in color than the spinner stage. 

Mayfly Adult Dun

Behavior: Duns take flight from the water surface to nearby vegetation. They're not strong fliers and can be clumsy, making them easy prey for waiting fish.

Fly Fishing Implication: Dry flies that mimic the dun are used by anglers to float on the water's surface, representing this vulnerable stage. When you tell someone you fly fish…this is what they think of.  They have no idea that the fishing here is likely only to be present 10% of the time a fish eats.  Either way, it is one of the most exhilarating methods and one of the most frustrating as well.
If you want to improve your dry fly fishing, try the OSSCP and Systematized Perfect Drift articles.

My Favorite Mayfly Dun: The Parachute Adams is by far my favorite Dun fly. It imitates every mayfly there is and if I headed out with one fly for dry fly fishing this is the one I would take. You can see it well, it's shape imitates the Dun phase and the fly lasts forever.  Listen to this Tip and Trick Episode to get more ideas to increase the versatility of the fly and your fly fishing!

Parachute Adams Dry Fly - Mayfly Imitation



Spinner (also called the Imago):

Description: The fully mature, reproductive stage of the insect.

Appearance: Generally, Spinners are more sleek and clear-winged than the dun, often with shinier and more robust bodies.

Mayfly Spinner

Behavior: Spinners engage in mating swarms, after which females return to the water to lay their eggs. Post-mating, they die and fall onto the water, becoming "spent spinners." 

Fly Fishing Implication: Anglers use spinner patterns, especially during a "spinner fall," when many dead or dying spinners are on the water, offering a feast for trout. These infamous spinner falls can be feeding frenzies. Observing the water for spent spinners and watching trout behavior can set you up to capitalize on the opportunity. 

My Favorite Mayfly Spinner Pattern: The Rusty Spinner is a perfect silhouette fly pattern. They are offered in other colors, but the size is important here, from what I’ve experienced.

Rusty Spinner - Mayfly Imitation

Imagine an insect's life as the parts of a Star Wars movie. The Nymph is an underwater drama, full of crawling aliens exploring the universe. The Emerger is the thrilling breakout scene, a struggle for change and freedom. The Dun introduces the beauty of planning in flight, a fleeting dance in the air, and the Spinner is the grand finale, filled with passion, reproduction, and the poignant end. And in this Star Wars, the fly fisher is both an audience and a participant, casting their lines in sync with each act. Welcome to the world of Mayflies, young Jedi Master!

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Christian Bacasa, Host of the Fly Fishing Insider Podcast

By Christian Bacasa
Host of the Fly Fishing Insider Podcast

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