Short answer: mayflies have upright, sail-like wings, a slender tapered body, and two or three long thread-like tails. If the wings stand up like a butterfly at rest, it's a mayfly. Tent-shaped wings mean caddis; flat wings mean stonefly.
Identifying a mayfly
Mayflies, known scientifically as Ephemeroptera, are crucial for the fly angler to recognise given their significance in the aquatic food chain.
Adults are easily identified by wings that stand upright like a butterfly's, a slender tapered body, and two or three long thread-like tails. The nymphal stage varies considerably — the slender, streamlined Baetis nymphs contrast with the flatter, leaf-like Heptagenia nymphs. Observing these characteristics alongside a field guide helps pinpoint the species.
Understanding the hatch
The term "hatch" refers to the period when aquatic insects emerge from their underwater nymphal stage to become winged adults, otherwise known as duns. With mayflies this process is fascinating.
The nymphs migrate toward the surface, and once there they break through their exoskeleton to reveal the subimago or dun — an immature winged adult. You can watch the duns floating down the water looking like miniature sailboats on a gentle cruise.
These duns then fly to nearby vegetation where they undergo another moult, shedding their skin one last time to become the fully mature spinner, or imago. Recognising a hatch involves noticing increased bird activity over the water, fish breaking the surface, and floating duns.
Shortly after, the adults return to the water to mate, which results in their death. Some stay in that mating form longer than others, but the lifespan is short — 24 hours or so — and the end of life is the spinner stage.
The full breakdown of all four stages is in Mayfly: Stages of an Insect.
Matching the hatch
The essence of fishing a mayfly hatch is matching it — selecting an artificial that closely resembles the emerging or adult insects. Keen observation is crucial.
If trout are taking insects just below the surface they might be feeding on emerging duns, so an emerger pattern is ideal. The big tell is in the feed: gulping, sipping, or splashing.
When feeding below the surface you'll often see larger splashing takes, because fish are chasing rising emergers through the column and breaking the surface as they do. Keen observation here is critical, and often if they're taking adults they'll take emergers too.
If trout are gulping adults off the top, a dry resembling the adult stage is effective. The silhouette, size, and colour should mimic the natural as closely as possible — during a Pale Morning Dun hatch, a light-coloured size 16 dry might be the ticket.
A CDC emerger is a favourite of mine for emerger work. It can be deadly.
I use a memorisation tool for pattern selection, following the acronym OSSCP — Observation, Silhouette, Size, Colour, Presentation. Observation and Presentation bracket everything by being the most powerful elements. Then, in order of importance, it's Silhouette, Size, then Colour.
You've likely heard the phrase "the flies are for the angler." What it means is that the real importance is observing what they're eating, then presenting something that looks relatively similar in the right location. The fly only needs to resemble the natural — it doesn't need to be an exact replica.
Two classic adult patterns: the Parachute Adams as a full adult, and the mayfly cripple.
We carry the Adams Classic Dry Fly and the Adams Parachute in the smaller sizes, plus the Baetis Cripple for the stuck-in-the-shuck silhouette. For the nymph stage, the Bead Head Pheasant Tail Assortment is the mayfly nymph imitation that works almost anywhere, and the Trico Mayfly BugCycle gives you one mayfly species across its life stages — a genuinely good way to learn how silhouette shifts as an insect develops.
Factors affecting the hatch
Several environmental factors influence the timing and intensity of a mayfly hatch. Water temperature is paramount — most species prefer a specific range for hatching, often between 50-65°F (10-18°C).
Weather plays a part too. Overcast days can extend hatching periods, while a sudden cold snap might delay or reduce intensity. Being aware of these factors, along with local hatch charts or your own logs, helps you predict when emergence is likely.
Wet nets!
If you liked this article, you'll enjoy Identifying the Blue Winged Olive, Identifying a Caddis, and How to Identify a Midge.
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