The Mayfly

Pale Morning Dun Adult (PMD)

On a late cloudy summer day, I was fishing the Yampa River.  The ominous clouds were impenetrable brewing a thunderstorm for the afternoon.  There stood another angler maybe 75 yards upstream from me.  A few rain drops drummed my Gore-Tex jacket.  A sudden cloudburst moved upon us instantaneously with a downpour so dense, I could hardly visibly make out the silhouette of the angler upstream.  Neither one of us abandoned our spots and withstood the mini monsoon.  As hastily as the rains hit, it departed bringing back sunshine and the most magnificent mayfly BWO hatch ever!

Mayflies are some of the most ornate insects in the fly fishing world.  There are many types of mayflies each with color variations and sizes.  Trout often are highly selective to mayflies and feed ravenous upon them.

Blue Winged Olive (BWO)

Mayflies have an incomplete life cycle of egg, nymph, and adult.  The eggs are deposited in the water by the females and hatch into young nymphs.  Mayflies have been classified into several families, which have quite different nymph habits.  Those with burrowing nymphs are the Ephemeridae.  Those with clinging nymphs are known as Heptagenidae.  Those with crawling, swimming nymphs are the Baetidae.

Mayflies spend most of their lives under water.  In the immature stages, they are called nymphs. The nymphs have bodies of ten segments and gills distributed variously depending on the different genera.  The thorax is very muscular and supports the growing wings under the dark wing casings.  Most nymphs have three tails, however, some species have only two.  The nymphs are very bashful and seek shelter to the bottom until it is time for them to emerge.  During this time, they lose their timidity and become considerably active in the water.

Mayflies consume diatoms and desmids in large quantities for food.  Their diet is predominantly vegetarian.  They have somewhat of a elongated, flattened body that goes through a number of instars, or molting stages to increase in size.  Growth is rapid with the nymph outgrowing and shedding his nymph skin as often as twenty times.

Pale Morning Dun Nymph

When the nymph reaches the surface to emerge, he splits his nymph shuck at the thorax, and the surface tension literally peels him out of his old skin.  The freshly hatched mayfly then pops out on the surface.  As his wings dry, he takes to the air.  The drying period is not long, usually lasting maybe a few seconds. During damp and drizzly days, mayflies cannot dry their wings off  as quickly and remain on the water for a longer period of time, much to the delight of a hungry trout.

This stage of the mayfly is known as the dun for most fisherman.  Duns may be recognized by their generally dull coloring and underdeveloped wing venation.  Most species make a direct flight to the shoreline brush to rest before the final molting to the spinner stage, where both males and females now have the ability to mate and lay eggs (ovipositing).  The final molting takes place from twenty minutes to three days depending upon the species. 

The adult mayfly is exquisite.  On the water, the mayfly look like miniature delicate sailboats.  Mayflies are the most diverse insect a fly fisherman needs to identify and understand.  There is definitely a science in learning the entomology.  Learn the stages, what flies to use, and matching the hatches.  The successful fly fisherman then can use the proper mayfly imitation and the skill to present their fly cleverly to the trout.

Illustration by Cat Toy