As the water warms in streams, rivers and lakes, there is an explosion of invertebrate activity, when the hoards of aquatic bugs that pass much of the year on the bottom are stimulated by the favorable thermal conditions which allow them to continue with their life cycle. Among the insects preparing to leave the safety of some protective nook, or transition into a stage that no longer perfectly matches the surroundings, are the mayflies, an exceptionally prolific and ecologically significant group of aquatic organisms.
Mayflies form a category, or order, of insects known as Ephemeroptera, which literally translates into the short-lived insects. This label is somewhat a misnomer, as most mayflies have a life span of one full year. Nearly this entire period, however, is spent underwater, initially in the embryonic form of an egg and then as a naiad passing from one nymph stage into another as they consume and convert microscopic matter in the water into body tissue.
During the final few days of its life, the mayfly exits the water, transitioning into an adult. It mates, lays eggs and then dies. Because of the adult’s extremely short life span, these insects lack a functional mouth and a digestive system during this phase of their existence. The energy needed to mate and lay eggs is provided by small stores of nutrients which the adults acquire as their body structure is rearranged during their metamorphosis.
There are dozens of species of mayflies that are common throughout the Adirondacks, and each has its own particular time and method for leaving the water. In some species, the nymph rises to the surface and slowly pulls its sub-adult body from its present exoskeleton. Other species quickly exit their external body covering and fly upward, providing a fish on the prowl with little opportunity to gulp down the emerging bug. Some species of mayflies are known to transition into a winged state while still submerged. These individuals quickly float upward and immediately take to the air upon reaching the surface. A few species of mayflies crawl out of the water and then experience the change into a winged insect after reaching the shore.
Unlike nearly all other insects, mayflies pass through a winged sub-adult stage known to entomologists as the subimagos form before developing into a mature adult, called the imagos state several hours later. The initial winged phase is one that better allows the mayfly to deal with hydrological issues, like surface tension, as it leaves an aquatic environment. This form is known to fly fisherman as the “dun” stage. In the imagos state, the mayfly is recognized by its gracefully arched body, its long, delicate, transparent wings and thread-like filaments projecting from the tip of its abdomen. A mayfly in this final stage, known to anglers as a “spinner”, is ideally adapted for the bouts of flight that are well known to those that spend time near streams and rivers from late May through early July.
In a highly synchronized manner, countless mayflies emerge from the water on the same day and assemble into a massive swarm, usually near a stretch of faster flowing water, as dusk approaches. The insects then mate during this aerial gathering. The females quickly flutter over the surface to deposit their eggs in various manners, depending on the species. Shortly after the eggs have been laid, all of the individuals composing the swarm die and drop near or into the water.
As would be expected, a massive swarm of adult mayflies hovering just above the surface and then plunging into the water triggers a feeding frenzy in the trout of the area. Because a trout tends to target the specific type of mayfly involved with each event, it often ignores other bugs that are dancing on the surface in the immediate vicinity. This is why an experienced fly fisherman uses a dry fly that duplicates exactly the species of mayfly involved with that hatch.
Since mayfly swarms are composed of so many individuals, a fair percentage always survives the feeding onslaught that occurs with fish, birds, and amphibians. On subsequent days, other hatches involve other species of mayflies, providing to the angler the challenge of matching the spinner of the day.
Over the course of the next several months, swarms of mayflies can be noticed over a river or stream, particularly near dusk. This is the time that always excites fly fisherman, but can lead to frustration when the fish ignore the lure at the end of the line and opt to take only those bugs on the water.
Photos: Above, fly fishing on the Ausable in Wilmington (John Warren photo); below, male subimago of Baetis tricaudatus (photo by Wikimedia user Jerry Friedman).