Mayflies
historically are considered the most important aquatic insects to anglers.
Part of this is because they are the most noticeable, and the prettiest. But
another large part is due to their concentrated hatches, which cause trout
to feed with abandon and with eyes that see no other insects.
Mayfly nymphs have adapted to different types of water. Slender and darting swimmers live in slow water; a few are found in riffles, but the most astonishing numbers thrive on flats where the current is peaceful and aquatic vegetation has taken root. They like to flit around in it, browsing on the stems and leaves of submerged plants. Robust crawler nymphs prefer faster water. They are found most often in riffles and runs, clambering among the cobble and stones. They have weightlifters' arms with which to hold on while they browse the thin layer of photosynthetic growth on bottom rocks. Flat clingers have the aerodynamics of an airplane's wing and live by clinging along the faces of rocks in the fastest riffles, rapids, and cascades. Like crawlers, they eat the same slippery layer of algae that we slip on and curse while wading. Burrower mayfly nymphs live in the slowest water, over sand, silt, or mud bottoms. Some wriggle into the substrate until only their eyes show, others dig U-shaped tunnels and live in them beneath the bottom, coming out only to feed at night.
When the nymph is mature, a mayfly nymph swims or floats to the surface, where its skin splits along the back and the dun emerges. The dun has upright wings, a long slender body, and two or three long tails. They look like brave little sailboats adrift on water too big for them. As soon as their wings are dry, duns leave the water and fly to nearby vegetation, if they are not eaten by trout first. They tend to emerge in great numbers, in small spaces, and trout do their heaviest feeding when a hatch is occurring.
There are three moments of vulnerability to the mayfly during a hatch: first, as the nymph makes its way to the surface; second, as it emerges through the surface film and the dun escapes the nymphal shuck; and third, as the dun rides the current, waiting for its wings to dry. The individual insect is largely helpless entirely at the whim of the current and the trout. The survival of the species is dependent of the sheer mass of individuals that attempts to emerge, a few of which are bound to survive even in the most dangerous circumstances. This sort of survival tactic is a delight to feeding trout.
Duns that survive perdition and escape to streamside vegetation cast a final thin skin and turn into spinners, the reproductive stage of the insect. Male spinners form swarms, often dancing in clouds above evening streams. When a female enters the cloud of males she is quickly coupled. Her eggs are fertilized in the air, then deposited onto the water. Female mayflies generally die spent upon the water; males may fall to the water or return to vegetation and die there, useless to trout.