Let’s discuss Mayflies: First we have several different species of Mayflies here in Missouri; Pale Morning Dun (PMD), Blue Wing Olive (BWO), Quill Gordon, Hendrickson, March Brown, Cahill, Trico, to name just a few. So your asking yourself, “How do I recognize the insect as a mayfly”? The may fly duns or young adults have wings that are straight up on their backs , similar to a sail on a ship. Mayflies live underwater as nymphs until they reach maturity, then they hatch into winged adults called Duns. The duns molt once more into spinners that mate, then die. It’s said that the spinners get their name from the “dance” they do during mating, which is an up and down flight. Before dying the females lay eggs in the river. When they die they fall into the river as “spent” flies, with wings now flat to the water surface. The mayflies are divided into four major groups on the behavior of their nymphs: Crawlers, Clingers, Swimmers, and Burrowers. The names of the nymph groups should give you an idea of how they react underwater.
How should we fish mayflies and what types of flies should we use. During their nymph stage dead drift or swim your nymph patterns, use a hares ear nymph or a pheasant tail nymph. Most mayfly nymphs have tails (see picture) so use a nymph pattern with a tail (unlike the Caddis nymph). The dun stage one can use parachute baetis, parachute adams, sparkle dun, or a comparadun. Fish them like you would any other dry fly. There are numerous dry fly patterns that represent the exact mayfly that you are seeing. There are several spinner patterns also. A spent (or dead) mayfly pattern usually has the wings spread out flat against the water surface. dead drift this pattern also.
Most of our mayfly hatches occur in the spring and the fall/winter months. Hatches usually occur during cloudy times. Since mayfly adults only live about two days, you could run through every fly pattern possibility in a days fishing!