Even though there are a few mayflies skipping the spinner stage and egg-lying as dun (like O.renana), the vast
majority of these species turn to imago. The spinners mate in flight, thus in this stage they do not touch the water and cannot become prey
of the fish. At least until the females dip the eggs with the typical up and down rhythmic gliding dance.
In this phase, the most interesting case for the fly angler is the fall of the spents. Once the mating is over and eggs are laid,
the insects drop dead into the water. They fall in large numbers and the presence of such amount of food stimulates the rise of the fish.
Left: Heptagenia flavia dun. Right, the same species as spinner.