Mayfly Life Cycle
Mayflies demonstrate an incomplete lifecycle, because they don't undergo metamorphosis.
1. Egg matures into a nymph.
2. The nymph lives on the bottom growing in stages called instars* until it matures.
3. The mature nymph (emerger) swims to the surface.
4. Sheds it's husk becoming a dun, (subimago) and floats along the surface until it's wings are dry enough to take flight. In the insect world this subimago stage is unique to Mayflies. That is insects that are fully winged before the adult stage.
5. The dun flies into the bushes, or trees along the river bank, where it sheds it's skin becoming an adult, (spinner), (imago).
6. The spinner leaves the trees, or bushes to begin the mating swarm.
7. The female spinner dips her eggs on the waters surface, and they fall to the streams bottom.
8. After mating both male, and female spinners fall to the waters surface, and die.
Males appear first beginning the mating ritual high above the water. They flutter and dance in an effort to attract a female. Soon females appear, diving into the swarm to pick out a mate. As mating progresses the swarm falls closer, and closer to the waters surface. Fluttering through the air males hold females in place with their claspers. With the mating process complete females, leave the males, and begin depositing their eggs on the water. Exhausted, males begin falling into the water. They struggle to stay alive for a time, holding their wings upright, but soon fall flat on surface. After depositing their eggs females soon follow.
This is a general description of a Mayflies life cycle. Not all nymphs swim to the surface to emerge, some crawl out of the water on rocks, sticks, or the river bank. Different types of mayflies molt in the trees, or bushes for varying amounts of time, some may take a day or two, some an hour or less. Some types of mayflies dive below the surface to deposit their eggs on the streams bottom.
*As invertebrates grow they shed their exoskeleton numerous times these periods of growth are called instars.