Stoneflies are insects that tend to live near water, preferring rivers and streams. They are a favorite for fly fishermen. According to the Upper Delaware River website, approximately 500 different species of stoneflies live in North America. The University of Montana claims about 1,900 species exist worldwide. Stoneflies undergo no metamorphosis, meaning they do not have a pupa stage, and are considered to have an incomplete life cycle. While the life cycle of the stonefly is the same across the species, the time span can differ.
Stonefly Egg Stage
Female stonefly eggs are dropped as an egg sac into the river or stream. Sometimes she will fly above the water. Other times she will crawl down to the stream's edge to drop the egg sac underwater. Eggs dissipate in the water and eventually mature into nymphs.
Stonefly Nymph Stage
After eggs develop into nymphs, stoneflies can take anywhere from three months to three years to mature into adulthood. During this time, they pass through stages, known as instars. As the nymphs grow, they shed their exoskeletons. Each period of shedding marks the end of an instar stage. Depending on the species, the nymph may undergo anywhere from 12 to 23 instars. As maturity approaches, the stonefly nymphs make their way to the water's edge and prepare to emerge. Males tend to mature faster and emerge sooner than females. Emerging occurs mostly at night.
Stonefly Adult Stage
Upon emerging from the water, the stonefly has one last instar. Upon its completion, the nymph is now an adult. The adult stage tends to only last for a few days up to a few weeks, depending on the species. Those that live only a short time never eat. Instead, they focus upon mating. Those that live longer will feed upon pollen, plants and growths on tree bark.
Stoneflies tend to mate in swarms. They are often found at this point swarming some sort of vegetation near the river or stream. This can be any plants or trees found nearby. Sometimes they will mate on the ground. Males will attract their mates by beating their bodies on the vegetation or ground. Some species of stonefly will mate repeatedly, creating multiple offspring.
The stonefly female will now have a sac of eggs. She returns to the water's edge and drops her egg sac under the water. Some species will mate repeatedly, creating multiple egg sacs to be dropped underwater. The process begins again.