The larva, sometimes also called nymph or naiad, is aquatic. Larvae breathe through abdominal gills. Their body shape varies according to species. Some are swimmers, some crawl through muddy bottoms, others are burrowers, and yet others live in fast-flowing streams and rivers.
Larvae undergo 12 to 20 moults before reaching their full adult form; depending on the species, this can take anywhere from one to three years. Larvae float to the surface when they moult for the last time. They appear as colourless insects with wings edged with hairs: this is the subimago, or pre-adult stage. They fly to rest on nearby vegetation. Some twenty hours later in a final moult called the imaginal moult, the fully mature adult, or imago, emerges. Their short lifespan, lasting only a few hours or days, is devoted essentially to mating and egg laying.
After mating in full flight, females lay several hundred eggs on the surface of lakes and streams. The eggs then sink and attach themselves to the bottom. The embryonic stage usually lasts a few weeks, but it can be interrupted by winter hibernation (diapause). Males die after mating and females after laying the eggs.