Alderflies, like Sialis fuliginosa, have an unfortunate life cycle, taking between one and two years to develop into adults from the egg stage, only to live for the next week or two, mating and laying eggs for the next generation.
Each female may lay several thousand eggs, in batches of a few hundred, on the vegetation overhanging slow-moving or still waters of ponds, lakes and rivers. On hatching, the tiny larvae drop directly into the water where they’ll live in the silt and detritus, using their strong mandibles to feed on worms, small crustaceans and the larvae of other insects. Here, they will develop, through a series of moults over a period of twelve or more months, before finally crawling out of the water to burrow into nearby moist soil to pupate.
The adults emerge from the pupal stage during May and June. Because their lifespan is so short, they feed little or not at all. They are weak, slow fliers, and tend to remain near the water where they had hatched.
