The alderfly is fairly common in and around water bodies like ponds, lakes and streams with silty sediments. The adults sun themselves on vegetation and objects next to the water bodies from which they emerge en masse. The alderfly has no association with alder trees, other than both being found beside water bodies. The alderfly belongs to a fairly primitive family of insects, Sialidae, that were the first insect group to develop a pupal stage. In the UK, there are three species of alderfly (Sialis fuliginosa, S. lutaria and S. nigripes) with all three being relatively common and widespread. S. fuliginosa is generally associated with flowing water, S. lutaria with more static water bodies and S. nigripes possibly with more calcareous waters.
The adults have a short and broad black/brown body (20 – 25 mm long) with two pairs of brown/grey wings with dark veins (22 – 34 mm wingspan) that are held roof wise over the head/body when at rest. The adults last for 2 to 3 days during which they do not feed. The adults are clumsy flyers and can easily get blown onto the surface of the water bodies around which they congregate. Following mating at night, the eggs are laid on vegetation and objects adjacent to the water body. On hatching, the larvae which are aquatic, drop into the water or onto the ground and crawl into the water. The larvae (17 mm in length) have 3 pairs of legs and 7 pairs of gills on the first seven segments of the abdomen, which are used for breathing as well as for swimming. The larvae have well developed mouth parts and are predatory. After 1 to 2 years the larvae emerge and pupate in sediment on the water body bank. The adults may be on the wing in any month of the year, however, the peak occurrence is in May and June.
Spatial distribution of UK Sialis species (NBN, 2018)
Alderfly UK relative abundance in each month (after NBN, 2018)
Flies that are good options when the alderfly is present:
