Alderfly (Sialis lutaria)

GenusSialis (1)
SizeLength of larva: 17 mm (2)
Length of adult: 20 mm (3)
Adult wingspan: 22-34 mm (3)

Common and widespread (1).

Alderflies are fairly primitive insects, and were the first insect group to develop a pupal stage(4). The robust body is brown or black, with two pairs of brown or grey wings, which are folded back over the body when at rest and feature dark veins. The larvae are aquatic and have large heads with powerful jaws; there are three pairs of legs and then a pair of limb-like feathery gills on each segment of the body, except the very last segment which is tipped with a long spike-like gill (3).

This widespread species is the commonest alderfly in Britain (1).

Adults are found close to ponds and slow-flowing streams where there is plenty of silt. The aquatic larvae live in mud and detritus (1).

Adults are found in May and June (1); they are weak fliers and last for just two to three days, during which time they do not feed (4). The emergence of adults is synchronised, so that large numbers of both sexes are present at the same time. Mating occurs at night, and females lay very large batches of eggs on plants that overhang the water. A number of females may choose to lay their eggs on the same plant, which may become covered in large expanses of alderfly eggs (4). After hatching, the predacious larvae fall into the water where they crawl around in search of food; they take other insects, small invertebrates and worms (4). After 1-2 years of development, the larva leaves the water, and digs into the soil or waterside debris to pupate. Two to three weeks later the adults emerge (2).

This species is not threatened at present.

Not relevant.

For more on invertebrates and their conservation see Buglife, the Invertebrate Conservation Trust at:

This information is awaiting authentication by a species expert, and will be updated as soon as possible. If you are able to help please contact:

  1. National Biodiversity Network Species Dictionary (Jan 2003):
  2. Macan, T. T. (1959) A guide to freshwater invertebrate animals. Longmans, Green & Co Ltd., London.
  3. Olsen, L., Sunesen, J., & Pedersen, B. V. (1999) Small freshwater creatures. Oxford University Press, Oxford.
  4. O'Tool, C. (2002) The new encyclopedia of insects and their allies. Oxford University Press, Oxford.