Alderfly (Sialis lutaria)

Alderfly climbing stem
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Alderfly fact file

Alderfly description

GenusSialis (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).

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

Alderfly biology

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 invertebrates_freshwater 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).


Alderfly range

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

You can view distribution information for this species at the National Biodiversity Network Atlas.

Alderfly habitat

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).


Alderfly status

Common and widespread (1).


Alderfly threats

This species is not threatened at present.


Alderfly conservation

Not relevant.

There may be further information about this species available via the National Biodiversity Network Atlas.

Find out more

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:


Stage in an animal's lifecycle after it hatches from the egg. Larvae are typically very different in appearance to adults; they are able to feed and move around but usually are unable to reproduce.
Pupal stage
Stage in an insect's development, when huge changes occur that reorganise the larval form into the adult form. In butterflies the pupa is also called a chrysalis.
The process of forming a pupa, the stage in an insect's development, when huge changes occur that reorganise the larval form into the adult form. In butterflies the pupa is also called a chrysalis.


  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.

Image credit

Alderfly climbing stem  
Alderfly climbing stem

© Richard Becker /

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