{{Taxobox
| color = pink
| name = Sialidae
| image = Sialis spPCCA20050708-9036A.jpg
| image_width = 240px
| regnum =
Animalia
| phylum =
Arthropoda
| classis =
Insecta
| ordo =
Megaloptera
| familia =
Sialidae
| familia_authority =
Leach, 1815
}}
Alderflies are
neuropterous insects of the
family Sialidae. They are closely related to the
dobsonflies, and have long filamentous
antennae and four large
wings, of which the
anterior pair is slightly longer than the posterior. There is only one genus, ''
Sialis'', in
North America and
Europe, and a few other small genera elsewhere (
Japan,
South America).
The females lay a vast number of eggs upon grass stems near water. The
larvae are aquatic, active, armed with strong sharp mandibles, and breathe by means of seven pairs of abdominal
branchial filaments. When full sized they leave the water and spend a quiescent
pupal stage on the land before
metamorphosis into the sexually mature insect.
''Sialis lutaria'' is the commonest alderfly in the
United Kingdom and across much of Europe.
Image:Sialis lutaria larva (aka).jpg|''Sialis lutaria'' larva
Image:Megaloptera-sialidae-sp.gif|''Sialoide sp.''