Member of the family Sialidae of the insect order Neuroptera (suborder Megaloptera). The alder flies are around 20 mm/0.8 in long and dark, with the general features typical of the order Neuroptera. Although they have an almost worldwide distribution, their numbers in terms of genera and species are very limited.

Sialis lutaria and S. fuliginosa are the only British members of the superfamily Sialoidea. The entire life cycle of S. lutaria spans about a year; the eggs are deposited in compact masses of 200–500 eggs on leaves, stones or other substrata, generally not far from water. The larvae are aquatic and are usually found in the muddy bottom of ponds, canals and slow-moving streams. They are carnivorous and with the help of their large projecting jaws they prey on other insect larvae and small worms. The adult insect, dark brown and about 26 mm/1 in long, is a poor flier; it spends the day at rest by the water and flies in swarms during the twilight hours; it feeds entirely on liquid food.