Alderfly, small (13-18 mm), dark, soft-bodied insect of order Megaloptera, family Sialidae, found in freshwater habitats bordered by alder. It is characterized by 2 pairs of heavily veined, membranous wings, chewing mouth parts, large eyes and long, many-segmented antennae. Adults may not feed and are short-lived. Alderflies are most active at midday.

They deposit 200-500 dark brown eggs in rows forming large masses on branches or other objects near water. Larvae hatch in 10-14 days and drop or make their way to water. Larvae are aquatic, preferring muddy bottoms. They have chewing mouth parts and are predaceous. The larval stage may require 2-3 years to complete. Fully grown larvae pupate in a cell in soil, moss, under stones, etc, usually near water. About one month is spent in the pupal period. Adults appear in early summer and are feeble fliers.

Of the 10 species found in Canada, Sialis velata is the most widely distributed (BC to Qué). Larvae and adults are food for many freshwater fish and provide bait for anglers.