Alderfly is the name given to neuropterous insects of the family Sialidae, related to the ant-lions, with long filamentous antennae and four large wings, of which the anterior pair is rather longer than the posterior.

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 a well-known British example. In America there are two genera, Corydalis and Chauliodes (more commonly called "Fishflies"), which are remarkable for their relatively gigantic size and for the immense length and sabre-like shape of the mandibles.