They are sizeable Megaloptera, with a body usually larger than 25 mm (1 inch). They often have long filamentous antennae, though in male fishflies they are characteristically feathered. Ocelli are present; the fourth tarsal segment is cylinder-shaped. The four large wings are translucent, smoky grey, or mixed, and the anterior pair is slightly longer than the posterior one.
The larvae are aquatic, active, armed with strong sharp mandibles, and breathe by means of abdominal branchial filaments. When full sized — which can take several years — they leave the water and spend a quiescent pupal stage on the land, in chambers dug under stones or logs, before metamorphosis into the sexually mature insect.