Alderflies are megalopteran insects of the family Sialidae. They are closely related to the dobsonflies and fishflies as well as to the prehistoric Euchauliodidae. All living alderflies - about 66 species altogether - are part of the subfamily Sialinae, which contains between one and seven extant genera according to different scientists' views. But in most classifications, all or almost all of these are treated as subgenera of Sialis.