The larvae of this holometabolous order are the only life stage which is aquatic. The abdomen of both dobsonflies and fishflies terminates in a pair of anal prolegs, each with two hooks, while the abdomen of alderflies terminates in a single median filament without hooks. In order to differentiate between fishflies and dobsonflies look at the ventral side of the abdomen.
Dobsonflies have cottony gill tufts beside the lateral appendages, fishflies lack such gills.