The life cycle of dobsonflies is characterized by a complete metamorphosis, with four developmental stages: egg, larva, pupa, and adult. Adult dobsonflies are usually found near freshwater, especially streams, either resting on vegetation or engaged in an awkward, fluttering flight. Sometimes adult dobsonflies can be abundant at night around lights, even far from water. The immature stages of dobsonflies are aquatic and are usually found beneath stones or other debris in swiftly flowing streams.
Dobsonflies have rather soft bodies. The adults of North American species generally have body lengths of 0.75-1.5 in (2-4 cm) and wing spans of 2 in (5 cm) or greater. These insects have four wings with distinctive, many-veined membranes. The wings are held tent-like over the back when the insect is at rest. Dobsonflies have piercing mouthparts. Male dobsonflies have large mandibles about three times longer than the head and projecting forward. Female dobsonflies have much smaller mandibles. Adult dobsonflies are active at night and are not believed to feed, so the function of the exaggerated mandibles of the male insects are unknown. Dobsonflies lay their eggs on vegetation near water, and the larvae enter the water soon after hatching.
Larval dobsonflies are sometimes known as hellgrammites and are predators of other aquatic invertebrates. Larval dobsonflies are quite large, often longer than 3 in (8 cm) or more, with distinctive, tracheal gills projecting from the segments of their large abdomen. Larval dobsonflies are sometimes used as bait for trout fishing.
Various species of dobsonflies occur in North America. The species Corydalus cornutus is common but not abundant in eastern parts of the continent, while the genus Dysmicohermes is widespread in western regions.