Attribution: Didier Descouens, [CC-BY-SA-3.0], via Wikimedia Commons
Up to 3 inches
Dobsonflies are important aquatic insects that live in fast moving streams. The adults are one of the largest insects in the U.S., and the males have large mandibles up to 1-inch or more in length. The males look frightening but are harmless. Adults are attracted to outdoor lights on buildings near waterways thereby prompting curiosity and concern by homeowners discovering one or more dobsonflies on the porch or deck. Dobsonflies, however, are not dangerous and do not bite.
The larvae, also called hellgrammites, live under stones at the bottom of streams. They are prized as fish bait, especially among fly fishermen.
No control measures are necessary when a dobsonfly is discovered outside a home or other building. It is simply a wayward insect attracted to the lights. You can simply move it to a nearby shrub in the shade where it will likely resume its journey come darkness.