No, this is not a prehistoric monster from a horror film (e.g. Them! or The Beginning of the End). It is an insect in the family Corydalidae. The common names of these insects are Fishflies or Dobsonflies. They are very important insects in healthy aquatic ecosystems.
Immature corydalids (larvae) live underwater in well oxygenated streams and are also called hellgrammites.
Adult corydalids (pictured here) are very large insects with wing lengths ranging from 2.5 cm (1 inch) to over 5 cm (2 inches) and wingspans of up to 14 cm (5.5 inches).
The individual in these pictures is probably a fishfly. Fishflies are smaller than dobsonflies, with wings that are usually less than 5 cm long. Adult dobsonflies and fishflies don't actually eat very much (if anything) and they are around for only a few weeks before they die. The larvae however are alive for up to five years, living underwater as predators of other insect larvae. The next time someone tells you that dobsonflies only live for a few weeks you can kindly explain to them that while they only spend a few weeks of their lives as adults, they are alive for up to five years before that.
My friend found this fishfly last May in her yard near Long Lake, MI
