That is a question that I get rather frequently during the summer. The question is usually accompanied by a photo and the statement that “I’ve never seen one before.” Each summer I receive photos of some of the same species that are unusual enough in appearance, size or behavior to attract attention. One of those now generating contacts is the eastern dobsonfly (Corydalus cornutus). The male of the species, although harmless, is ferocious-looking due to the long pincers growing from the head. The female has much shorter pincers, but hers are stronger and she can inflict a painful bite. When threatened, both male and female will spread their pincers to intimidate predators. The adult insects don’t eat and only live a few days to mate and lay eggs, which is probably why many people have not observed them.
The larval stage of the dobsonfly, called a hellgrammite, is aquatic and can spend two to three years living under rocks on the bottom of a stream or river. Reaching 2 to 3 inches in length, hellgrammites resemble a centipede but with a more rugged exterior and a head with pincers. They feed on other aquatic insect larvae. Anglers sometimes use hellgrammites as fish bait, but they must be handled carefully as they can bite with their sharp pincers. Requiring well oxygenated water, the presence of hellgrammites can indicate good stream health. Adult dobsonflies are usually observed near the stream or river where they lived as hellgrammites.
Dobsonflies are related to lacewings and ant lions. You can probably see the resemblance if you are familiar with either of those species. The size of a dobsonfly and the long pincers on the males are probably why they attract interest. An adult male can reportedly reach a length of 5 inches. The adults can fly and are often attracted to lights at night. If you're lucky enough to see one, remember to avoid the pincers on the female!
Photo above of male dobsonfly by Whitney Cranshaw, Colorado State Univ., Bugwood.org