The Hellgrammite is the larval form of the Dobsonflly, Corydalus Cornutus. They are typically about 2.5 inches long, and live in and around fast moving streams and rivers. They have 1/4 inch long mandibles that can cause a bloody pinch on a human finger, as well as hooked forelegs that they use to hold on to rocks in a current-or the aforementioned finger. How would said finger come in contact with above Hellgrammite you ask? It seems Hellgrammites are a favorite food of just about every tasty fresh water sport fish in the US. And as such, a popular bait. As a kid fishing with my dad in Maine, I believed that the Hellgrammites weren't actually bait, but that they attacked the fish and put them on the hook. We handled them with needle-nose pliers.
As adult Dobsonflies, they only live for a few days, but gain longer pincers. In the males they are used for grasping the female for mating. In the females, they are shorter, but more likely to be used to inflict a nasty bite.