Eastern dobsonfly (Corydalus cornutus) larva I found while hanging out in the river in Northeastern Pennsylvania (US) last year. Dobsonfly larvae, also known as hellgrammites, are one of the top invertebrate predators in fast-moving streams. They tend to hide under rocks and use their sharp mandibles to capture and eat prey. These mandibles can also deliver a painful (but harmless) bite, so definitely be careful handling them! Larvae can get up to 3.5 inches (9 cm) long. Since they're large and relatively common, many of the local fishermen like to use them as bait (which may or may not be problematic).
These insects spend 1-3 years as larvae and pupate outside of the stream under a rock or log. The adults are huge. The male can be up to 5.5 inches (14 cm) long and has massive mandibles that are intimidating but harmless. Adults aren't known to feed, and they only live for about a week, just enough time to mate and lay eggs.