Also known as crawlers by local fishermen/women, these are the underwater larvae of dobsonflies. And they are in turn one of the largest non-lepidopteran found in our area.
Hellgrammites are formable, hence the creepy name, but the male dobsonflies with their enlarged mandibles are pretty intense themselves. Like stag beetles, these enlarged horns are used to compete for the attention of the females. Another instance in which female-choice has driven the males of a species to an extreme look as in the bright red of a male cardinal.
Adult dobsonflies are nocturnal carnivores found along local streams. They prey on other non-hellgrammite aquatic larva found in stream riffles and probably eat a few of their own as well.
Both images are from WikiMedia