Hellgramites are the larval stage of an insect, the Dobsonfly. Dobsonflies can be up to 4" long. They have a soft body, long, lacy wings, and long mandibles (jaws). Dobsonflies lay 2-3 thousand eggs on a rock or branch just above the water. The young larvae drop into the water.
Hellgramites are important food for bass and other fish. You see few adult Dobsonflies because fish have eaten so many Hellgramites!