Hellgrammite larva of the Dobsonfly. Serious FISH food. The first time ya see a Dobson Fly you will remember it. They are huge and appear to mean business. Common in some of the rivers and streams that I fly fish. Certain patterns mostly subsurface can get ya some very good action on a 5wt or 6wt rod. At times the "hatch" can be plentiful, other times scarce every year is different. That's the wonder of fishing. Be prepared.
my 2 cents