"meaty, beaty, big & bouncy..."
Hellgrammites are the aquatic larval stage of the dobson fly and normally live three years before emergence. As they near maturity they crawl out of the water and burrow in the soil or decaying wood where pupation takes place. The adult dobson is a monstrous flying beast with a 4-6" wingspan. They are seldom seen as they emerge at dark, fly little and live only two or three days to deposit their eggs on limbs above the stream.
Hellgrammites primarily burrow into weeds & gravel in search of prey, but are in the stream year-round and are often available to fish. And they are definitely a mouthful! On the Flint River, they are a favorite of the shoal bass and the bait fisherman. You can find them deep in the submerged grass and moss (along with stoneflies and many other aquatics). If you wonder why the shoalies are often down in the rocks burrowing in the moss, this is why.
The hellgrammite has large lateral gill filaments, gripping hooks on the last abdominal section, and large biting mandibles on the head (and they will bite!). Mainly crawlers and clingers, they swim poorly and tend to curl up and tumble when swept by the current. My favorite imitation in a sparsely-hackled weighted, black wooly bugger tied on a long shank hook, fished dead-drift near the bottom.