The Dobson fly larva is a prized bait for smallmouth and channel cats. Like crayfish, they are found under rocks on the stream bed. They can be a few inches in length, and when disturbed they roll up and tumble down stream. The one pictured is a male. The jaws of the female are not nearly as large. They too, however ,are rather formidable. Those of the larva are similar in size to those of the female adult. There are many hellgrammite patterns that are effective in catching smallies with the flyrod. In my little experience with using them, they work fairly well for channel cats too. The Woolly Bugger mimics the hellgrammite fairly well.
They are somewhat common in the Vermilion River.