This is what a Hellgrammite larva will grew up to be.

http://jareddavidsonphotography.blogspot.com/2015/05/dobson-fly-adult-hellgrammite.html

Interesting.

My grandpa and I used to catch coffee cans full of these in the Llano and James river and fish with them. Always caught good size fish using them as bait.

Here is what they look like in the river under rocks.

http://www.fly-fishing-discounters.com/hellgrammite.html

We would go to White's Place on Brazos River, before the Granbury Dam was built. Right at Falls' Creek. My dad sent all of us kids to gather them so he could catfish with them. Always caught fish good. They will give you a blood blister from the pinch.

This was photographed near Frederickburg, near Llano. My family has a camp we visit every summer and we caught the adults all over right beside the Pedernales River. Whats funny though is I have never found the larva, only the adults.