I've gotten into tying these lately. I want to try them for Bass, Wipers, and Trout this year. They are a mix between a tube fly and an articulated leech.
I tie them a little different from this guy. But its the same idea. Here's a great tutorial on how to make these bad boys...
Derek Fergus came up with the idea for this fly. There's a great interview with Fergus on The Caddis Fly: Oregon Fly Fishing Blog