The giant Salmonflies of the Western mountains are legendary for their proclivity to elicit consistent dry-fly action and ferocious strikes.
This "specimen" is actually two caddis larvae fighting each other over a case. The case is a hollow tube; one larva would go in the back end, presumably bite the other, and chase it out. The invader crawled forward into the case while the other one fled, and then it went around to the back and bit the first one. They did this several times, and I recorded it on video.
Caddisfly larvae of this family can easily leave and re-enters their cases. I caught two of them playing musical chairs or something with this one... funny!