Another good day for the Cooper Bug

I am becoming very intrigued by this fly.  The fish went crazy over it today.  But the amazing thing was the distance they were moving to grab it.  Cooler water temps meant rainbows were the more active fish, brown trout were noticeably absent today and the river I fished is predominantly a brown trout fishery.  Subtle takes were not on the menu this afternoon.  The rainbows shot out of their lies and smashed this fly.  On three occasions I had 4x tippet snap on the take.

As far as insect activity goes there were a few straggling Hendricksons coming off, but that hatch peaked a week or two ago and should be all but over.  I did not observe any caddis activity (at or above the surface), but I know there was some earlier in the week.  The only other insects were a few small stone flies crawling about on mid-stream rocks. What are these fish taking this fly for??? ( I have a guess)  

 

There may have been a behavioral drift of green rock worms (Rhyacophila) occurring.  I was snagging these little buggers left and right.  In my opinion this fly bears little resemblance to the natural but I guess the fish disagree.  I even went as far as to imitate the obviously prevalent caddis larva with one of my rock worm patterns that did a better job "matching the hatch".

In the end the Beadhead Cooper bug out fished it three to one.  Without exception the larger fish of the day were caught on this fly.