The Poly-Caddis (not the only existing pattern with that name), is GB’s synthetic take-off on the Elk Hair Caddis. It hails from that early-1970s era when some pretty interesting fly designs were coming to the forefront. If I recall right, GB used macrame yarn for the wing originally (although he stopped short of going all-yarn as with the Rackelhanen, a pattern which pre-dated the Poly-Caddis by several years).
The Poly-caddis is still a great fly, and the use of the synthetic wing allows a pile of color/texture possibilities (and makes tiny sizes fairly easy to tie). The dubbed (or thread-only) body also allows for tiny sizes to be tied more easily. Of course, synthetic-based caddis are common these days, but the original design (perhaps slightly updated with an X-wrap hackle), still does the job just fine.
Here’s how to tie it:
The base pattern can also be modified a bit further, using un-dyed sparkle yarn for the wing, to make a “diving caddis” (see the lower left fly in the top photo). GB has also taken to bending the thorax region of the hook upward a few degrees to get the wing to lie back even flatter along the back of the fly (see the below pic for an updated version of the Poly-Caddis that GB calls the “Z-Caddis”). Play around as you see fit, it is an easy pattern to modify (indeed, that’s how it came about— as a modification of another pattern).