One of my favorite nymphs to fish in the winter is a Red Fox Squirrel nymph. It can imitate a stonefly and it just plain looks buggy. The ones I have tried to tie have always looked crappy until my recent instruction. After learning to use a dubbing loop, I was able to get that shaggy look this pattern is known for. You can fish them as big as 6, and I have never fished them smaller than a 16 but I'm sure an 18 would catch fish. The ones pictured are 10's.
Rubber legs and beads are optional. Here is the recipe
Hook: 2x-3x long nymph hook
Weight; Bead and or lead
Tail: Red fox squirrel back fur
Rib: Gold wire
Abdomen: Red fox squirrel belly fur and antron dubbing
Thorax: Red fox squirrel back fur and antron dubbing
Legs: Soft brown saddle hackle (optional rubber legs)
The light, or belly fur dubbing, is the abdomen and the darker dubbing the thorax. I reversed them on this fly. We will see if it makes a difference.