Al's Rat is a very effective midge pupa imitation, developed by All Miller for catching finicky trout on Pennsylvania's limestone streams. I'm going to tie one here on a Dai Riki #310 Straight Eye hook in size 22. There's something about the straight eye that just looks right with this pattern.
Make sure you mash the barb as these small hooks can be rather difficult to remove. Although Mr. Miller used Monochord for this pattern, I'm going to use UTC 70 Denier in dark brown.
Start your thread on the hook shank leaving a hook eye long space behind the hook eye. Cover a third of the shank before wrapping back to the tie in point, then wrap back rearward another third and return once again. What you're doing here is creating a thin but tapered body. Now give your bobbin a clockwise spin to cord the thread then make adjacent wraps up the hook shank to add micro segmentation to the already tapered body.
Natural colored muskrat fur is traditionally used for the thorax of the fly. You only need the smallest amount. With the thorax complete, whip finish the fly with 4 or 5 turns and snip or cut your tying thread free. Trim any overly long pieces of muskrat fur if you'd like.
And that's basically it. As simple as this fly is, it's fooled many a picky trout. Most anglers fish it in the film but some have lots of success fishing it deep as a trailing fly. Try them not only in brown, but also in green, red, gray and even black.