This time of year, the hatches are definitely on the decline, the fish are edging deeper, and picking a fly that works can be a challenge. So it is best to turn to a fly that works year round. Carpenter ants and red ants are generally to be found year round, and most fish appreciate the snack when forage is in short supply.
Hook: Mustad 9671 wet fly hook
Body: melted monofilament, colored with black (or red) waterproof marker
Legs: wet fly grade black (or ginger) hackle (dyed or natural)
This is not a hard fly to make, as long as one's focus doesn't stray in the process. Do be very careful, because serious burns, however small they may be, can result from careless actions while forming the body. Place a piece of heavy monofilament in a pair of hemostats, leaving 3 - 4 inches free. Light the end with a lighter or other flame source and allow it to burn toward the hemostats. As the monofilament melts, apply it to the hook set in the vice. Be careful pace the molten nylon in two separate lumps on the shaft, leaving a distinct gap between them. Rotate the vice as applying to assure a round symmetric shape. Allow to cool and color with waterproof felt tip marker.
Apply a short but solid foundation between the lumps and tie in a small wet grade hackle of matching color. Wrap 4 or 5 turns and tie off. Finish with a wrap knot between the lumps. Clip the top and bottom of the hacle so that only the sides extend from the body.
This is a fast sinking fly and should be fished with a strike indicator on the tippet. Basically, the fish strike on the fall more than any other time it's in the water. It can be swum, but it's not nearly as effective that way. Various nymphs are more suited for swimming, so concentrate on casting to spots near cover where the fly can sink unimpeded without getting hung up. Care should be taken not to throw the fly into heavy cover, since it is such a fast sinking fly and has a tendency to hang on limbs more than a normal nymph.
In moving water, fish it as you would any weighted nymph. Cast and play the line to avoid drag as long as possible. Don't be afraid to use the fly in deeper water. Being tree dwellers, it's not too uncommon for carpenter ants to fall from limbs onto the water and be carried out by the wind, where they are then swamped by waves, current, etc. Also, this fly has a similar profile to a number of auquatic insects, such as some immature dragon flies. Don't be afraid to cast it in deep water where the fish may be holding.