Mercer's Gidget can be used to imitate a midge (particularly a chironomid) or even a small mayfly; although Mercer also has another, similar pattern called the Micro Mayfly for that specific hatch. (You can read about the Micro Mayfly in Mercer's book Creative Fly Tying). With the copper/brass bead, it will sink faster than his Midgeling, making it more useful in "faster" currents and/or deeper waters. But, given the size, it's not going to sink by itself in faster/deeper water. Think split shot or as a dropper behind a heavier nymph in all but the most benign flows.