What insect does this traditional pattern look like? Some think it suggests two mating midges. Whatever. It has peacock herl, a material whose trout catching ability should never be underestimated. When and where should you use it? During midge hatches (use size 14-18) or anytime you feel like casting a dry fly and there is no hatch in progress.
How to Fish
Dress the fly with floatant and use standard dry fly presentations.