It is, in my opinion, the most deadly effective dry fly pattern for when trout are highly selective and keyed on small mayflies, particularly baetis. I'm usually a fan of the size 18 parachute Adams, as I believe that, presented right, that bug will tackle 90 percent of dry fly challenges. But when I have encountered that other 10 percent, it's the Mole fly that saves the day. I've seen it many times: show the fish 10 different dry flies, and finally solve the riddle with the Mole. I should probably lead with the Mole. But it's fun to watch its effectiveness with some context.