After a mayfly mates (the spinner stage), it falls to the water dead. Wings splay flat, body limp. The Spent Wing pattern is tied to imitate this exact silhouette — wings tied at 90° angles to the hook, often with sparse hackle so the fly rides flush.
Common variants: Rusty Spinner (the classic adult mayfly spinner imitation), Trico Spinner (size 22 black-and-white), and color-specific spinners matching whichever mayfly is in the air. Fish them during evening spinner falls in slow water and back-eddies where dead bugs collect.