The famous nocturnal Hex hatch of the Midwest (and a few other lucky locations) stirs to the surface mythically large brown trout that only touch streamers for the rest of the year.
Vestigial: An anatomical structure which has become dysfunctional or reduced after hundreds of generations of non-use is vestigial, and whatever remains of it is a vestige. The human tailbone is a classic example.
These other words reference the same concept: vestige, vestiges.