True flies have reduced the second pair of wings found in most insects to halteres for balance shown in Figure C below. Cavefish lose both eyes and pigment, because both would require energy to create, and they are useless in the lightless habitats of caves, as shown in Figure B below. You are probably very familiar with this fine example of a vestigial behavior: goosebumps raise the sparse hairs on your arms even though they are no longer sufficiently dense to insulate you from the cold by trapping warm air next to your skin; in most mammals, this reflex is still quite functional, as shown in Figure D below. Most vestigial structures are homologous to similar, functioning structures in closely related species and, as such, support both common ancestry and natural selection.