Many living aquatic creatures that are prey to game fish absorb ultraviolet light and emit light in the ultraviolet and visible spectrums (fluoresce), making them far more colorful, radiant and visible than artificial flies. This visual phenomenon is why aquarium hobbyists add ultraviolet light emitting bulbs, enhancing the fluorescing light and colors of the fish and other living organisms in the tank. This is true in both freshwater and saltwater environments.
Thus, as a wary predator searches for his next meal, the visible features triggering the most aggressive response are movement, silhouette and a full spectrum of radiant light and colors as emitted from the living prey. It is for this reason that dull and lifeless imitations are often rejected.