These two photographs of a flower show how the evening primrose looks in two different wavelengths. The upper panel shows the flower as humans see it in visible light. The lower panel shows the primrose in UV, and reveals the "honey guides" (the dark areas) invisible to the human eye but seen by insects. The dark lines and patches guide the insect to the collection of nectar stored in the center and to the pollen on the anthers.
Image courtesy Dr. Jeremy Burgess, Science Source/Photo Researchers.