At Pimlico Race Course in Baltimore every May, the winning horse in the Preakness Stakes is draped with a blanket covered with what appear to be the Maryland state flower, the black-eyed Susan. But the flower doesn't bloom until later in the season. Those crafting the victory blanket must resort to using yellow Viking daisies — and painting the centers black.

That might fool race fans, but bees can see through the ruse. With eyes equipped to detect ultraviolet light, a bee can pick out an additional band in the black-eyed Susan's bull's-eye. The insect's livelihood depends on it. At the center of the target is the flower's nutritional payload, nectar and pollen, which also glows in UV light.

As with other members of the sunflower family, black-eyed Susan flower heads are composed of two kinds of florets. The dark center is made up of numerous disc florets, each of which contains male and female reproductive components. When a bee or other pollinator fertilizes a disc floret, it develops a single seed that ripens and falls from the flower head in the autumn. Seeds can remain viable for more than 30 years.

Circling the disc florets are bright yellow ray florets, which flag down pollinators and act as landing strips. The inner portion of each ray floret contains several compounds that absorb UV rays. The outer portion reflects UV rays, contributing a visually energetic outer ring to the pattern — provided you're a bee.

Black-eyed Susan, Rudbeckia hirta. illustration by Patterson Clark

Human color receptors can detect red, green and blue light. Red and green wavelengths reflected by ray florets combine to produce yellow light.

Ultraviolet light, simulated here with violet, is both reflected and absorbed by the flower. Special filters, films and cameras can make this detectable to humans.

Bees' eyes cannot detect red light, but they can see green, blue and ultraviolet. Because pigments in the ray florets absorb blue light, they would appear green to a bee — except where UV rays are also reflected, simulated here with magenta.

Sources: Journal of Comparative Physiology, Phytochemistry, Pimlico Race Course

Color perception in humans and bees. illustration by Patterson Clark

UV absorbed

UV reflected



B  G  R

Ray florets


UV  B  G

Ray florets