Nectar guide

Rewards commonly take the form of nectar, pollen, or both, but various plants produce oil,[1] resins,[2] scents,[3] or waxes.

These patterns are sometimes visible to humans; for instance, the Dalmatian toadflax (Linaria genistifolia) has yellow flowers with orange nectar guides.

Under ultraviolet, the flowers have a darker center, where the nectaries are located, and often specific patterns upon the petals as well.

This page on butterflies shows an animated comparison of black-eyed Susan (Rudbeckia hirta) flowers in visible and UV light.

[citation needed] The ultraviolet color, invisible to humans, has been referred to as bee violet, and mixtures of greenish (yellow) wavelengths (roughly 540 nm[6]) with ultraviolet are called bee purple by analogy with purple in human vision.