Nectar guide

Nectar guides are markings or patterns seen in flowers of some angiosperm species, that guide pollinators to their rewards. Rewards commonly take the form of nectar, pollen, or both, but various plants produce oil,[1] resins,[2] scents,[3] or waxes. Such patterns also are known as "pollen guides" and "honey guides", though some authorities argue for the abandonment of such terms in favour of floral guides (see for example Dinkel & Lunau[4]). Pollinator visitation can select for various floral traits, including nectar guides through a process called pollinator-mediated selection.

Two images comparing the appearance of a Mimulus flower in visible and ultraviolet light
Images of a Mimulus flower in visible light (left) and ultraviolet light (right) showing a dark nectar guide that is visible to bees but not to humans

These patterns are sometimes visible to humans; for instance, the Dalmatian toadflax (Linaria genistifolia) has yellow flowers with orange nectar guides.[5] However, in some plants, such as sunflowers, they are visible only when viewed in ultraviolet light. Under ultraviolet, the flowers have a darker center, where the nectaries are located, and often specific patterns upon the petals as well. This is believed to make the flowers more attractive to pollinators such as honey bees and other insects that can see ultraviolet. This page on butterflies shows an animated comparison of black-eyed Susan (Rudbeckia hirta) flowers in visible and UV light.

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.[7]

References
  1. Buchmann, SL. (1987). "The ecology of oil flowers and their bees". Annual Review of Ecology and Systematics. 18 (1): 343–70. doi:10.1146/annurev.es.18.110187.002015.
  2. Reis Mariza G.; de Faria, Aparecida D.; Bittrich, Volker; do Carmo, Maria; Amaral E.; Marsaioli, Anita J. : The Chemistry of Flower Rewards : J. Braz. Chem. Soc., Vol. 11, No. 6, 600-608, 2000.
  3. Teichert, Holger; Pollination biology of cantharophilous and melittophilous Annonaceae and Cyclanthaceae in French Guiana; Doctoral dissertation at University of Ulm, 2008
  4. Dinkel T., Lunau K.: How drone flies (Eristalis tenax L., Syrphidae, Diptera) use floral guides to locate food sources. Journal of Insect Physiology Volume 47, Issue 10, September 2001, Pages 1111-1118
  5. "Dalmatian Toadflax (Internet Archive)" (PDF). Archived from the original (PDF) on 2007-03-17.
  6. Briscoe, Adriana D.; Chittka, Lars. The Evolution of Color Vision in Insects. Annu. Rev. Entomol. 2001. 46:471–510
  7. Charles D. Michener (1974). The Social Behavior of the Bees: A Comparative Study. Harvard University Press. ISBN 0-674-81175-5.
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