The Diascia (Scrophulariaceae) window: an orientation cue for oil-collecting bees

The Diascia (Scrophulariaceae) window: an orientation cue for oil-collecting bees. Flowers of Diascia section Racemosae are characterized by an unusual translucent ‘window’ at the base of the upper corolla lip which is bright yellow with maroon flecks and varies from a nearly flat to a deeply cone-shaped depression. The inner epidermal cells of the window are also variable in shape ranging from lenticular to long papillate. In contrast to the surrounding corolla which strongly reflects ultraviolet light, the window tissue is strongly ultraviolet absorptive due to the presence of ultraviolet absorbing flavonoids in the epidermal cells. Observations in natural and experimental situations suggest that the window is used as an orientation cue by oil-collecting bees.