Abstract
Nectar is the most common floral reward for flower-visiting flies, bees, bats and birds. Many flowers hide nectar in the floral tube and preclude sensing of nectar by flower-visitors from a distance. Even in those flowers that offer easily accessible nectar, the nectaries are mostly inconspicuous to the human eye and the amount of nectar is sparse. It is widely accepted that many flowers display nectar guides in order to direct flower-visitors towards the nectar. Using false colour photography, covering ultraviolet, blue and green ranges of wavelength, revealed a yet unknown conspicuousness of nectar, nectaries and false nectaries for bees due to concordant reflection in the ultraviolet range of wavelength. Nectars, many nectaries and false nectaries have glossy surfaces and reflect all incident light including UV-light. In most cases, this is not particularly conspicuous to the human eye, but highly visible for UV-sensitive insects, due to the fact that the glossy areas are often positioned in UV-absorbing central flower parts and thus produce a strong UV-signal. The optical contrast produced by the glossiness of small smooth areas in close proximity to nectar holders represents a widespread yet overlooked floral cue that nectarivorous flower-visitors might use to locate the floral nectar.
Highlights
Nectar is the most common floral reward for flower-visiting flies, bees, bats and birds
We found that flowers and inflorescences of many plant species had glossy areas that exhibited a striking visual inter-species similarity, with glossy parts in close proximity to the floral center (Figs. 1–3)
The colour, false colour, and UV-photos of the non-homologous glossy areas of flowers document a common property, which is based on the total reflection of light, in a UV-absorbing direct surrounding causing highest conspicuousness in the UV, and visibility to bees (Figs. 1–3)
Summary
Nectar is the most common floral reward for flower-visiting flies, bees, bats and birds. Many flowers hide nectar in the floral tube and preclude sensing of nectar by flower-visitors from a distance. Even in those flowers that offer accessible nectar, the nectaries are mostly inconspicuous to the human eye and the amount of nectar is sparse. Many nectaries and false nectaries have glossy surfaces and reflect all incident light including UV-light In most cases, this is not conspicuous to the human eye, but highly visible for UV-sensitive insects, due to the fact that the glossy areas are often positioned in UV-absorbing central flower parts and produce a strong UV-signal. Despite the limited experimental evidence for visually detectable nectar and nectaries, some researchers take it for granted that nectar is glossy and nectar-seeking flower-visitors might detect nectar by means of this cue[24,30]
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