The evolution of floral mimicry: identifying traits that visually attract pollinators

Abstract

Summary Some orchids have been proposed to be Batesian floral mimics imitating flowers of sympatric rewarding species to attract pollinators. It is not yet well understood which traits are critical for pollinator attraction, although colour, shape and scent have all been implicated. We conducted field‐based behavioural experiments using pairwise combinations of plastic flowers differing in spectral and shape properties offered to long‐proboscid tabanid flies (Philoliche aethiopica) at two sites – one where the fly‐pollinated orchid mimic Disa pulchra occurs with its pink‐flowered model, the iris Watsonia lepida and another where the flies forage on a blue‐flowered rewarding plant Agapanthus campanulatus. Flies intensively visited and probed plastic flowers of colours indistinguishable in a fly vision model from those of the rewarding plants. Inflorescence architecture and brightness of plastic flowers made little difference to fly attraction, but those that matched the shape and nectar guides of Watsonia flowers were significantly more attractive. Flowers of the three focal plant species are weakly scented and divergent in scent chemistry. This study shows that traits that mimic, in order of importance, the spectra, shape and nectar guide patterns of flowers of rewarding plants would be under strong selection in food‐deceptive orchids as they maximize attractiveness to their pollinators. Our experiments also help to explain why deceptive orchids in general often seem to match the flowers of sympatric rewarding plants more in visual attributes than in scent chemistry.