Pollination of the carrion flowers of an African stapeliad (Ceropegia mixta: Apocynaceae): the importance of visual and scent traits for the attraction of flies

Flowers that mimic carrion or faeces exhibit unusual traits, the evolution and functional significance of which remain poorly understood. Odour is an important pollinator attractant, but visual traits and interactions between visual and scent traits have seldom been considered. We studied pollination of the carrion flowers of Ceropegia mixta [= Orbea variegata], analysed floral traits and used manipulative experiments to explore the contributions of visual and scent traits to pollinator attraction. Flowers were pollinated primarily by Musca domestica (Muscidae), with lesser contributions by Calliphoridae and Sarcophagidae flies. The floral odour (analysed using gas chromatography-mass spectrometry) was dominated by oligosulphides and phenol. Comparison of floral and abiotic background colours (analysed using reflectance spectrometry) using a fly colour vision model suggested that flowers would be chromatically indistinguishable from the background. Comparison of fly arrival rates at concealed (but still scented) versus exposed flowers showed that flies can locate flowers without visual cues, but visitation was higher when the flowers were visible. Experiments using model flowers with odour supplied by real flowers (to explore the significance of dark flowers and dark spots on a pale background, which both occur frequently in flowers that mimic carrion or faeces) showed that scented black flowers attracted significantly more flies than similarly scented human-yellow flowers, while the presence or size of black spots on the corolla had no effect on the attraction of flies. Our results suggest that there is a visual component to fly attraction, but some traits, such as the mottled patterning, may not have evolved to enhance pollinator attraction.