Response of bee-flies to the shape and pattern of model flowers: implications for floral evolution in a Mediterranean herb

1. The functional significance of floral traits in Linum pubescens (Linaceae), a Mediterranean annual that is pollinated almost exclusively by the bee-fly Usia bicolor (Bombyliidae), was investigated. The flies feed on both pollen and nectar, and use the flowers as mating rendezvous sites in the afternoon.
2. Choice experiments with model flowers were used to determine the response of U. bicolor to visual cues, such as shape, size, colour and pattern.
3. Bee-flies strongly preferred models with a dissected outline over models with a simple outline. They also preferred pink models over other colours, and larger models over smaller models.
5. Flies landing on models with converging lines ('nectar guides') tended to follow the lines to the point where they meet in the centre of the model, while flies landing on plain models showed undirected behaviour, often moving to the edge of the model.
6. Flies were strongly attracted to flowers of L. pubescens which had a fly glued on to one of the petals, as well as flowers with a dark spot painted onto one of the petals. In addition, models with a dark spot were strongly preferred over plain models during the afternoon when flies exhibit mating behaviour. This evidence suggests that the dark centre of the L. pubescens flower may function as an attractant to mate-seeking bee-flies.