Fly pollination of Gorteria diffusa (Asteraceae), and a possible mimetic function for dark spots on the capitulum.

We investigated the functional significance of raised black spots on the ray florets of Gorteria diffusa (Asteraceae) in South Africa. Field observations showed that G. diffusa is pollinated by a small bee-fly, Megapalpus nitidus (Bombyliidae), which is strikingly similar to the raised spots that occur on some of the ray florets. Removal of the spots resulted in a significant decrease in the rate of fly visits to capitula, but did not significantly affect seed set. Replacement of the spots with simple ink spots also significantly reduced the rate of pollinator visits, suggesting that flies respond to details in the structure of the spots. Investigations using scanning electron microscopy showed that the spots of G. diffusa consist of a complex of different cell types. Differences in epidermal sculpturing may partly explain the UV reflectance pattern of these spots, which is similar to that of the flies. Male flies are strongly attracted to the spots, as well as to other flies sitting in the capitula, although female flies also visit the capitula. We conclude that the spots of G. diffusa mimic resting flies, thereby eliciting mate-seeking and aggregation responses in fly pollinators. Similar dark spots have evolved in unrelated South African Gazania, Dimorphotheca, and Pelargonium species pollinated by bee-flies.