Food or mate ?

South Africa exhibits a high incidence of fly mimicry, with Gazania, Dimorphotheca and pelargonium, species displaying convergent evolution (Johnson and Midgley, 1997). In particular the African daisy Gorteria diffusa, while completely unrelated to other fly mimicking species, depends on the mimicry for pollination. The flowers of G. diffusa actively mimic female Megapalpus capensis flies through olfactory and visual signals. While M. capensis flies are not generally the only visitors to the daisy, they are the main visitor, and are sometimes the only successful pollinator throughout the flowering season (De Jager and Ellis, 2013). G. diffusa are unable to pollinate themselves, so rely exclusively on the success of pollinators.

G. diffusa displaying fly mimicry. Adapted from www.plantsci.cam.ac.uk

The black spots on the daisy are raised and shiny, exuding the scent of female M. capensis flies. Removal of these black spots results in a significant decrease in M. capensis visitations, as does the replacement of these spots with simple ink spots (Johnson and Midgley, 1997). Investigations of these spots using electron microscopy has shown that they consist of a wide variety of complex cell types, and specific epidermal sculpturing may explain the UV reflectance pattern which is remarkably similar to that of M. capensis. This leads to the conclusion that the flies are responding to the incredibly fine detail in the spots. The number of spots per capitulum is varied amongst the species, and can even be varied among the capitula of a single plant, but after analysis it was found that M. capensis does not discriminate between flowers with higher or lower number of spots (Johnson and Midgley, 1997). While male M. capensis are strongly attracted to the flowers because of the presence of the black spots, the females also visit the flowers. 

SEM photographs of G. diffusa ray floret. Adapted from Johnson and Midgley, 1997.

G. diffusa are commonly referred to as sexually deceptive flowers, this is incorrect because they also offer a pollen reward. It is believed that the species has retained its floral reward after evolving fly mimicry because the black spots are used to attract M. capensis to the flower, where it can then feed and pollinate. The pollen reward is hypothesised to be the primary reward with the black spots an added ‘incentive’, which has provided the daisy with an efficient pollination mechanism (Johnson and Midgley, 1997). An alternate hypothesis is that the flies associate the black spots with a floral reward, and that is why those flowers receive a higher incidence of visitation. But this is unlikely, mostly because even young flies are attracted to the spots and there are many instances where the male flies will land on the flower, attempt copulation with the spots, then fly away without feeding on the pollen (Johnson and Midgley, 1997).