How to stand out to a mate - Flash colour of Photinus fireflies maximizes signal contrast against ambient light

During twilight, flying male Photinus fireflies flash in search for conspecific females, which sit on the vegetation below and flash back in response. However, before darkness sets in, the green vegetation in the habitat makes the detection of flashes more difficult. Twilight active fireflies tend to emit more yellow flashes than nocturnal fireflies and it has been proposed that firefly flash colour evolved to enhance the contrast of firefly flashes against their signalling background. There is indirect evidence for this contrast hypothesis, but it remains to be rigorously tested. This is the first study to directly test the contrast hypothesis by recording ambient light spectra in three different habitats throughout the daily flash activity of Photinus pyralis and P. australis and comparing these ambient light spectra to the male flash spectra of the two species. The contrast hypothesis predicts that during peak activity, firefly light colour will maximally contrast with the signalling background. Our data show that male flash spectra fit into a gap in the ambient light spectrum that opens during peak flashing activity of each species, effectively enhancing the visual contrast between male flashes and their background and making males more visible to their potential mates.