Tuning of photoreceptor spectral sensitivity in fireflies (Coleoptera : Lampyridae)

Sexual communication between male and female fireflies involves the visual detection of species-specific bioluminescent signals. Firefly species vary spectrally in both their emitted light and in the sensitivity of the eye, d epending on the time when each is active. Tuning of spectral sensitivity in three firefly species that occupy different photic niches was investigated using light and electron microscopy, microspectrophotometry, and intracell ular recording to characterize the location and spectral absorption of the screening pigments that filter incoming light, the visual pigments that rec eive this filtered light, and the visual spectral sensitivity. Twilight-act ive species had similar pink screening pigments, but the visual pigment of Photinus pyralis peaked near 545 nm, while that of P. scintillans had a lam bda(max) near 557 nm. The night-active Photuris versicolor had a yellow scr eening pigment that was uniquely localized, while its visual pigment was si milar to that of P. pyralis. These results show that both screening and vis ual pigments vary among species. Modeling of spectral tuning indicates that the combination of screening and visual pigments found in the retina of ea ch species provides the best possible match of sensitivity to bioluminescen t emission. This combination also produced model sensitivity spectra that c losely resemble sensitivities measured either with electroretinographic or intracellular techniques. Vision in both species of Photinus appears to be evolutionarily tuned for maximum discrimination of conspecific signals from spectrally broader backgrounds. Ph. versicolor, on the other hand, appears to have a visual system that offers a compromise between maximum sensitivi ty to, and maximum discrimination of, their signals.