Habitat preference in two European fireflies is reflected in their colour vision

Colour vision has so far been demonstrated in only a single firefly species, the European Lampyris noctiluca. Lamprohiza splendidula is another widespread European species that often co-occurs with L. noctiluca. First, in field surveys, we quantitatively demonstrated that the former species prefers open habitats, while the latter shows a stronger preference for densely vegetated areas. Second, by measuring compound eye spectral sensitivity with electroretinography and assuming dichromatic colour vision, we used a basic psychophysical colour space model to quantify the physiological experience sensed by L. splendidula and L. noctiluca males viewing the females' bioluminescent signal with eyes adapted to spectral conditions of a densely vegetated and open areas illuminated by the late twilight sky, when the activity of these fireflies is maximal. Dark-adapted eyes of both species were mostly sensitive to the yellowish green spectral region (559-568 nm), well matching the bioluminescence's peak wavelength of the females (556 nm). Recordings on green-adapted eye preparations revealed the presence of a second, blue-sensitive photoreceptor (423-456 nm). We also found that the spectral separation between the blue- and green-sensitive receptors was greater in L. splendidula than in L. noctiluca. According to our colour vision modeling, differences in the spectral sensitivities of the two species' visual systems may help maintain colour vision performance and facilitate female detection in their respective preferred habitats.