Relationships between pupil working range and habitat luminance in flies and butterflies

Abstract

The luminance range over which the pupil mechanism operates was measured with pupil reflectometry in 11 species of butterflies and 13 species of dipteran flies. The different species were selected to be as different as possible regarding the range of ambient luminances in which they are active. Habitat luminance ranges were also measured and correlated to luminances in the experimental situation. The pupil mechanism in butterflies operates in the centre of the luminance range in which the different species are active. Three distinct groups of butterflies with pupil sensitivities matched to their specific types of activity pattern were identified: species active only in direct sunlight, species active also in shaded places and species extending their activity into dawn and dusk. Quite differently, the pupil mechanisms of dipteran flies operate in the upper end of the ambient luminances, and in some species well above the luminances normally encountered by the animal. All fly pupils start to close roughly at the same luminance, irrespective of the luminances in which the species are active. The results suggest that the most important role for the pupil mechanism in many of the butterfly species is to maximize acuity over a wide range of luminances, whereas in flies it is to avoid saturation of transduction units and thereby maximize the photoreceptor's signal-to-noise ratio at high light intensities.