Circadian and light-dependent control of the pupil mechanism in tipulid flies

Abstract

Infrared reflectometry of the deep pseudopupil was used for measuring the absolute sensitivity and the dynamics of the pupil mechanism in the acone apposition eye of tipulid flies. The following species were studied: Tipula luteipennis, T. fusca, T. pagana, T. paludosa and T. staegeri. Pupil responses plotted against light intensity produced S-shaped $\text{R}\text{log}\text{I}$ curves that were fitted by Lipetz equations with I₅₀-values (intensity corresponding to 50% of the maximal response) ranging between 10^17.4 and 10^20.2 photons m⁻² str⁻¹ s⁻¹. The sensitivity of the pupil is considerably higher (1–2 log units) during daytime than at night in all examined species. Differences in the sensitivity between different tipulid species are explained by the different light intensities in their natural environment. The dynamic range of the pupil mechanism is exceptionally large. In T. luteipennis, the pupil operates over a total range of about 8.5 log units of light intensity. This implies that the pupil mechanism of T. luteipennis is functional at light intensities ranging from bright sunlight down to moonlight. The speed of the pupillary movements are slower at night than during the day. During daytime, light adaptation is completed in about 30 min and dark adaptation in about 50 min. At night, the corresponding values are 45 and 60 min. The differences in speed and sensitivity of the pupil mechanism between day and night are due to a circadian rhythm, setting the control range of the pupil aperture.