CIRCADIAN AND LIGHT-DEPENDENT CONTROL OF THE PUPIL MECHANISM IN TIPULID FLIES

Infrared reflectometry of the deep pseudopupil was used for measuring the absolute sensitivity and the dynamics of the pupil mechanism in th e acone apposition eye of tipulid flies. The following species were st udied: Tipula luteipennis, T. fusca, T. pagana, T. paludosa and T. sta egeri. Pupil responses plotted against light intensity produced S-shap ed R/log I curves that were fitted by Lipetz equations with I-50-value s (intensity corresponding to 50% of the maximal response) ranging bet ween 10(17.4) and 10(20.2) photons m(-2) str(-1) s(-1). The sensitivit y of the pupil is considerably higher (1-2 log units) during daytime t han at night in all examined species. Differences in the sensitivity b etween different tipulid species are explained by the different Light intensities in their natural environment. The dynamic range of the pup il mechanism is exceptionally large. In T. luteipennis, the pupil oper ates over a total range of about 8.5 log units of light intensity. Thi s 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 d ay. During daytime, light adaptation is completed in about 30 min and dark adaptation in about 50 min. At night, the corresponding values ar e 45 and 60 min. The differences in speed and sensitivity of the pupil mechanism between day and night are due to a circadian rhythm, settin g the control range of the pupil aperture.