PUPIL ADJUSTMENTS IN THE EYE OF THE COMMON BACKSWIMMER

The pupil mechanism in the acone apposition eye of the semi-aquatic co mmon backswimmer Notonecta glauca (Hemiptera) was investigated with in frared reflectometry of the pseudopupil. This method allows non-invasi ve continuous measurements of pupil responses in the living animal. Th e dynamic range of the pupil sensitivity is about 7 log units during d aytime and 6 log units at night. During the day, the sensitivity range of the pupil covers the normal daylight intensities in the animal's h abitat, just under the water surface (I-50=10(19.2) photons m(-2) sr(- 1) s(-1)). At night, the sensitivity is 1 log unit lower (I-50=10(20.2 ) photons m(-2) sr(-1) s(-1)), ensuring that the pupil is maximally op en when light intensities are low. During daytime, light adaptation is completed in slightly less than 40 min, and dark adaptation takes app roximately 50 min. The pupil response is only slightly slower at night , The speed of the response as well as the pupil sensitivity are depen dent on the preceding adaptation history, An endogenous circadian rhyt hm determines the control range of the pupil aperture. However, the rh ythm is easily disturbed, especially within a 3h period before dusk an d dawn. The results are compared with corresponding results from other insects with the same type of pupil mechanism.