Differences in the physiological properties of the median and the lateral eyes and their possible meaning for the entrainment of the scorpion's circadian rhythm

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Abstract
Circadian adaptation phenomena were investigated in the visual system of the scorpion Androctonus australis L. by ERG[electroretinogram]-recordings with implanted electrodes during long-time experiments. In constant darkness, the median eyes change their sensitivity endogenously, under the control of a circadian oscillator. The period of this rhythm is composed of a subjective day state, and of a subjective night state having a sensitivity level .apprx. 3 log units higher than that of the day state. In the lateral eyes, circadian sensitivity oscillations range over only .apprx. 0.3 log unit. Compared with the synchronous oscillations in the median eyes, these changes in sensitivity are negligible. During the sensitive night state, the lateral eyes are .apprx. 1 log unit more sensitive than the median eyes. The time courses of the changes in sensitivity are almost symmetrical during the transition phases from day state to night state and from night state to day state. The slope of this transition differs in different animals and may last 2-3 h. In a wide but not complete range, the time course of the transition period, e.g., from day state to night state, is sufficient to compensate for the diminishing light during dusk. The possible influence of the circadian adaptation of the median eyes, over the information processing of Zeitgeber stimuli, are calculated from experimental data. The consequences of these results and of the physiological differences between the lateral and the median eyes are discussed with respect to their significance in the entrainment of the scorpion''s circadian rhythm.

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