Diurnal changes in photoreceptor sensitivity in a reflecting superposition eye

Summary

• 1. Intracellular recordings of intensity response functions (V/log I curves) were made at different adaptation states at different times during a 24th period, from single photoreceptors in the central region of the eye of the Australian crayfishCherax destructor (Clark 1936).

• 2. The intensity that elicits a criterion response (50% of the maximum) is less at night than in the day in both the dark-adapted (DA) and lightadapted (LA) states.

• 3. Within the first 30 s of light-adaptation there is an extremely fast initial decrease in sensitivity, while in the latter stages of dark-adaptation there is a slow but steady increase, neither of which can be correlated with the position of the screening pigments.

• 4. The time courses for the movement of screening-pigment on lightor dark-adaptation during both day and night were measured using light microscopy. As found by de Bruin and Crisp (1957) in several species of prawn and Frixione et al. (1979) in the crayfishProcambarus, the proximal pigment migrates faster than the distal pigment on light-adaptation. On dark-adaptation, the distal pigment migrates faster than the proximal pigment, the latter only carrying out the first stage of dark-adaptation during the day.

• 5. It is concluded that there are three variable systems affecting sensitivity in a reflecting superposition eye: (a) The effective rhabdom cross-sectional area as determined by the position of proximal pigment; (b) the blur circle diameter and intensity determined by the position of the distal pigments and the width of the clear zone, and (c) transduction gain.