Image quality and acceptance angle in a reflecting superposition eye

Summary

• 1. Intracellular recordings of acceptance angles were made at different adaptation states and at different times of day, from retinula cells in the central region of the reflecting superposition eye of the freshwater crayfish,Cherax destructor (Clark 1936). A number of cells were held for longer than 24 h.

• 2. The acceptance angle is greater in the dark-adapted (DA) state than in the light-adapted (LA) state, but this difference is significantly smaller at night than during the day. Mean values were 5 ° (LA day and night), 19 ° (DA day) and 13 ° (DA night).

• 3. The anatomy was studied with proper attention to the time of day. Light microscopy shows that there is an increase of 10–15% in the clear-zone width in the DA state at night compared to the other states. This is achieved by a proximal movement of the rhabdom layer.

• 4. The position of the distal screening pigment depends on the ambient light level. It is around the distal cones in the dark and moves into the clear zone on light-adaptation. The proximal screening pigment, however, attains only the first stage of dark-adaptation (Frixione et al. 1979) during the day. Complete dark-adaptation (movement below the basement membrane) occurs only at night. The distal reflecting pigment remains around the cones at all times, and the proximal reflecting pigment forms a cup around the base of the rhabdom during both day and night.

• 5. The smaller acceptance angle in the DA state at night, compared to that of the DA day state, indicates that the receptors lie closer to the best superposition focus in this state, when the animal is in air.

• 6. The effect of the change in clear-zone width and the performance of the eye in air and under-water are examined using a model of the eye.