Fine structure and spectral sensitivities of retinular cells in the dorsal sector of compound eyes in the dragonfly Aeschna

Abstract

1. Retinular fine structure observed with the electron microscope and receptor potentials of single retinular cells responding to equal quantum narrow band monochromatic stimuli between 327 and 615 nm have been studied in the dorsal sector of the compound eye of Aeschna cyanea. 2. Each retinnla comprises 8 cells: 5 distal retinular cells, 2 proximal retinular cells and 1 small cell without a rhabdomere. Nevertheless, the small cell sends an axon to the lamina. In terms of microvillus directions seen in cross section the distal rhabdom has three parts with an angular difference between them of 120°. The proximal rhabdom lacks one of these three parts and is made up of two, again separated in microvillus direction by 120° (240°). 3. Basically the dorsal area of the eye has two types of receptor cells: a UV type (λmax 356 nm) and a green type (λmax 475–519 nm with a secondary peak at 356 nm). However, other response types, most likely derived from the green cells, were fairly frequently recorded: blue type, double type (green plus blue) and shifting type (alternately green and blue). 4. Close control of stimulus direction shows that green cells giving a single peak at 475–519 nm to axial or near axial light rays, develop a second peak at 458 nm when the stimulus direction deviates more strongly from axial. 5. Comparison of structural and electrophysiological evidence suggests that the distal retinular cells are the green receptors, the proximal units the UV receptors but direct evidence is needed.