The identification of spectral receptor types in the retina and lamina of the dragonflySympetrum rubicundulum

Abstract

Photoreceptors and monopolar cells in the ventral eye ofSympetrum rubicundulum have been recorded from intracellularly and stained with Lucifer yellow. Units with four types of spectral sensitivity were found having λmax at 340, 410, 490–540 and 620 nm. On the basis of a significant difference in half bandwidth ofS(λ), the green receptors are separable into two subgroups with λmax at 490 and 540 nm. The fluorescence marking reveals that R5/8 and R2/3 are the green receptors; R1/4 is either a UV or an orange cell. Discrimination between the members of the three matched pairs R2 & 3, R5 & 8, and R1 & 4 has not been possible. R7 is the violet receptor, and R6 is probably an additional green receptor; these are the receptors with long visual fibres. No receptors in the ventral eye besides the orange (620 nm) are sensitive to polarized light, whereas UV receptors in the dorsal eye are highly sensitive to polarized light. The polarized light sensitivity of the orange receptors is interpreted as an adaptation to increase the contrast between a conspecific animal and shorter wavelength light with a predominantly horizontal E-vector, such as is provided by reflections on the water's surface.