Pigment migration and spectral sensitivity in the compound eye of moths

Abstract

The spectral sensitivity of dark adapted eyes of the moth Manduca sexta (Lepidoptera: Sphingidae) was measured between 350 and 650 nm by determining the relative number of photons necessary to elicit a retinal potential of 50 μV. The spectral sensitivity was determined in eyes with the screening pigment: a) in the extreme distal position, b) in a proximal position, and c) with the pigment removed. Sensitivity maxima were found at about 350 to 370, 450 and 530 nm, irrespective of pigment position. Pigment movement is associated with a change in light attenuation of 2 to 3 log units at all wavelengths between 350 and 650 nm. Only small variations, about 0.6 log units, in screening effect were observed throughout the measured spectrum, and therefore the effect of pigment movement on spectral sensitivity is small. There is a slightly larger decline in sensitivity of eyes with the pigment removed compared to eyes with the pigment in the distal position at wavelengths above 530 nm. This result suggests that the distally located pigments have a slight screening effect.