[97] Microspectrophotometric investigation of insect visual pigments

This chapter discusses the microspectrophotometric investigation of insect visual pigments. Microspectrophotometric measurements are used for the identification and characterization of visual pigments, with most of the work in this field relating to the rods and cones of the vertebrate retina. The absorbance spectra and dichroic properties of the light-absorbing structures can be determined by microspectrophotometric investigations. There are several other optical methods that measure light reactions in the eye and which enable one to determine the spectral sensitivity. For vertebrate visual pigments, the direct measurement of absorbance spectra as the absorbance difference between a pigmented structure and a pigment-free surrounding is possible in many cases. Visual pigments in insect photoreceptors exhibit dichroic properties—that is, the absorption probability for light polarized with the e-vector parallel to the long axis of the microvilli is higher than for light polarized perpendicular to that axis.