Spectral sensitivity of screening pigment migration in the compound eye ofManduca sexta

Summary

• 1. The pupillary response in the compound eye ofManduca sexta results from pigment migration in pigment cells located distal to the retina. We used reflectance photometry to measure the spectral sensitivity of the pupillary response (Figs. 1–4), and compared pupillary action spectra with the ERG spectral sensitivity function and the absorption spectra of retinal visual pigments (Figs. 3–5). Our study was provoked by the report of Hamdorf and Höglund (1981) that the pupillary response of a related moth is maximal in the violet and ultraviolet, but is insensitive to wavelengths greater than 530 nm. Since, in sharp contrast, retinal sensitivity peaks in the green, and drops off at shorter wavelengths, they concluded that the pupillary response is not coupled to retinal excitation, but may result from intrinsic photosensitivity of the pigment cells themselves. Our aim was to test this conclusion by more accurately measuring the spectral response of the pigment cells.

• 2. The spectral sensitivity of the pupillary response from 460–690 nm matches the ERG action spectrum (Fig. 5), and both correspond to the absorption spectrum of the long wavelength photopigment (P520, λ max = 520 nm) present in the retina (Fig. 3).

• 3. There is a second peak in pupillary action spectra between 410 and 450 nm that varies from eye to eye in its height relative to the maximum at 520 nm (Fig. 4). The short wavelength peak corresponds to the violet receptors of the retina, whose existence has been inferred from selective adaptation experiments and from the presence of a violet sensitive photopigment (P440, λmax ≅440 nm). A violet peak is not prominent in the dark-adapted ERG spectral sensitivity function (Fig. 5). The variable violet peak in pupillary action spectra may reflect variation in relative input to the pigment cells from the green and violet receptors.

• 4. We conclude that the pupillary response is probably driven by way of the retinal receptors, rather than depending on a photosensitive system within the pigment cells themselves. As in other insects, the pupillary response inManduca provides the possibility for non-invasively measuring visual function.