Wavelength discrimination in the hummingbird hawkmoth Macroglossum stellatarum.

Abstract:
Despite the strong relationship between insect vision and the spectral properties of flowers, the visual system has been studied in detail only in few insect pollinator species. For instance, wavelength discrimination thresholds have been determined in two species only: the honeybee (Apis mellifera) and the butterfly Papilio xuthus. Here, we present the wavelength discrimination thresholds (Δλ) for the hawkmoth Macroglossum stellatarum. We compare the data with those found for the honeybee, the butterfly P. xuthus and the predictions of a colour discrimination model. After training moths to feed from a rewarded disk illuminated with a monochromatic light, we tested them in a dual choice situation, in which they had to choose between the light of the training wavelength and a novel unrewarded wavelength. To characterise the Δλ function, we decreased the difference between wavelengths in subsequent tests. We also varied the light intensity to test its effect on the discrimination capacity. In agreement with the predictions of the model, we found two expected minima of discrimination where photoreceptor sensitivities overlap, as well as a minor third unpredicted minimum around the peak of the blue photoreceptor. M. stellatarum is capable to discriminate lights with a wavelength difference of 1 to 2 nm. These discrimination minima are similar to those found for the tetrachromatic P. xuthus, and are better than those of the honeybee. The moth is also capable to use achromatic information to discriminate between lights of long wavelengths.