Spectral characteristics and regionalization of the eyes of Diptera, especially Tabanidae

The physical origin of the color patterns in the eyes of Tabanidae is well understood: a stack of layers in the facet lenses acts as an interference reflectance filter (Bernard & Miller 1968). This causes a reduced transmittance in specific wavelength bands. Because a facet lens focuses light into the underlying rhabdomeres, where the visual pigment molecules can absorb the incident light, the spectrally affected transmittance presumably causes a modified spectral sensitivity of the photoreceptors. We have investigated this conjecture by measuring reflectance spectra from the golden-greenish shining facets of the blinding breeze fly Chrysops relictus. By combining the associated transmittance spectrum with various visual pigment spectra, we conclude that noticeable shifts in photoreceptor sensitivity spectra can only occur with visual pigments that have absorption spectra peaking around 540 nm, i.e. in the green, in accordance with ERG measurements of the related horsefly Haematopota pluvialis (Kirschfeld 1986).