Interference filters in the corneas of Diptera.

Many Dipteran compound eyes, especially the Tabanidae, show colored reflection patterns ivhen illuminated with white light. For instance, the female horsefly Hybomitra lasiophthalma compound eye, when viewed from the same direction as the illumination, displays a stripsd pattern consisting of 5 dark stripes, predominantly bluish, and 4 bright stripes, predominantly orangish. The pattern consists of reflections originating from a small locally flat part of each corneal facet surface and from facet intersections. The colored reflections were found to be a constant feature of living eyes, fixed eyes, and of isolated corneas. Patterns of other colored Dipteran eyes, rather than having a striped appearance, may be uniform in appearance, or be subdivided into irregular areas, or even may consist of alternating rows of two facet types, of different colors. We have studied the structure of such Dipteran corneal facets with optical and electron microscopes. This study reveals a specialized system of alternating dense and rare layers located fust beneath the front corneal surface. Variation in color and intensity of reflections over such eyes, for fixed viewing and illumination angles, was found to be caused by variations in parameters of the specialized layer system. A theoretical model was used to gain insight into the behavior of the layer system. Parameters of the layer system tuere related to reflection properties. The calculations show these layer systems to be interference filters composed of quarter-wavelength layers. The behavior of reflection properties as a function of incident angle and wavelength is supported by the calculations. We feel that the corneal interference filters probably play the same role in compound eyes that colored oil droplets (absorption filters) play in some vertebrate eyes, that is, to perform a contrast filtering function for vision.