Vision through colored eyes.

Abstract

The conspicuous eye-color patterns of tabanid flies have been frequently portrayed. The eye colors originate from cornea color filters, which, on.the one hand, cause colorful reflections and, on the other, alter the spectral composition of transmitted light. In this study, spectral transmission measurements of single cornea lenses were performed in order to study the visual function of cornea color filters. Spectral transmission curves allow a quantitative description of the difference between input stimulus and utilized stimulus caused by the transmission properties of this stimulus-conducting system. The external appearance of dipteran compound eyes can be explained by different reasons: 1) The eye pigments, viewed through the dioptric apparatus, result in dull colors which are usually reddish or brownish in flies Ill. 2) Layered cornea lenses cause colorful metallic reflections or interference colors [2-4, 22] so that additional reflections from eye pigments may not be recognized for the most part. 3) The cuticle of facet intersections causes reflections of pigment colors of interference colors. In most Diptera, the color of the compound eyes is rather uniform and dull. However, metallic green and even metallic multicolored compound eyes have been reported for 72 species out of 23 families of Diptera [5, 6]. Phasecontrast light microscopy and transmission electron microscopy of the metallic eyes of tabanid flies demonstrated the presence of a multiple layering of alternating layers of high and low refractive index near the front surface of the cornea lens. The variation in color and intensity of the reflections depends on the angles of viewing and illumination. The measured reflection properties of single cornea lenses of intact eyes corresponded to calculated reflection properties, which were based on the assumption that the layer systems function as interference filters composed of quarterwavelength layers [2, 3, 7]. This study examines the spatial arrangement of the various colored facets in four species with very conspicuous eye-color patterns: three tabanid flies (Chrysops relictus Meigen, Haematopota pluvialis L., Heptatoma pellucens E; Tabanidae, Diptera) and one dolichopodid fly (Hercostomus germanus Wiedemann; Dolichopodidae, Diptera), and presents spectral transmission curves of single cornea lenses. The external appearance of the colored eye patterns was described by means of observations of living specimens with a Wild M3B stereomicroscope under various illuminations. The spectral transmission of single cornea lenses was recorded with a Bentham spectrometer system (monochromator M 300 EA, photomultiplier DH 3) which was combined with a UV-transmitting objective (Zeiss Ultrafluar 32 x). The measuring aperture (q) 0.12 mm) was adjusted to the facet center. Illumination was provided by a 75-W Zeiss Xenon lamp. The light beam was adjusted at right angles to the facet surface and to the objective surface. The diameter of the measuring spot amounted to 10 gin. Raw data were taken at 5-nm increments between limits of 265 and 800 nm. For the preparation of isolated corneas, living flies were decapitated. The cornea was prepared in fly Ringer solution (150raM NaC1, 10raM KC1, 2raM CaCl 2. Fixation may strongly influence the transmission properties of cornea lenses [8, 9]. A small retouching brush was used to clean the inner cornea surface. Several cuts were made into the cornea, before it was spread in fly Ringer solution between two cover slips which were mounted on the opening of a slide. Water evaporation was prevented by sealing with Vaseline. The measuring light transmitted the top cover slip, Ringer solution, the facet center in the physiological orthodrome direction, Ringer solution, and the bottom cover slip. Reference measurements were taken from the same preparation, each in close proximity to the original measurement, but without cornea facets in the light path. The spectral transmission was calculated from original and reference measurements. In order to reduce spectral artifacts, the light path was carefully adjusted at right angles to the very center of the facets. Only minor effects were found if this method was systematically varied in order to trace spectral artifacts. The spatial arrangement of colored cornea lenses differs greatly between the tested species. The spectral transmission of cornea lenses corresponding to dark brown facets in the three tabanid species is approx. 100% throughout the range of wavelengths tested, and has no distinct minimum. The human-visible color of the metallic, colored facets corresponds to the wavelength position of the transmission minimum, as if looking at the reflection and transmission of interference filters. The spectral transmission curves of the main types of cornea lenses in the species tested are shown in Fig. 1. Measuring very fresh preparations, the calculated values of spectral transmission often surpass the 100% level which is intrinsic to the focusing effect of the lenses.