The bright zone, a specialized dorsal eye region in the male blowfly Chrysomyia megacephala

1. In the compound eye of the male Chrysomyia megacephala the facets in the ventral part of the eye are only ca. 20 µm in diameter, but increase abruptly to ca. 80 µm above the equator of the eye. Correspondingly there is a large and abrupt increase in the rhabdomere diameter from 2 to as much as 5 µm. The far-field radiation pattern of the eye shows that, despite the large change in ommatidial dimensions, the resolution of the eye remains approximately constant across the equator: angular sensitivity of the photoreceptors and sampling raster are similar ventrally and dorsally. The main result of the large dorsal facets is a more than tenfold increase in light capture. Thus this eye provides a clear example of an insect where large dorsal facets have evolved not for higher acuity, but rather for higher light capture. 2. Sensitivity is increased even more by a seventh photoreceptor cell joining neural superposition, as reported before for the dorsal eye of male houseflies. All seven photoreceptors have the same spectral sensitivity. 3. Angular sensitivities in the dorsal eye are more Gaussian-shaped than the flat-topped profile expected for large rhabdomere diameters. This is explained by the anatomical finding that the dorsal rhabdomeres taper strongly. It is suggested that the combination of high photon capture and rounded angular sensitivities is advantageous for monitoring movement and position of small objects. 4. Finally some of the constraints involved in constructing specialized dorsal eye regions for detection of small objects are considered.