Charting Drosophila’s non-ideal neural superposition eyes

More than half a century ago, Buchner (1971) evaluated the visual axes of the ommatidia in a Drosophila ’s neural superposition eyes, using the pseudopupil method. The spatial distribution of the visual axes was reported in a stereographic projection chart. Analysis of this chart shows that the ommatidial lattice is well described by a horizontal-vertical (H-V)-coordinate system, which allows assessment of the partial interommatidial angles between the vertical columns and horizontal rows of ommatidia, Δ φ h and Δ φ v , respectively. Their angular ratio ρ a = Δ φ h /Δ φ v is expected in an ideal neural superposition eye to be √3, but this appears to hold only in a central band of the eye. Dorsally and ventrally ρ a is much smaller, while posteriorly it is much larger. The quite different gradients of Δ φ h and Δ φ v over the eye cause distinct regional deviations from ideal neural superposition. Instead of the interommatidial angle Δ φ , considered to be the central factor determining the eyes’ spatial resolving power, the local values of Δ φ h and Δ φ v will presumably be the important determinants for the spatial resolving power, at least for the vertical and horizontal motion-sensitive neurons in the fly brain. Various ways to chart the visual space of a compound eye are demonstrated and discussed.