Figure 3.

Retinal specialization in the ventral periphery of three semi-aquatic insect species. Investigation of retinal ultrastructure using electron microscopy has revealed three very informative examples for specializations in the ventral periphery of the insect retina, each providing unique adaptations to the life close to linearly polarized water surfaces. (A–E) The ventral retina of the water strider Gerris lacustris (A) is perfectly adapted for filtering out horizontally polarized surface glare. A morphologically distinct ventral region is clearly visible (B, marked in yellow). The cellular composition of ommatidia changes drastically at the boundary between dorsomedial and ventral retina: only in the ventral part, the proximal cell with two vertically oriented rhabdomeres spans the entire thickness of the retina (double headed red arrows in B), whereas an additional, distal cell with one horizontally oriented rhabdomere is found on top of the distal cell across the dorsomedial retinal field (double headed blue arrow in B). Electron microscopy sections through the distal part of the Gerris retina shown in (D), with a dorsal ommatidium on the left, and a ventral ommatidium on the right. Summary of cells and rhabdomere orientations at the interface of dorsal and ventral Gerris ommatidia shown in (E). (F–I) Zonation of the ventral retina in the back swimmer Notonecta glauca (F) Three distinct zones can be distinguished within the ventral retina of Notonecta, based on the rhabdomere orientations of the inner photoreceptors (named R7 and R8, according to Drosophila nomenclature), visualized by electron microscopy (G). Orientation of R7 vs. R8 rhabdomeres differ from parallel and horizontal (most dorsally), to different orthogonal configurations (H). The relative position of the three zones within the ventral retina and their inner photoreceptor rhabdomere orientations are shown in (I). (J–N) Alternating rows of ommatidial subtypes in long-legged flies (Dolichopodidae): alternating rows of shiny red and green colored facets in Dolichopus nitidus shown in (K). Analysis of retinal ultrastrure using electron microscopy revealed specific differences in R7 vs. R8 rhabdomere orientation between “Type A” ommatidia (red) and “Type B” ommatidia (green): parallel and vertically oriented (Type A) vs. orthogonal (Type B) shown in (L). This subtype-specific difference is achieved by alternating changes in R7 cell rhabdomere orientation. An electron microscopy section through a Type B ommatidium (left) and a Type A ommatidium (right) in (M). (A,F,J) Reproduced from Wikimedia under Creative Commons licenses; (K) reproduced from (Stavenga et al., 2017) under Creative Commons licenses; (D) reproduced with permission from (Schneider and Langer, 1969); (I) reproduced with permission from (Schwind, 1983b); (M) and (N) reproduced with permission from (Trujillo-Cenóz and Bernard, 1972).