Twenty-eight retinas but only twelve eyes: an anatomical analysis of the larval visual system of the diving beetle Thermonectus marmoratus (Coleoptera: Dytiscidae).

The larvae of the sunburst diving beetle, Thermonectus marmoratus (Coleoptera: Dytiscidae), are highly efficient visually guided predators. Their visual system consists of a cluster of six stemmata and one eye patch on each side of the head capsule. Histological investigations show that the organization of individual stemmata differs strongly from any eye that has previously been described. Based on general morphology, ultrastructural data, and the presence of actin-rich areas that are typical for rhabdoms, we find that each eye is characterized by several retinas. The most dorsal eye on each side is relatively long and tubular, and we have identified three spatially distinct areas with retinula cells: 1) a band of two rows of rhabdoms along the medial side of the eye tube; 2) a flattened cone-shaped region towards the bottom of the tube that is formed by the rhabdoms of retinula cells that are oriented perpendicular to the light path; and 3) two horizontal rows of long rhabdoms parallel to the light path at the base of the tube. A second large eye is organized similarly but lacks the medial band. The remaining four eyes are nearly spherical and each has two distinct retinas. The 12 eyes hence account for a total of 26 retinas, and two further retinas are present in eye patches lacking lenses. Our anatomical findings suggest that this is an example of a visual system in which specific visual tasks are distributed among the eyes, and which relies on a variety of highly specialized retinas.