Hunting ecology predicts eye arrangements in the modular visual system of spiders

Vision is one of the most important senses used by animals and contributes to fundamental behaviors, including foraging, navigation, and mate detection and selection.¹ Although much is known about how eye position and orientation correlate to ecology in the context of binocularity,² animals with multipartite visual systems (more than two eyes) remain comparatively neglected. Spiders are highly successful predators that occupy a range of ecological niches and usually possess eight eyes. Here, we use three-dimensional geometric morphometrics and evolutionary modeling to test whether eye positions, orientations, and interocular angles correlate with hunting strategies in 52 species across the spider phylogeny. We demonstrate that eye configurations diversified from an ancestral medial cluster, as seen in modern trapdoor spiders, to a halo-like configuration in orb-weavers, and to the frontal clustering of eyes in several members of derived spider lineages. We show that visual hunters have the highest disparity and evolutionary rates in the configuration of their eyes but display a distinct morphological signature with multiple eye pairs concentrated at the front of the carapace. Moreover, we quantify the extent to which eye configuration is modular and show that the position and orientation of the eye pairs evolve semi-independently of each other. Our findings demonstrate that modularity in the spider visual system facilitates not only the specialization of individual eyes but also the whole architecture of the visual system, in line with different hunting strategies, body plans, and ecological niches.