Corneal Mapping and the Ecological Role of Vision in Toe Biters

Belostomatids, commonly known as toe biters, are ambush predatory insects that reside in stagnant waters across the globe. Toe biters go through several nymphal stages of life and gradually increase in body size until reaching a sexually mature adult stage. Nymphs and adults are both predatory and employ similar ambush predation strategies to secure prey, but sexual selection and mating behaviors are only performed by fully matured adults. Additionally, these insects present an uncommon case of reverse sex roles, in which females seek out and compete for male mates, while males are the sole contributors of parental investment in care and protection of the fertilized eggs. Many insects with traditional sex rolespresent males with a specific region of increased visual acuity for identifying and pursuing potential female mates. This ledus to question ifthe reverse sex roles of toe biters gives rise to differences in the visual systems, and thus visual ecologies, of adult males and females. We hypothesized that vision plays a primary role in mate selection and, therefore, that there is variation in visual acuity morphology between adult and nymph eyes. Considering this main hypothesis that vision and mate selection are linked, we predicted that female toe biters possess eyes with regions ofgreater visual acuity than males, which would be evident if females possess larger eyes, a greater number of corneal facets in each eye, and/or larger diameters of the individual corneal facets whencompared to male eyes. To test our hypothesis, we performed corneal dissections from toe biters in each of these three groups (adult males, adult females, and last-stage nymphs) and compiled a dataset of transmission microscopy images of each cornea. With these data, we were able to count the number of ommatidia in each eye, measure the surface area of each cornea, as well as measure the diameter of individual facets within each cornea. We found no significant differences in ommatidial counts or individual facet diameters between nymphs and adults, indicating that visual acuity is very similar between these two groups. Similarly, we found no significant differences in ommatidial counts, total eye area, or individual facet diameters between females and males. These findings led us to reject our hypothesis that vision plays a major role in the sexually dimorphic mating behaviors of adult toe biters. Rather, these findings suggest that vision is more likely involved in the similar ambush predatory behaviors performed by all three groups -- nymphs, adult males, and adult females -- and provide a foundation for future studies of the visual function and evolution of Belostoma flumineum.