A sexual dimorphism in the spatial vision of band-winged grasshoppers

Visual acuity (VA) --- a measurement of the fineness or coarseness of vision --- correlates with the size of an animal, with larger species often possessing sharper vision. However, it is unknown whether the same relationship between visual acuity and size holds within a species when individuals differ consistently and substantially in size, such as through a sexual size dimorphism. Here we examine the visual acuity of three species of sexually dimorphic band-winged grasshoppers, in which females are the larger sex (Arphia pseudonietana, Dissosteira carolina, and Spharagemon equale; total n = 98). Using a radius of curvature estimation method, we find that females have [~]21% finer vision in the most acute region and axis of the eye than do males. Further explorations of the eyes of the species showing the greatest size dimorphism (D. carolina) suggest that this VA dimorphism is driven by females having larger eyes with more ommatidia. In contrast to many flying insects where males have finer vision to acquire mates, our study is one of the first to demonstrate a female-biased sexual dimorphism in acuity. Given the number of species in which females are larger than males, our results suggest that differences in VA between the sexes may be more common than currently appreciated.