Selection drives divergence of eye morphology in sympatric Heliconius butterflies

When populations experience different sensory conditions, natural selection may favor sensory system divergence, affecting peripheral structures and/or downstream neural pathways. We characterized the outer eye morphology of sympatric Heliconius butterflies from different forest types and their first-generation reciprocal hybrids to test for adaptive visual system divergence and hybrid disruption. In Panama, Heliconius cydno occurs in closed forests, whereas Heliconius melpomene resides at the forest edge. Among wild individuals, H. cydno has larger eyes than H. melpomene, and there are heritable, habitat-associated differences in the visual brain structures that exceed neutral divergence expectations. Notably, hybrids have intermediate neural phenotypes, suggesting disruption. To test for similar effects in the visual periphery, we reared both species and their hybrids in common garden conditions. We confirm that H. cydno has larger eyes and provide new evidence that this is driven by selection. Hybrid eye morphology is more H. melpomene-like despite body size being intermediate, contrasting with neural trait intermediacy. Overall, our results suggest that eye morphology differences between H. cydno and H. melpomene are adaptive and that hybrids may suffer fitness costs due to a mismatch between the peripheral visual structures and previously described neural traits that could affect visual performance.

Allometry and ecology shape eye size evolution in spiders

Eye size affects many aspects of visual function, but eyes are costly to grow and maintain. The allometry of eyes can provide insight into this trade-off, but this has mainly been explored in species that have two eyes of equal size. By contrast, animals possessing larger visual systems can exhibit variable eye sizes within individuals. Spiders have up to four pairs of eyes whose sizes vary dramatically, but their ontogenetic, static, and evolutionary allometry has not yet been studied in a comparative context. We report variable dynamics in eye size across 1,098 individuals in 39 species and 8 families, indicating selective pressures and constraints driving the evolution of different eye pairs and lineages. Supplementing our sampling with a recently published phylogenetically comprehensive dataset, we confirmed these findings across more than 400 species; found that ecological factors such as visual hunting, web building, and circadian activity correlate with eye diameter; and identified significant allometric shifts across spider phylogeny using an unbiased approach, many of which coincide with visual hunting strategies. The modular nature of the spider visual system provides additional degrees of freedom and is apparent in the strong correlations between maximum/minimum investment and interocular variance and three key ecological factors. Our analyses suggest an antagonistic relationship between the anterior and posterior eye pairs. These findings shed light on the relationship between spider visual systems and their diverse ecologies and how spiders exploit their modular visual systems to balance selective pressures and optical and energetic constraints.

Eye morphology in four species of tiger beetles (Coleoptera: Cicindelidae)

Compound eyes undoubtedly represent the widespread eye architecture in the animal kingdom. The insects' compound eye shows a wide variety of designs, and insects use their visual capacity to accomplish several tasks, including avoiding enemies, searching for food and shelter, locating a mate, and acquiring information about the environment and its surroundings. Broad literature data support the concept that visual ability lies in the way the eyes are built. Since the resolution and sensitivity of the compound eye are partly determined by the density of the ommatidia and the size of the facets. Morphological parameters of the compound eyes could influence the function of the visual organ and its capacity to process information, also representing a sensitive indicator of different habitat demands. In this study, we compared compound eyes' parameters in four closely related species of tiger beetles to disclose differences arising from different habitats. Furthermore, to investigate whether there are consistent intersexual differences, we also compared the most relevant parameters of the eye in males and females of four selected species. Our results show sex-related and interspecific differences that occur in examined species.

Host-trailing satellite flight behaviour is associated with greater investment in peripheral visual sensory system in miltogrammine flies

Insect sensory systems are the subjects of different selective pressures that shape their morphology. In many species of the flesh fly subfamily Miltogramminae (Diptera: Sarcophagidae) that are kleptoparasitic on bees and wasps, females perch on objects close to the host nests and, once a returning host is detected, they follow it in flight at a fixed distance behind until reaching the nest. We hypothesized that such satellite (SAT) flight behaviour, which implies a finely coordinated trailing flight, is associated with an improved visual system, compared to species adopting other, non-satellite (NON-SAT) strategies. After looking at body size and common ancestry, we found that SAT species have a greater number of ommatidia and a greater eye surface area when compared to NON-SAT species. Ommatidium area is only affected by body size, suggesting that selection changes disproportionately (relative to body size variation) the number of ommatidia and as a consequence the eye area, instead of ommatidium size. SAT species also tend to have larger ocelli, but their role in host-finding was less clear. This suggests that SAT species may have a higher visual acuity by increasing ommatidia number, as well as better stability during flight and motion perception through larger ocelli. Interestingly, antennal length was significantly reduced in SAT species, and ommatidia number negatively correlated with antennal length. While this finding does not imply a selection pressure of improved antennal sensory system in species adopting NON-SAT strategies, it suggests an inverse resource (i.e. a single imaginal disc) allocation between eyes and antennae in this fly subfamily.