Resolving the Trade-off Between Visual Sensitivity and Spatial Acuity-Lessons from Hawkmoths

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.

Subtle morphological changes in the visual and antennal sensory system of bees and wasps across an urbanisation gradient

Increased temperature and fragmentation of green spaces in urban areas could drive variations in functional traits of insects. Such morphological shifts may occur for sensory systems, which were previously reported to be prone to change with habitat characteristics in non-urban contexts. Here, we measured traits related to the visual and antennal sensory systems in the bees Halictus scabiosae and Osmia cornuta and the wasp Polistes dominula along an urbanisation gradient within Milan (Italy). We hypothesised that fragmentation could filter for better visual properties, and that higher temperature could filter for fewer thermoreceptors and more olfactory hairs. While controlling for body size, results show subtle but appreciable responses to urbanisation in one or more traits in all species, though not always supporting our hypotheses. O. cornuta shows marginally higher ommatidia density and smaller ommatidia diameter (associated with better visual resolution) in more fragmented sites, as well as marginally fewer thermoreceptors in hotter sites, in agreement with our two predictions. On the other hand, H. scabiosae has marginally smaller antennae and P. dominula has smaller eyes at warmer locations, and the wasp also has smaller antennae and 9th flagellomeres in more fragmented areas. Perhaps higher temperatures accelerate development of sensory system at higher speed than the rest of body in these two species. Our results represent the first evidence of urbanisation effects on the visual and antennal sensory systems of bees and wasps and underline how such effects may involve a much broader bouquet of traits then previously observed.

Comparison of dynamic visual acuity after implantation of toric bifocal or trifocal intraocular lens in age-related cataract patients: a randomized controlled trial

Purpose

To investigate the dynamic visual acuity (DVA) after implantation of toric bifocal or trifocal intraocular lens in age-related cataract patients.

Methods

This was a prospective randomized controlled trial. Of one hundred and twenty-four patients enrolled and randomized to receive unilateral phacoemulsification and toric trifocal (939 M/MP, Carl Zeiss Meditec AG, Jena, Germany) or toric bifocal (909 M, Carl Zeiss Meditec AG, Jena, Germany) intraocular lenses (IOL) implantation, ninety-nine patients completed the follow-up and were included in final analysis. Postoperatively, uncorrected and corrected distance (UDVA and CDVA), intermediate (UIVA and DCIVA) and near (UNVA and DCNVA) static visual acuity, manifest refraction and uncorrected and corrected distance DVA (UDDVA and CDDVA) at 20, 40 and 80 degrees per second (dps) were evaluated at one week, one month and three months.

Results

Three months postoperatively, the UDVA were 0.13 ± 0.11 and 0.14 ± 0.13 in the toric trifocal and bifocal IOL group, respectively. Significant better UIVA (trifocal, 0.17 ± 0.13 vs. bifocal, 0.23 ± 0.13, p = 0.037) and DCIVA (trifocal, 0.16 ± 0.11 vs. bifocal, 0.20 ± 0.12, p = 0.048) were observed in patients implanting toric trifocal than bifocal IOL at three months postoperatively. Patients implanted with toric bifocal IOL obtained better CDDVA at 80 dps (0.5607 ± 0.2032) than the trifocal group (0.6573 ± 0.2450, p = 0.039) at three months. Postoperative UDDVA and CDDVA at 20, 40 and 80 dps were significantly associated with age (p < 0.05, respectively) and postoperative static visual acuity (p < 0.05, respectively).

Conclusion

Toric trifocal IOL provides better static intermediate visual acuity, and toric bifocal IOL implantation provides better distance dynamic visual acuity at high speed.

Visual acuity of Empoasca onukii (Hemiptera, Cicadellidae)

Empoasca onukii is a common tea plant pest with a preference for the color yellow. Past work has shown that host leaf color is a key cue for habitat location for E. onukii. Before studying the effect of foliage shape, size, or texture on habitat localization, it is necessary to determine the visual acuity and effective viewing distance of E. onukii. In this study, a combination of 3D microscopy and X-ray microtomography showed that visual acuity did not significantly differ between females and males, but there were significant differences in the visual acuity and optical sensitivity among five regions of E. onukii's compound eyes. The dorsal ommatidia had the highest visual acuity at 0.28 cycles per degree (cpd) but the lowest optical sensitivity (0.02 μm²sr), which indicated a trade-off between visual resolution and optical sensitivity for E. onukii. The visual acuity determined from the behavioral experiment was 0.14 cpd; E. onukii exhibited low-resolution vision and could only distinguish the units in a yellow/red pattern within 30 cm. Therefore, visual acuity contributes to the limited ability of E. onukii to distinguish the visual details of a distant target, which might be perceived as a lump of blurred color of intermediate brightness.

Parallel evolution of opsin visual pigments in hawkmoths by tuning of spectral sensitivities during transition from a nocturnal to a diurnal ecology

Light environments differ dramatically between day and night. The transition between diurnal and nocturnal visual ecology has happened repeatedly throughout evolution in many species. However, the molecular mechanism underlying the evolution of vision in recent diurnal-nocturnal transition is poorly understood. Here, we focus on hawkmoths (Lepidoptera: Sphingidae) to address this question by investigating five nocturnal and five diurnal species. We performed RNA-sequencing analysis and identified opsin genes corresponding to the ultraviolet (UV), short-wavelength (SW) and long-wavelength (LW)-absorbing visual pigments. We found no significant differences in the expression patterns of opsin genes between the nocturnal and diurnal species. We then constructed the phylogenetic trees of hawkmoth species and opsins. The diurnal lineages had emerged at least three times from the nocturnal ancestors. The evolutionary rates of amino acid substitutions in the three opsins differed between the nocturnal and diurnal species. We found an excess number of parallel amino acid substitutions in the opsins in three independent diurnal lineages. The numbers were significantly more than those inferred from neutral evolution, suggesting that positive selection acted on these parallel substitutions. Moreover, we predicted the visual pigment absorption spectra based on electrophysiologically determined spectral sensitivity in two nocturnal and two diurnal species belonging to different clades. In the diurnal species, the LW pigments shift 10 nm towards shorter wavelengths, and the SW pigments shift 10 nm in the opposite direction. Taken together, our results suggest that parallel evolution of opsins may have enhanced the colour discrimination properties of diurnal hawkmoths in ambient light.

Night skies through animals' eyes-Quantifying night-time visual scenes and light pollution as viewed by animals

A large proportion of animal species enjoy the benefits of being active at night, and have evolved the corresponding optical and neural adaptations to cope with the challenges of low light intensities. However, over the past century electric lighting has introduced direct and indirect light pollution into the full range of terrestrial habitats, changing nocturnal animals' visual worlds dramatically. To understand how these changes affect nocturnal behavior, we here propose an animal-centered analysis method based on environmental imaging. This approach incorporates the sensitivity and acuity limits of individual species, arriving at predictions of photon catch relative to noise thresholds, contrast distributions, and the orientation cues nocturnal species can extract from visual scenes. This analysis relies on just a limited number of visual system parameters known for each species. By accounting for light-adaptation in our analysis, we are able to make more realistic predictions of the information animals can extract from nocturnal visual scenes under different levels of light pollution. With this analysis method, we aim to provide context for the interpretation of behavioral findings, and to allow researchers to generate specific hypotheses for the behavior of nocturnal animals in observed light-polluted scenes.

Color, activity period, and eye structure in four lineages of ants: Pale, nocturnal species have evolved larger eyes and larger facets than their dark, diurnal congeners

The eyes of insects display an incredible diversity of adaptations to enhance vision across the gamut of light levels that they experience. One commonly studied contrast is the difference in eye structure between nocturnal and diurnal species, with nocturnal species typically having features that enhance eye sensitivity such as larger eyes, larger eye facets, and larger ocelli. In this study, we compared eye structure between workers of closely related nocturnal and diurnal above ground foraging ant species (Hymenoptera: Formicidae) in four genera (Myrmecocystus, Aphaenogaster, Temnothorax, Veromessor). In all four genera, nocturnal species tend to have little cuticular pigment (pale), while diurnal species are heavily pigmented (dark), hence we could use cuticle coloration as a surrogate for activity pattern. Across three genera (Myrmecocystus, Aphaenogaster, Temnothorax), pale species, as expected for nocturnally active animals, had larger eyes, larger facet diameters, and larger visual spans compared to their dark, more day active congeners. This same pattern occurred for one pale species of Veromessor, but not the other. There were no consistent differences between nocturnal and diurnal species in interommatidial angles and eye parameters both within and among genera. Hence, the evolution of eye features that enhance sensitivity in low light levels do not appear to have consistent correlated effects on features related to visual acuity. A survey across several additional ant genera found numerous other pale species with enlarged eyes, suggesting these traits evolved multiple times within and across genera. We also compared the size of the anterior ocellus in workers of pale versus dark species of Myrmecocystus. In species with larger workers, the anterior ocellus was smaller in pale than in dark species, but this difference mostly disappeared for species with smaller workers. Presence of the anterior ocellus also was size-dependent in the two largest pale species.

Allometric scaling of a superposition eye optimizes sensitivity and acuity in large and small hawkmoths

Animals vary widely in body size within and across species. This has consequences for the function of organs and body parts in both large and small individuals. How these scale, in relation to body size, reveals evolutionary investment strategies, often resulting in trade-offs between functions. Eyes exemplify these trade-offs, as they are limited by their absolute size in two key performance features: sensitivity and spatial acuity. Due to their size polymorphism, insect compound eyes are ideal models for studying the allometric scaling of eye performance. Previous work on apposition compound eyes revealed that allometric scaling led to poorer spatial resolution and visual sensitivity in small individuals, across a range of insect species. Here, we used X-ray microtomography to investigate allometric scaling in superposition compound eyes-the second most common eye type in insects-for the first time. Our results reveal a novel strategy to cope with the trade-off between sensitivity and spatial acuity, as we show that the eyes of the hummingbird hawkmoth retain an optimal balance between these performance measures across all body sizes.

Fiddler crab electroretinograms reveal vast circadian shifts in visual sensitivity and temporal summation in dim light

Many animals with compound eyes undergo major optical changes to adjust visual sensitivity from day to night, often under control of a circadian clock. In fiddler crabs, this presents most conspicuously in the huge volume increase of photopigment-packed rhabdoms and the widening of crystalline cone apertures at night. These changes are hypothesised to adjust the light flux to the photoreceptors and to alter optical sensitivity as the eye moves between light- and dark-adapted states. Here, we compare optical sensitivity in fiddler crab eyes (Gelasimus dampieri) during daytime and night via three electroretinogram (ERG) experiments performed on light- and dark-adapted crabs.

1) Light intensity required to elicit a threshold ERG response varied over six orders of magnitude, allowing more sensitive vision for discriminating small contrasts in dim light after dusk. During daytime, the eyes remained relatively insensitive, which would allow effective vision on bright mudflats, even after prolonged dark adaptation.

2) Flicker fusion frequency (FFF) experiments indicated that temporal summation is employed in dim light to increase light-gathering integration times and enhance visual sensitivity during both night and day.

3) ERG responses to flickering lights during 60 mins of dark adaptation increased at a faster rate and greater extent after sunset compared to daytime. However, even brief, dim and intermittent light exposure strongly disrupted dark-adaptation processes.

Together, these findings demonstrate effective light adaptation to optimise vision over the large range of light intensities that these animals experience.

Spatial tuning of translational optic flow responses in hawkmoths of varying body size

To safely navigate their environment, flying insects rely on visual cues, such as optic flow. Which cues insects can extract from their environment depends closely on the spatial and temporal response properties of their visual system. These in turn can vary between individuals that differ in body size. How optic flow-based flight control depends on the spatial structure of visual cues, and how this relationship scales with body size, has previously been investigated in insects with apposition compound eyes. Here, we characterised the visual flight control response limits and their relationship to body size in an insect with superposition compound eyes: the hummingbird hawkmoth Macroglossum stellatarum. We used the hawkmoths’ centring response in a flight tunnel as a readout for their reception of translational optic flow stimuli of different spatial frequencies. We show that their responses cut off at different spatial frequencies when translational optic flow was presented on either one, or both tunnel walls. Combined with differences in flight speed, this suggests that their flight control was primarily limited by their temporal rather than spatial resolution. We also observed strong individual differences in flight performance, but no correlation between the spatial response cutoffs and body or eye size.

Species and sex differences in eye morphometry and visual responsivity of two crepuscular sweat bee species (Megalopta spp., Hymenoptera: Halictidae)

Visually dependent dim-light foraging has evolved repeatedly, broadening the ecological niches of some species. Many dim-light foraging lineages evolved from diurnal ancestors, requiring immense visual sensitivity increases to compensate for light levels a billion times dimmer than daylight. Some taxa, such as bees, are anatomically constrained by apposition compound eyes, which function well in daylight but not in starlight. Even with this constraint, the bee genus Megalopta has incredibly sensitive eyes, foraging in light levels up to nine orders of magnitude dimmer than diurnal relatives. Despite many behavioural studies, variation in visual sensitivity and eye morphometry has not been investigated within and across Megalopta species. Here we quantify external eye morphology (corneal area and facet size) for sympatric species of Megalopta, M. genalis and M. amoena, which forage during twilight. We use electroretinograms to show that males, despite being smaller than females, have equivalent visual sensitivity and increased retinal responsivity. Although males have relatively larger eyes compared with females, corneal area and facet size were not correlated with retinal responsivity, suggesting that males have additional non-morphological adaptations to increase retinal responsiveness. These findings provide the foundation for future work into the neural and physiological mechanisms that interface with morphology to influence visual sensitivity, with implications for understanding niche exploitation.