Polarized vision in the eyes of the most effective predators: dragonflies and damselflies (Odonata)

Polarization is a property of light that describes the oscillation of the electric field vector. Polarized light can be detected by many invertebrate animals, and this visual channel is widely used in nature. Insects rely on light polarization for various purposes, such as water detection, improving contrast, breaking camouflage, navigation, and signaling during mating. Dragonflies and damselflies (Odonata) are highly visual insects with polarization sensitivity for water detection and likely also navigation. Thus, odonates can serve as ideal models for investigating the ecology and evolution of polarized light perception. We provide an overview of the current state of knowledge concerning polarized light sensitivity in these insects. Specifically, we review recent findings related to the ecological, morphological, and physiological causes that enable these insects to perceive polarized light and discuss the optical properties responsible for the reflection of polarized light by their bodies and wings. Finally, we identify gaps in the current research and suggest future directions that can help to further advance our knowledge of polarization sensitivity in odonates.

Behavioral responses of free-flying Drosophila melanogaster to shiny, reflecting surfaces

Active locomotion plays an important role in the life of many animals, permitting them to explore the environment, find vital resources, and escape predators. Most insect species rely on a combination of visual cues such as celestial bodies, landmarks, or linearly polarized light to navigate or orient themselves in their surroundings. In nature, linearly polarized light can arise either from atmospheric scattering or from reflections off shiny non-metallic surfaces like water. Multiple reports have described different behavioral responses of various insects to such shiny surfaces. Our goal was to test whether free-flying Drosophila melanogaster, a molecular genetic model organism and behavioral generalist, also manifests specific behavioral responses when confronted with such polarized reflections. Fruit flies were placed in a custom-built arena with controlled environmental parameters (temperature, humidity, and light intensity). Flight detections and landings were quantified for three different stimuli: a diffusely reflecting matt plate, a small patch of shiny acetate film, and real water. We compared hydrated and dehydrated fly populations, since the state of hydration may change the motivation of flies to seek or avoid water. Our analysis reveals for the first time that flying fruit flies indeed use vision to avoid flying over shiny surfaces.

Expression and function of opsin genes associated with phototaxis in Zeugodacus cucurbitae Coquillett (Diptera: Tephritidae)

BACKGROUND

Zeugodacus cucuribitae is a major agricultural pest that causes significant damage to varieties of plants. Vision plays a critical role in phototactic behavior of herbivorous insects. However, the effect of opsin on the phototactic behavior in Z. cucuribitae remains unknown. The aim of this research is to explore the key opsin genes that associate with phototaxis behavior of Z. cucurbitae.

RESULTS

Five opsin genes were identified and their expression patterns were analyzed. The relative expression levels of ZcRh1, ZcRh4 and ZcRh6 were highest in 4-day-old larvae, ZcRh2 and ZcRh3 were highest in 3rd-instar larvae and 5-day-old pupae, respectively. Furthermore, five opsin genes had the highest expression levels in compound eyes, followed by the antennae and head, whereas the lower occurred in other tissues. The expression of the long-wavelength-sensitive (LW) opsins first decreased and then increased under green light exposure. In contrast, the expression of ultraviolet-sensitive (UV) opsins first increased and then decreased with the duration of UV exposure. Silencing of LW opsin (dsZcRh1, dsZcRh2, and dsZcRh6) and UV opsin (dsZcRh3 and dsZcRh4) reduced the phototactic efficiency of Z. cucurbitae to green light by 52.27%, 60.72%, and 67.89%, and to UV light by 68.59% and 61.73%, respectively.

CONCLUSION

The results indicate that RNAi inhibited the expression of opsin, thereby inhibiting the phototaxis of Z. cucurbitae. This result provides theoretical support for the physical control of Z. cucurbitae and lays the foundation for further exploration of the mechanism of insect phototaxis. © 2023 Society of Chemical Industry.

Motion-sensitive neurons activated by chromatic contrast in a butterfly visual system

A pattern of two equally bright colours contains only chromatic contrast. Unlike in flies, such a pattern elicits strong optokinetic responses in the butterfly Papilio xuthus. To investigate the neural basis of chromatic motion vision, we performed single-cell electrophysiology. We found spiking neurons exhibiting direction-selective motion sensitivity in the second optic ganglion, the medulla. We analysed the response characteristics of these neurons using two-colour stripe patterns moving vertically. We systematically manipulated the intensities of the colours so that the set of presented patterns included an isoluminant condition for the butterfly. Moving patterns containing only chromatic contrast still elicited a response in the neurons. The neurons' sensitivity profile is similar to that of the behavioural responses. Post-recording dye injection revealed that the neurons have dendrites in the ventral lateral protocerebrum and axonal processes in the medulla, suggesting a feedback role. Presumably, the neurons contribute to subtracting wide-field motion to facilitate the detection of small moving targets. This article is part of the theme issue 'Understanding colour vision: molecular, physiological, neuronal and behavioural studies in arthropods'.

Polarization vision in invertebrates: beyond the boundaries of navigation

Invertebrates possess the unique ability to see polarized light. This allows them to exploit the rich polarization information embedded in their natural environments: patterns in plants, high contrast on water surfaces, distinctive signatures of conspecifics, and the celestial polarization pattern around the sun. From this wide repertoire of polarization signals, studies have primarily focused on understanding how celestial polarization information is converted into an internal compass. This review highlights several studies which suggest that spatio-temporal polarization information is utilized by insects for additional functions, such as signaling, detection, contrast enhancement, and host assessment. It concludes by evaluating recent technological advances for uncovering the full repertoire of polarization-sensitivity in invertebrates.

Swallowtail Butterflies Use Multiple Visual Cues to Select Oviposition Sites

Simple Summary

Butterflies must not only identify host plants on which to lay their eggs—which they achieve using chemical cues—but also select suitable leaves on that plant that will support the growth of their larval offspring. Here, we asked whether swallowtail butterflies lay eggs on particular leaves of a Citrus tree and, if so, which cues they use to select the leaves. We first observed that butterflies indeed select just a few leaves on which to lay eggs. These leaf preferences were observed across many individuals, implying that they were not idiosyncratic, and the butterflies descended directly towards the leaves from some distance, suggesting that they were using visual rather than chemical cues. We then investigated which visual cues are used by the butterflies, and found that the number of eggs laid upon a leaf was correlated with its height on the tree, flatness, green reflectance, brightness, and degree of polarization. These five features may be important both for selecting young leaves and those which are situated well for egg-laying. An outstanding question for future study is how visual and chemical cues interact in this context.

Abstract

Flower-foraging Japanese yellow swallowtail butterflies, Papilio xuthus, exhibit sophisticated visual abilities. When ovipositing, females presumably attempt to select suitable leaves to support the growth of their larval offspring. We first established that butterflies indeed select particular leaves on which to lay eggs; when presented with a single Citrus tree, butterflies significantly favored two out of 102 leaves for oviposition. These preferences were observed across many individuals, implying that they were not merely idiosyncratic, but rather based on properties of the leaves in question. Because the butterflies descended towards the leaves rather directly from a distance, we hypothesized that they base their selection on visual cues. We measured five morphological properties (height, orientation, flatness, roundness, and size) and four reflective features (green reflectance, brightness, and degree and angle of linear polarization). We found that the number of eggs laid upon a leaf was positively correlated with its height, flatness, green reflectance, and brightness, and negatively correlated with its degree of polarization, indicating that these features may serve as cues for leaf selection. Considering that other studies report ovipositing butterflies’ preference for green color and horizontally polarized light, butterflies likely use multiple visual features to select egg-laying sites on the host plant.

Stark trade-offs and elegant solutions in arthropod visual systems

Vision is one of the most important senses for humans and animals alike. Diverse elegant specializations have evolved among insects and other arthropods in response to specific visual challenges and ecological needs. These specializations are the subject of this Review, and they are best understood in light of the physical limitations of vision. For example, to achieve high spatial resolution, fine sampling in different directions is necessary, as demonstrated by the well-studied large eyes of dragonflies. However, it has recently been shown that a comparatively tiny robber fly (Holcocephala) has similarly high visual resolution in the frontal visual field, despite their eyes being a fraction of the size of those of dragonflies. Other visual specializations in arthropods include the ability to discern colors, which relies on parallel inputs that are tuned to spectral content. Color vision is important for detection of objects such as mates, flowers and oviposition sites, and is particularly well developed in butterflies, stomatopods and jumping spiders. Analogous to color vision, the visual systems of many arthropods are specialized for the detection of polarized light, which in addition to communication with conspecifics, can be used for orientation and navigation. For vision in low light, optical superposition compound eyes perform particularly well. Other modifications to maximize photon capture involve large lenses, stout photoreceptors and, as has been suggested for nocturnal bees, the neural pooling of information. Extreme adaptations even allow insects to see colors at very low light levels or to navigate using the Milky Way.

Retinal organization and visual abilities for flower foraging in swallowtail butterflies

Papilio butterflies' ability to forage for flowers relies upon multiple visual cues such as color, brightness, and motion. Papilio learns the color of rewarding flowers and detects it at a distance. Its color vision is based on four photoreceptor classes: UV, blue, green, and red, providing sensitive wavelength discrimination. These four receptor classes also contribute to the perception of brightness and polarization. Papilio's motion vision is based on a different set of receptors: green, red, and broad band. This implies that two visual pathways exist in Papilio. The contribution of several receptor classes not only for chromatic vision but also achromatic vision likely enhances the butterfly's ability to detect flowers in complex visual environments.

Polarized light sensitivity in Pieris rapae is dependent on both color and intensity

There is an ever increasing number of arthropod taxa shown to have polarization sensitivity throughout their compound eyes. However, the downstream processing of polarized reflections from objects is not well understood. The small white butterfly, Pieris rapae, has been demonstrated to exploit foliar polarized reflections, specifically the degree of linear polarization (DoLP), to recognize host plants. The well-described visual system of P. rapae includes several photoreceptor types (red, green, blue) that are sensitive to polarized light. Yet, the roles and interaction among photoreceptors underlying the behavioral responses of P. rapae to stimuli with different DoLP remain unknown. To investigate potential neurological mechanisms, we designed several two-choice behavioral bioassays, displaying plant images on paired LCD monitors, which allowed for independent control of polarization, color and intensity. When we presented choices between stimuli that differed in either color or DoLP, both decreasing and increasing the intensity of the more attractive stimulus reduced the strength of preference. This result suggests that differences in color and DoLP are perceived in a similar manner. When we offered a DoLP choice between plant images manipulated to minimize the response of blue, red, or blue and red photoreceptors, P. rapae shifted its preference for DoLP, suggesting a role for all of these photoreceptors. Modeling of P. rapae photoreceptor responses to test stimuli suggests that differential DoLP is not perceived solely as a color difference. Our combined results suggest that Prapae females process and interpret polarization reflections in a way different from that described for other polarization-sensitive taxa.

Spectral organization of the compound eye of a migrating nymphalid, the chestnut tiger butterfly Parantica sita

Several butterflies of family Nymphalidae perform long-distance migration. Extensive studies of migration in the iconic monarch butterfly Danaus plexippus have revealed that vision plays a crucial role in migratory orientation. Differences in the migratory patterns of butterflies suggest that not all species are exposed to the same visual conditions and yet, little is known about how the visual system varies across migratory species. Here, we used intracellular electrophysiology, dye injection and electron microscopy to assess the spectral and polarization properties of the photoreceptors of a migrating nymphalid, Parantica sita Our findings reveal three spectral classes of photoreceptors including ultraviolet, blue and green receptors. The green receptor class contains three subclasses, which are broad, narrow and double-peaking green receptors. Ultraviolet and blue receptors are sensitive to polarized light parallel to the dorso-ventral axis of the animal, while the variety of green receptors are sensitive to light polarized at 45 deg, 90 deg and 135 deg away from the dorso-ventral axis. The polarization sensitivity ratio is constant across spectral receptor classes at around 1.8. Although P. sita has a typical nymphalid eye with three classes of spectral receptors, subtle differences exist among the eyes of migratory nymphalids, which may be genus specific.

Parallel processing of polarization and intensity information in fiddler crab vision

Many crustaceans are sensitive to the polarization of light and use this information for object-based visually guided behaviors. For these tasks, it is unknown whether polarization and intensity information are integrated into a single-contrast channel, whereby polarization directly contributes to perceived intensity, or whether they are processed separately and in parallel. Using a novel type of visual display that allowed polarization and intensity properties of visual stimuli to be adjusted independently and simultaneously, we conducted behavioral experiments with fiddler crabs to test which of these two models of visual processing occurs. We found that, for a loom detection task, fiddler crabs process polarization and intensity information independently and in parallel. The crab's response depended on whichever contrast was the most salient. By contributing independent measures of visual contrast, polarization and intensity provide a greater range of detectable contrast information for the receiver, increasing the chance of detecting a potential threat.