Detectors for polarized skylight in insects: a survey of ommatidial specializations in the dorsal rim area of the compound eye

Bumblebees learn polarization patterns

Foraging insect pollinators such as bees must find and identify flowers in a complex visual environment. Bees use skylight polarization patterns for navigation [1–3], a capacity mediated by the polarization-sensitive dorsal rim area (DRA) of their eye [4, 5]. While other insects use polarization sensitivity to identify appropriate habitats [6], oviposition sites, and food sources [7], to date no nonnavigational functions of polarization vision have been identified in bees. Here we investigated the ability of bumblebees (Bombus terrestris) to learn polarization patterns on artificial “flowers” in order to obtain a food reward. We show that foraging bumblebees can learn to discriminate between two differently polarized targets, but only when the target artificial “flower” is viewed from below. A context for these results is provided by polarization imaging of bee-pollinated flowers, revealing the potential for polarization patterns in real flowers. Bees may therefore have the ability to use polarization vision, possibly mediated by their polarization-sensitive DRA, both for navigation and to learn polarization patterns on flowers, the latter being the first nonnavigational function for bee polarization vision to be identified.

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Bumblebees (Bombus terrestris) learn polarization patterns on artificial “flowers”

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Polarization patterns were only learned from downward-facing, pendant “flowers”

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Polarization vision in bumblebees is not restricted to sun-compass navigation

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Polarization patterns of petals may be a component of floral signaling

Walking Drosophila align with the e-vector of linearly polarized light through directed modulation of angular acceleration

Understanding the mechanisms that link sensory stimuli to animal behavior is a central challenge in neuroscience. The quantitative description of behavioral responses to defined stimuli has led to a rich understanding of different behavioral strategies in many species. One important navigational cue perceived by many vertebrates and insects is the e-vector orientation of linearly polarized light. Drosophila manifests an innate orientation response to this cue ('polarotaxis'), aligning its body axis with the e-vector field. We have established a population-based behavioral paradigm for the genetic dissection of neural circuits guiding polarotaxis to both celestial as well as reflected polarized stimuli. However, the behavioral mechanisms by which flies align with a linearly polarized stimulus remain unknown. Here, we present a detailed quantitative description of Drosophila polarotaxis, systematically measuring behavioral parameters that are modulated by the stimulus. We show that angular acceleration is modulated during alignment, and this single parameter may be sufficient for alignment. Furthermore, using monocular deprivation, we show that each eye is necessary for modulating turns in the ipsilateral direction. This analysis lays the foundation for understanding how neural circuits guide these important visual behaviors.

Diversity and wiring variability of visual local neurons in the Drosophila medulla M6 stratum

Local neurons in the vertebrate retina are instrumental in transforming visual inputs to extract contrast, motion, and color information and in shaping bipolar-to-ganglion cell transmission to the brain. In Drosophila, UV vision is represented by R7 inner photoreceptor neurons that project to the medulla M6 stratum, with relatively little known of this downstream substrate. Here, using R7 terminals as references, we generated a 3D volume model of the M6 stratum, which revealed a retinotopic map for UV representations. Using this volume model as a common 3D framework, we compiled and analyzed the spatial distributions of more than 200 single M6-specific local neurons (M6-LNs). Based on the segregation of putative dendrites and axons, these local neurons were classified into two families, directional and nondirectional. Neurotransmitter immunostaining suggested a signal routing model in which some visual information is relayed by directional M6-LNs from the anterior to the posterior M6 and all visual information is inhibited by a diverse population of nondirectional M6-LNs covering the entire M6 stratum. Our findings suggest that the Drosophila medulla M6 stratum contains diverse LNs that form repeating functional modules similar to those found in the vertebrate inner plexiform layer.

Diurnal dung beetles use the intensity gradient and the polarization pattern of the sky for orientation

To escape competition at the dung pile, a ball-rolling dung beetle forms a piece of dung into a ball and rolls it away. To ensure their efficient escape from the dung pile, beetles rely on a 'celestial compass' to move along a straight path. Here, we analyzed the reliability of different skylight cues for this compass and found that dung beetles rely not only on the sun but also on the skylight polarization pattern. Moreover, we show the first evidence of an insect using the celestial light-intensity gradient for orientation. Using a polarizer, we manipulated skylight so that the polarization pattern appeared to turn by 90 deg. The beetles then changed their bearing close to the expected 90 deg. This behavior was abolished if the sun was visible to the beetle, suggesting that polarized light is hierarchically subordinate to the sun. When the sky was depolarized and the sun was invisible, the beetles could still move along straight paths. Therefore, we analyzed the use of the celestial light-intensity gradient for orientation. Artificial rotation of the intensity pattern by 180 deg caused beetles to orient in the opposite direction. This light-intensity cue was also found to be subordinate to the sun and could play a role in disambiguating the polarization signal, especially at low sun elevations.

Homothorax and Extradenticle alter the transcription factor network in Drosophila ommatidia at the dorsal rim of the retina

A narrow band of ommatidia in the dorsal periphery of the Drosophila retina called the dorsal rim area (DRA) act as detectors for polarized light. The transcription factor Homothorax (Hth) is expressed in DRA inner photoreceptors R7 and R8 and is both necessary and sufficient to induce the DRA fate, including specialized morphology and unique Rhodopsin expression. Hth expression is the result of Wingless (Wg) pathway activity at the eye margins and restriction to the dorsal eye by the selector genes of the Iroquois complex (Iro-C). However, how the DRA is limited to exactly one or two ommatidial rows is not known. Although several factors regulating the Drosophila retinal mosaic are expressed in DRA ommatidia, the role of Hth in this transcriptional network is uncharacterized. Here we show that Hth functions together with its co-factor Extradenticle (Exd) to repress the R8-specific factor Senseless (Sens) in DRA R8 cells, allowing expression of an ultraviolet-sensitive R7 Rhodopsin (Rh3). Furthermore, Hth/Exd act in concert with the transcriptional activators Orthodenticle (Otd) and Spalt (Sal), to activate expression of Rh3 in the DRA. The resulting monochromatic coupling of Rh3 between R7 and R8 in DRA ommatidia is important for comparing celestial e-vector orientation rather than wavelengths. Finally, we show that Hth expression expands to many ommatidial rows in regulatory mutants of optomotorblind (omb), a transcription factor transducing Wg signaling at the dorsal and ventral eye poles. Therefore, locally restricted recruitment of the DRA-specific factor Hth alters the transcriptional network that regulates Rhodopsin expression across ommatidia.

Genetic dissection of photoreceptor subtype specification by the Drosophila melanogaster zinc finger proteins elbow and no ocelli

The elbow/no ocelli (elb/noc) complex of Drosophila melanogaster encodes two paralogs of the evolutionarily conserved NET family of zinc finger proteins. These transcriptional repressors share a conserved domain structure, including a single atypical C2H2 zinc finger. In flies, Elb and Noc are important for the development of legs, eyes and tracheae. Vertebrate NET proteins play an important role in the developing nervous system, and mutations in the homolog ZNF703 human promote luminal breast cancer. However, their interaction with transcriptional regulators is incompletely understood. Here we show that loss of both Elb and Noc causes mis-specification of polarization-sensitive photoreceptors in the 'dorsal rim area' (DRA) of the fly retina. This phenotype is identical to the loss of the homeodomain transcription factor Homothorax (Hth)/dMeis. Development of DRA ommatidia and expression of Hth are induced by the Wingless/Wnt pathway. Our data suggest that Elb/Noc genetically interact with Hth, and we identify two conserved domains crucial for this function. Furthermore, we show that Elb/Noc specifically interact with the transcription factor Orthodenticle (Otd)/Otx, a crucial regulator of rhodopsin gene transcription. Interestingly, different Elb/Noc domains are required to antagonize Otd functions in transcriptional activation, versus transcriptional repression. We propose that similar interactions between vertebrate NET proteins and Meis and Otx factors might play a role in development and disease.

Integration of polarization and chromatic cues in the insect sky compass

Animals relying on a celestial compass for spatial orientation may use the position of the sun, the chromatic or intensity gradient of the sky, the polarization pattern of the sky, or a combination of these cues as compass signals. Behavioral experiments in bees and ants, indeed, showed that direct sunlight and sky polarization play a role in sky compass orientation, but the relative importance of these cues are species-specific. Intracellular recordings from polarization-sensitive interneurons in the desert locust and monarch butterfly suggest that inputs from different eye regions, including polarized-light input through the dorsal rim area of the eye and chromatic/intensity gradient input from the main eye, are combined at the level of the medulla to create a robust compass signal. Conflicting input from the polarization and chromatic/intensity channel, resulting from eccentric receptive fields, is eliminated at the level of the anterior optic tubercle and central complex through internal compensation for changing solar elevations, which requires input from a circadian clock. Across several species, the central complex likely serves as an internal sky compass, combining E-vector information with other celestial cues. Descending neurons, likewise, respond both to zenithal polarization and to unpolarized cues in an azimuth-dependent way.

Null point of discrimination in crustacean polarisation vision

The polarisation of light is used by many species of cephalopods and crustaceans to discriminate objects or to communicate. Most visual systems with this ability, such as that of the fiddler crab, include receptors with photopigments that are oriented horizontally and vertically relative to the outside world. Photoreceptors in such an orthogonal array are maximally sensitive to polarised light with the same fixed e-vector orientation. Using opponent neural connections, this two-channel system may produce a single value of polarisation contrast and, consequently, it may suffer from null points of discrimination. Stomatopod crustaceans use a different system for polarisation vision, comprising at least four types of polarisation-sensitive photoreceptor arranged at 0°, 45°, 90° and 135° relative to each other, in conjunction with extensive rotational eye movements. This anatomical arrangement should not suffer from equivalent null points of discrimination. To test whether these two systems were vulnerable to null points, we presented the fiddler crab Uca heteropleura and the stomatopod Haptosquilla trispinosa with polarised looming stimuli on a modified LCD monitor. The fiddler crab was less sensitive to differences in the degree of polarised light when the e-vector was at -45°, than when the e-vector was horizontal. In comparison, stomatopods showed no difference in sensitivity between the two stimulus types. The results suggest that fiddler crabs suffer from a null point of sensitivity, while stomatopods do not.

Opsin expression, physiological characterization and identification of photoreceptor cells in the dorsal rim area and main retina of the desert locust, Schistocerca gregaria

For compass orientation many insects rely on the pattern of sky polarization but some species also exploit the sky chromatic contrast. Desert locusts, Schistocerca gregaria, detect polarized light through a specialized dorsal rim area (DRA) in their compound eye. To better understand retinal mechanisms underlying visual navigation, we compared opsin expression, spectral and polarization sensitivities and response-stimulus intensity functions in the DRA and main retina of the locust. In addition to previously characterized opsins of long-wavelength-absorbing (Lo1) and blue-absorbing visual pigments (Lo2), we identified an opsin of a UV-absorbing visual pigment (LoUV). DRA photoreceptors exclusively expressed Lo2, had peak spectral sensitivities at 441 nm and showed high polarization sensitivity (PS 1.3-31.7). In contrast, ommatidia in the main eye coexpressed Lo1 and Lo2 in five photoreceptors, expressed Lo1 in two proximal photoreceptors, and Lo2 or LoUV in one distal photoreceptor. Correspondingly, we found broadband blue- and green-peaking spectral sensitivities in the main eye and one narrowly tuned UV peaking receptor. Polarization sensitivity in the main retina was low (PS 1.3-3.8). V-log I functions in the DRA were steeper than in the main retina supporting a role in polarization vision. Desert locusts occur as two morphs, a day-active gregarious and a night-active solitarious form. In solitarious locusts sensitivities in the main retina were generally shifted to longer wavelengths, particularly in ventral eye regions, supporting a nocturnal life style at low light levels. The data support the role of the DRA in polarization vision and suggest trichromatic colour vision in the desert locust.

Interactions of the polarization and the sun compass in path integration of desert ants

Desert ants, Cataglyphis fortis, perform large-scale foraging trips in their featureless habitat using path integration as their main navigation tool. To determine their walking direction they use primarily celestial cues, the sky's polarization pattern and the sun position. To examine the relative importance of these two celestial cues, we performed cue conflict experiments. We manipulated the polarization pattern experienced by the ants during their outbound foraging excursions, reducing it to a single electric field (e-)vector direction with a linear polarization filter. The simultaneous view of the sun created situations in which the directional information of the sun and the polarization compass disagreed. The heading directions of the homebound runs recorded on a test field with full view of the natural sky demonstrate that none of both compasses completely dominated over the other. Rather the ants seemed to compute an intermediate homing direction to which both compass systems contributed roughly equally. Direct sunlight and polarized light are detected in different regions of the ant's compound eye, suggesting two separate pathways for obtaining directional information. In the experimental paradigm applied here, these two pathways seem to feed into the path integrator with similar weights.

Death Valley, Drosophila, and the Devonian toolkit

Most experiments on the flight behavior of Drosophila melanogaster have been performed within confined laboratory chambers, yet the natural history of these animals involves dispersal that takes place on a much larger spatial scale. Thirty years ago, a group of population geneticists performed a series of mark-and-recapture experiments on Drosophila flies, which demonstrated that even cosmopolitan species are capable of covering 10 km of open desert, probably in just a few hours and without the possibility of feeding along the way. In this review I revisit these fascinating and informative experiments and attempt to explain how-from takeoff to landing-the flies might have made these journeys based on our current knowledge of flight behavior. This exercise provides insight into how animals generate long behavioral sequences using sensory-motor modules that may have an ancient evolutionary origin.

Compound eye adaptations for diurnal and nocturnal lifestyle in the intertidal ant, Polyrhachis sokolova

The Australian intertidal ant, Polyrhachis sokolova lives in mudflat habitats and nests at the base of mangroves. They are solitary foraging ants that rely on visual cues. The ants are active during low tides at both day and night and thus experience a wide range of light intensities. We here ask the extent to which the compound eyes of P. sokolova reflect the fact that they operate during both day and night. The ants have typical apposition compound eyes with 596 ommatidia per eye and an interommatidial angle of 6.0°. We find the ants have developed large lenses (33 µm in diameter) and wide rhabdoms (5 µm in diameter) to make their eyes highly sensitive to low light conditions. To be active at bright light conditions, the ants have developed an extreme pupillary mechanism during which the primary pigment cells constrict the crystalline cone to form a narrow tract of 0.5 µm wide and 16 µm long. This pupillary mechanism protects the photoreceptors from bright light, making the eyes less sensitive during the day. The dorsal rim area of their compound eye has specialised photoreceptors that could aid in detecting the orientation of the pattern of polarised skylight, which would assist the animals to determine compass directions required while navigating between nest and food sources.

Polarotaxis and scototaxis in the supratidal amphipod Platorchestia platensis

Talitrid amphipods use many cues for orientation during forays between temporary burrows and feeding areas, and for locating beaches when submerged, with visual cues being particularly important. Little evidence exists for polarized light among these visual cues despite extensive orientation by celestial and underwater polarized light in other crustaceans and in insects. We used electroretinography to assess spectral sensitivity in the eye of the beach flea Platorchestia platensis, and behavioral studies to test whether linearly polarized light serves as an orientation cue. Two spectral classes were present in the P. platensis eye with maxima at 431 and 520 nm. Non-uniform orientation of amphipods in the laboratory arena required either light/dark or polarized cues. Scototactic movements depended on arena conditions (day/night, wet/dry), while orientation under linearly polarized light was wavelength-dependent and parallel to the e-vector. Subsequent tests presented conflicting and additive scototactic and polarotactic cues to differentiate among these responses. In dry conditions, orientation parallel to the polarization e-vector overcame a dominant negative scototaxis, confirming that polarotaxis and scototaxis are separate orientation responses in this species. These behavioral results demonstrate talitrid amphipods can perceive and orient to linearly polarized light, and may use it to orient toward preferred zones on beaches.

Life as a cataglyphologist--and beyond

Rüdiger Wehner's lifelong research activities centered on Cataglyphis have rendered these thermophilic desert ants model organisms in the study of animal navigation. The present account describes how the author encountered Cataglyphis and established a study site at Maharès, Tunisia; how he increasingly focused his research on the neuroethological analysis of the ant's navigational toolkit; and finally, how he extended these studies to thermophilic desert ants in other deserts of the world, to Ocymyrmex in southern Africa and Melophorus in central Australia. By including aspects of functional morphology, physiology, and ecology in his research projects, he has favored-and advocated-an organism-centered approach. Beyond "cataglyphology," he was engaged in substantial teaching both at his home university in Zürich and overseas, writing a textbook, running a department, and working as a Permanent Fellow at the Institute for Advanced Study in Berlin.

Homing abilities of the Australian intertidal ant, Polyrhachis sokolova

The pressure of returning and locating the nest after a successful foraging trip is immense in ants. To find their way back home, ants use a number of different strategies (e.g., path integration, trail-following) and rely on a range of cues (e.g., pattern of polarised skylight, landmark panorama) available in their environment. How ants weigh different cues has been a question of great interest and has primarily been addressed in the desert ants from Africa and Australia. We here identify the navigational abilities of an intertidal ant, Polyrhachis sokolova that lives on mudflats where nests and foraging areas are frequently inundated with tidal water. We find that these solitary foraging ants rely heavily on visual landmark information for navigation but they are also capable of path integration. By displacing ants with and without vector information at different locations within the local familiar territory we created conflicts between information from the landmarks and the path integrator. The homing success of full-vector ants, compared to the zero-vector ants, when displaced 5 m behind the feeder indicate that vector information had to be coupled with landmark information for successful homing. To explain the differences in the homing abilities of ants from different locations we determined the navigational information content at each release station and compared it to that available at the feeder location. We report here the interaction of multiple navigation strategies in the context of the information content in the environment.

Bio-inspired polarized skylight-based navigation sensors: a review

Animal senses cover a broad range of signal types and signal bandwidths and have inspired various sensors and bioinstrumentation devices for biological and medical applications. Insects, such as desert ants and honeybees, for example, utilize polarized skylight pattern-based information in their navigation activities. They reliably return to their nests and hives from places many kilometers away. The insect navigation system involves the dorsal rim area in their compound eyes and the corresponding polarization sensitive neurons in the brain. The dorsal rim area is equipped with photoreceptors, which have orthogonally arranged small hair-like structures termed microvilli. These are the specialized sensors for the detection of polarized skylight patterns (e-vector orientation). Various research groups have been working on the development of novel navigation systems inspired by polarized skylight-based navigation in animals. Their major contributions are critically reviewed. One focus of current research activities is on imitating the integration path mechanism in desert ants. The potential for simple, high performance miniaturized bioinstrumentation that can assist people in navigation will be explored.

Opsin evolution and expression in arthropod compound eyes and ocelli: insights from the cricket Gryllus bimaculatus

Background

Opsins are key proteins in animal photoreception. Together with a light-sensitive group, the chromophore, they form visual pigments which initiate the visual transduction cascade when photoactivated. The spectral absorption properties of visual pigments are mainly determined by their opsins, and thus opsins are crucial for understanding the adaptations of animal eyes. Studies on the phylogeny and expression pattern of opsins have received considerable attention, but our knowledge about insect visual opsins is still limited. Up to now, researchers have focused on holometabolous insects, while general conclusions require sampling from a broader range of taxa. We have therefore investigated visual opsins in the ocelli and compound eyes of the two-spotted cricket Gryllus bimaculatus, a hemimetabolous insect.

Results

Phylogenetic analyses place all identified cricket sequences within the three main visual opsin clades of insects. We assign three of these opsins to visual pigments found in the compound eyes with peak absorbances in the green (515 nm), blue (445 nm) and UV (332 nm) spectral range. Their expression pattern divides the retina into distinct regions: (1) the polarization-sensitive dorsal rim area with blue- and UV-opsin, (2) a newly-discovered ventral band of ommatidia with blue- and green-opsin and (3) the remainder of the compound eye with UV- and green-opsin. In addition, we provide evidence for two ocellar photopigments with peak absorbances in the green (511 nm) and UV (350 nm) spectral range, and with opsins that differ from those expressed in the compound eyes.

Conclusions

Our data show that cricket eyes are spectrally more specialized than has previously been assumed, suggesting that similar adaptations in other insect species might have been overlooked. The arrangement of spectral receptor types within some ommatidia of the cricket compound eyes differs from the generally accepted pattern found in holometabolous insect taxa and awaits a functional explanation. From the opsin phylogeny, we conclude that gene duplications, which permitted differential opsin expression in insect ocelli and compound eyes, occurred independently in several insect lineages and are recent compared to the origin of the eyes themselves.

Structure and spectral sensitivity of photoreceptors of two anchovy species: Engraulis japonicus and Engraulis encrasicolus

The morphology, fine structure and spectral sensitivity of retinal photoreceptors of two anchovy species were investigated using light and electron microscopy and microspectrophotometry. Distinct regional specialisation of cones was observed. Long and short (bilobed) cones were observed in the horizontal retinal belt, including the nasal and temporal retinal zones. Only triple cones with two long lateral components, one small central component were observed in the dorsal and ventro-nasal retinal regions. The long cones presented various lamellar organisation patterns: (1) in parallel along the cell axis in the central retina, (2) oriented transversely at the base of the outer segment, and (3) tilted longitudinally while extending to the tip of the cone in the retinal periphery. In the short cones, the lamellae were always oriented along the cell axis, and their planes were perpendicular to the lamellae in the long cones, providing a structural basis for the detection of polarisation of incident light. The lamellae in all the outer segments of the triple cones are arranged perpendicular to the long cell axis. In both species, the long and short cones from the ventro-temporal retina were slender and more densely packed, and the outer segments of the long cones lay far more sclerad compared with the outer segments of the bifid cones. Microspectrophotometry revealed that in both species the lateral components of the triple cones displayed a maximum absorbance wavelength (λ(max)) of approximately 502nm, while the short central components were more shortwave sensitive (λ(max)=475nm). The λ(max) of all long and short cones in the ventro-temporal zone was 492nm, compared to 502nm in other retinal regions. Anchovies are unique among vertebrates in that they contain clear structural basis for both colour and polarisation vision in the same retina.

Receptive field properties and intensity-response functions of polarization-sensitive neurons of the optic tubercle in gregarious and solitarious locusts

Many migrating insects rely on the plane of sky polarization as a cue to detect spatial directions. Desert locusts (Schistocerca gregaria), like other insects, perceive polarized light through specialized photoreceptors in a dorsal eye region. Desert locusts occur in two phases: a gregarious swarming phase, which migrates during the day, and a solitarious nocturnal phase. Neurons in a small brain area, the anterior optic tubercle (AOTu), are critically involved in processing polarized light in the locust brain. While polarization-sensitive intertubercle cells [lobula-tubercle neuron 1 (LoTu1) and tubercle-tubercle neuron 1 (TuTu1)] interconnect the AOTu of both hemispheres, tubercle-lateral accessory lobe tract (TuLAL1) neurons transmit sky compass signals to a polarization compass in the central brain. To better understand the neural network underlying polarized light processing in the AOTu and to investigate possible adaptations of the polarization vision system to a diurnal versus nocturnal lifestyle, we analyzed receptive field properties, intensity-response relationships, and daytime dependence of responses of AOTu neurons in gregarious and solitarious locusts. Surprisingly, no differences in the physiology of these neurons were found between the two locust phases. Instead, clear differences were observed between the different types of AOTu neurons. Whereas TuTu1 and TuLAL1 neurons encoded E-vector orientation independent of light intensity and would thus be operational in bright daylight, LoTu1 neurons were inhibited by high light intensity and provided strong polarization signaling only under dim light conditions. The presence of high- and low-intensity polarization channels might, therefore, allow solitarious and gregarious locusts to use the same polarization coding system despite their different activity cycles.

Glass scales on the wing of the swordtail butterfly Graphium sarpedon act as thin film polarizing reflectors

The wings of the swordtail butterfly Graphium sarpedon (the Common Bluebottle) have blue/green-colored patches that are covered on the underside by two types of scales: white and glass scales. Transmission and scanning electron microscopy revealed that the white scales are classically structured: the upper lamina, with prominent ridges and large open windows, is well separated by trabeculae from a flat, continuous lower lamina. In the glass scales, the upper lamina, with inconspicuous ridges and windows, is almost flat and closely apposed to the equally flat lower lamina. The glass scales thus approximate ideal thin films, in agreement with the observation that they reflect light directionally and are iridescent. Reflectance and transmittance spectra measured from the glass scales with a microspectrophotometer agree with spectra calculated for an ideal non-absorbing thin film. Imaging scatterometry of single, isolated glass scales demonstrated that the reflected light can be strongly polarized, indicating that they function as polarizing reflectors.

Evolution of eye morphology and rhodopsin expression in the Drosophila melanogaster species subgroup

A striking diversity of compound eye size and shape has evolved among insects. The number of ommatidia and their size are major determinants of the visual sensitivity and acuity of the compound eye. Each ommatidium is composed of eight photoreceptor cells that facilitate the discrimination of different colours via the expression of various light sensitive Rhodopsin proteins. It follows that variation in eye size, shape, and opsin composition is likely to directly influence vision. We analyzed variation in these three traits in D. melanogaster, D. simulans and D. mauritiana. We show that D. mauritiana generally has larger eyes than its sibling species, which is due to a combination of larger ommatidia and more ommatidia. In addition, intra- and inter-specific differences in eye size among D. simulans and D. melanogaster strains are mainly caused by variation in ommatidia number. By applying a geometric morphometrics approach to assess whether the formation of larger eyes influences other parts of the head capsule, we found that an increase in eye size is associated with a reduction in the adjacent face cuticle. Our shape analysis also demonstrates that D. mauritiana eyes are specifically enlarged in the dorsal region. Intriguingly, this dorsal enlargement is associated with enhanced expression of rhodopsin 3 in D. mauritiana. In summary, our data suggests that the morphology and functional properties of the compound eyes vary considerably within and among these closely related Drosophila species and may be part of coordinated morphological changes affecting the head capsule.

Segregation of visual inputs from different regions of the compound eye in two parallel pathways through the anterior optic tubercle of the bumblebee (Bombus ignitus)

Visually guided behaviors require the brain to extract features of the visual world and to integrate them in a context-specific manner. Hymenopteran insects have been prime models for ethological research into visual behaviors for decades but knowledge about the underlying central processing is very limited. This is particularly the case for sky-compass navigation. To learn more about central processing of visual information in general and specifically to reveal a possible polarization vision pathway in the bee brain, we used tracer injections to investigate the pathways through the anterior optic tubercle, a prominent output target of the insect optic lobe, in the bumblebee Bombus ignitus. The anterior optic tubercle of the bumblebee is a small neuropil of 200 μm width and is located dorsolateral to the antennal lobe at the anterior surface of the brain. It is divided into a larger upper and a smaller lower subunit, both of which receive input from the optic lobe and connect to the lateral accessory lobe, and the contralateral tubercle, via two parallel pathways. The lower subunit receives input from the dorsal rim area (DRA) of the compound eye. The bumblebee DRA shares structural similarities with polarization-sensitive DRAs of other insects and looks similar to that of honeybees. We identified several neurons within this pathway that could be homologous to identified polarization-sensitive neurons in the locust brain. We therefore conclude that the pathway through the lower subunit of the anterior optic tubercle could carry polarization information from the periphery to the central brain.

The retinal mosaics of opsin expression in invertebrates and vertebrates

Color vision is found in many invertebrate and vertebrate species. It is the ability to discriminate objects based on the wavelength of emitted light independent of intensity. As it requires the comparison of at least two photoreceptor types with different spectral sensitivities, this process is often mediated by a mosaic made of several photoreceptor types. In this review, we summarize the current knowledge about the formation of retinal mosaics and the regulation of photopigment (opsin) expression in the fly, mouse, and human retina. Despite distinct evolutionary origins, as well as major differences in morphology and phototransduction machineries, there are significant similarities in the stepwise cell-fate decisions that lead from progenitor cells to terminally differentiated photoreceptors that express a particular opsin. Common themes include (i) the use of binary transcriptional switches that distinguish classes of photoreceptors, (ii) the use of gradients of signaling molecules for regional specializations, (iii) stochastic choices that pattern the retina, and (iv) the use of permissive factors with multiple roles in different photoreceptor types.

The polarization compass dominates over idiothetic cues in path integration of desert ants

Desert ants, Cataglyphis, use the sky's pattern of polarized light as a compass reference for navigation. However, they do not fully exploit the complexity of this pattern, rather – as proposed previously – they assess their walking direction by means of an approximate solution based on a simplified internal template. Approximate rules are error-prone. We therefore asked whether the ants use additional cues to improve the accuracy of directional decisions, and focused on ‘idiothetic’ cues, i.e. cues based on information from proprioceptors. We trained ants in a channel system that was covered with a polarization filter, providing only a single e-vector direction as a directional ‘celestial’ cue. Then we observed their homebound runs on a test field, allowing full view of the sky. In crucial experiments, the ants were exposed to a cue conflict, in which sky compass and idiothetic information disagreed, by training them in a straight channel that provided a change in e-vector direction. The results indicated that the polarization information completely dominates over idiothetic cues. Two path segments with different e-vector orientations are combined linearly to a summed home vector. Our data provide additional evidence that Cataglyphis uses a simplified internal template to derive directional information from the sky's polarization pattern.

The monarch butterfly genome yields insights into long-distance migration

We present the draft 273 Mb genome of the migratory monarch butterfly (Danaus plexippus) and a set of 16,866 protein-coding genes. Orthology properties suggest that the Lepidoptera are the fastest evolving insect order yet examined. Compared to the silkmoth Bombyx mori, the monarch genome shares prominent similarity in orthology content, microsynteny, and protein family sizes. The monarch genome reveals a vertebrate-like opsin whose existence in insects is widespread; a full repertoire of molecular components for the monarch circadian clockwork; all members of the juvenile hormone biosynthetic pathway whose regulation shows unexpected sexual dimorphism; additional molecular signatures of oriented flight behavior; microRNAs that are differentially expressed between summer and migratory butterflies; monarch-specific expansions of chemoreceptors potentially important for long-distance migration; and a variant of the sodium/potassium pump that underlies a valuable chemical defense mechanism. The monarch genome enhances our ability to better understand the genetic and molecular basis of long-distance migration.

Genetic dissection reveals two separate retinal substrates for polarization vision in Drosophila

BACKGROUND

Linearly polarized light originates from atmospheric scattering or surface reflections and is perceived by insects, spiders, cephalopods, crustaceans, and some vertebrates. Thus, the neural basis underlying how this fundamental quality of light is detected is of broad interest. Morphologically unique, polarization-sensitive ommatidia exist in the dorsal periphery of many insect retinas, forming the dorsal rim area (DRA). However, much less is known about the retinal substrates of behavioral responses to polarized reflections.

SUMMARY

Drosophila exhibits polarotactic behavior, spontaneously aligning with the e-vector of linearly polarized light, when stimuli are presented either dorsally or ventrally. By combining behavioral experiments with genetic dissection and ultrastructural analyses, we show that distinct photoreceptors mediate the two behaviors: inner photoreceptors R7+R8 of DRA ommatidia are necessary and sufficient for dorsal polarotaxis, whereas ventral responses are mediated by combinations of outer and inner photoreceptors, both of which manifest previously unknown features that render them polarization sensitive.

CONCLUSIONS

Drosophila uses separate retinal pathways for the detection of linearly polarized light emanating from the sky or from shiny surfaces. This work establishes a behavioral paradigm that will enable genetic dissection of the circuits underlying polarization vision.

Three cone opsin genes determine the properties of the visual spectra in the Japanese anchovy Engraulis japonicus (Engraulidae, Teleostei)

A complement of cone visual pigments was identified in the Japanese anchovy Engraulis japonicus, one of the engraulid fish species that has a retina specialized for polarization and color vision. The nature of the chromophore bound to opsin proteins was investigated using high performance liquid chromatography (HPLC). The opsin genes were then cloned and sequenced, and the absorption spectra of different types of cones were obtained by microspectrophotometry (MSP). Two green (EJ-RH2-1, EJ-RH2-2) and one red (EJ-LWS) cone opsin genes were identified and are presumably related to the Vitamin A1-based visual pigments (i.e., rhodopsins) with λmax values of 492, 474 and 512 nm for EJ-RH2-1, EJ-RH2-2, and EJ-LWS, respectively. The long and short cones from the ventro-temporal retinal zone consisted of a pure population of RH2 class gene-based pigments (λmax value of 492 nm). The long and short cones from other retinal areas and the lateral components of the triple cones possessed a mixture of RH2 and LWS class gene-based pigments that exhibited a λmax value approximately 502 nm. The central component of the triple cones contained only RH2 class gene-based pigments (λmax value of 474 nm). Thus, E. japonicus possesses a middle-wave range of spectral sensitivity and acquires different color vision systems in distinct visual fields. .

The lizard celestial compass detects linearly polarized light in the blue

The present study first examined whether ruin lizards Podarcis sicula are able to orientate using plane polarized light produced by a LCD screen. Ruin lizards were trained and tested indoors, inside an hexagonal Morris water maze, positioned under the LCD screen producing white polarized light with a single E-vector, which provided an axial cue. White polarized light did not include wavelengths in the UV. Lizards orientated correctly either when tested with E-vector parallel to the training axis or after 90° rotation of the E-vector direction, and thus validating the apparatus. Further experiments examined whether in ruin lizards there is a preferential region of the light spectrum to perceive the E-vector direction of polarized light. For this purpose, lizards reaching learning criteria under white polarized light were subdivided into 4 experimental groups. Each group was respectively tested for orientation under a different spectrum of plane polarized light (named red, green, cyan and blue) with equalized photon flux density. Lizards tested under blue polarized light orientated correctly, whereas lizards tested under red polarized light were completely disoriented. Green polarized light was barely discernible by lizards, and thus insufficient for a correct functioning of their compass. When exposed to cyan polarized light, lizard orientation performances were optimal, indistinguishable from lizards detecting blue polarized light. Overall, the present results demonstrate that perception of linear polarization in the blue is necessary - and sufficient - for a proper functioning of the sky polarization compass of ruin lizards. This may be adaptively important, since detection of polarized light in the blue improves functioning of the polarization compass under cloudy skies, i.e. when the alternative celestial compass based on detection of the sun disk is rendered useless because the sun is obscured by clouds.

Flying Drosophila orient to sky polarization

Insects maintain a constant bearing across a wide range of spatial scales. Monarch butterflies and locusts traverse continents [1, 2], and foraging bees and ants travel hundreds of meters to return to their nests [1, 3, 4], whereas many other insects fly straight for only a few centimeters before changing direction. Despite this variation in spatial scale, the brain region thought to underlie long-distance navigation is remarkably conserved [5, 6], suggesting that the use of a celestial compass is a general and perhaps ancient capability of insects. Laboratory studies of Drosophila have identified a local search mode in which short, straight segments are interspersed with rapid turns [7, 8]. However, this flight mode is inconsistent with measured gene flow between geographically separated populations [9-11], and individual Drosophila can travel 10 km across desert terrain in a single night [9, 12, 13]-a feat that would be impossible without prolonged periods of straight flight. To directly examine orientation behavior under outdoor conditions, we built a portable flight arena in which a fly viewed the natural sky through a liquid crystal device that could experimentally rotate the polarization angle. Our findings indicate that Drosophila actively orient using the sky's natural polarization pattern.

A distinct layer of the medulla integrates sky compass signals in the brain of an insect

Mass migration of desert locusts is a common phenomenon in North Africa and the Middle East but how these insects navigate is still poorly understood. Laboratory studies suggest that locusts are able to exploit the sky polarization pattern as a navigational cue. Like other insects locusts detect polarized light through a specialized dorsal rim area (DRA) of the eye. Polarization signals are transmitted through the optic lobe to the anterior optic tubercle (AOTu) and, finally, to the central complex in the brain. Whereas neurons of the AOTu integrate sky polarization and chromatic cues in a daytime dependent manner, the central complex holds a topographic representation of azimuthal directions suggesting a role as an internal sky compass. To understand further the integration of sky compass cues we studied polarization-sensitive (POL) neurons in the medulla that may be intercalated between DRA photoreceptors and AOTu neurons. Five types of POL-neuron were characterized and four of these in multiple recordings. All neurons had wide arborizations in medulla layer 4 and most, additionally, in the dorsal rim area of the medulla and in the accessory medulla, the presumed circadian clock. The neurons showed type-specific orientational tuning to zenithal polarized light and azimuth tuning to unpolarized green and UV light spots. In contrast to neurons of the AOTu, we found no evidence for color opponency and daytime dependent adjustment of sky compass signals. Therefore, medulla layer 4 is a distinct stage in the integration of sky compass signals that precedes the time-compensated integration of celestial cues in the AOTu.

Spectral sensitivity of the principal eyes of sunburst diving beetle, Thermonectus marmoratus (Coleoptera: Dytiscidae), larvae

The principal eyes of sunburst diving beetle, Thermonectus marmoratus, larvae are among the most unusual eyes in the animal kingdom. They are composed of long tubes connecting bifocal lenses with two retinas: a distal retina situated a few hundred micrometers behind the lens, and a proximal retina that is situated directly beneath. A recent molecular study on first instar larvae suggests that the distal retina expresses a long-wavelength-sensitive opsin (TmLW), whereas the proximal retina predominantly expresses an ultraviolet-sensitive opsin (TmUV II). Using cloning and in situ hybridization we here confirm that this opsin distribution is, for the most part, maintained in third instar larvae (with the exception of the TmUV I that is weakly expressed only in proximal retinas of first instar larvae). We furthermore use intracellular electrophysiological recordings and neurobiotin injections to determine the spectral sensitivity of individual photoreceptor cells. We find that photoreceptors of the proximal retina have a sensitivity curve that peaks at 374-375 nm. The shape of the curve is consistent with the predicted absorbance of a single-opsin template. The spectral response of photoreceptors from the distal retina confirms their maximum sensitivity to green light with the dominant λ-peak between 520 and 540 nm, and the secondary β-peak between 340 and 360 nm. These physiological measurements support molecular predictions and represent important steps towards understanding the functional organization of the unusual stemmata of T. marmoratus larvae.

Central neural coding of sky polarization in insects

Many animals rely on a sun compass for spatial orientation and long-range navigation. In addition to the Sun, insects also exploit the polarization pattern and chromatic gradient of the sky for estimating navigational directions. Analysis of polarization-vision pathways in locusts and crickets has shed first light on brain areas involved in sky compass orientation. Detection of sky polarization relies on specialized photoreceptor cells in a small dorsal rim area of the compound eye. Brain areas involved in polarization processing include parts of the lamina, medulla and lobula of the optic lobe and, in the central brain, the anterior optic tubercle, the lateral accessory lobe and the central complex. In the optic lobe, polarization sensitivity and contrast are enhanced through convergence and opponency. In the anterior optic tubercle, polarized-light signals are integrated with information on the chromatic contrast of the sky. Tubercle neurons combine responses to the UV/green contrast and e-vector orientation of the sky and compensate for diurnal changes of the celestial polarization pattern associated with changes in solar elevation. In the central complex, a topographic representation of e-vector tunings underlies the columnar organization and suggests that this brain area serves as an internal compass coding for spatial directions.

Evidence for instantaneous e-vector detection in the honeybee using an associative learning paradigm

Many insects use the polarization pattern of the sky for obtaining compass information during orientation or navigation. E-vector information is collected by a specialized area in the dorsal-most part of the compound eye, the dorsal rim area (DRA). We tested honeybees' capability of learning certain e-vector orientations by using a classical conditioning paradigm with the proboscis extension reflex. When one e-vector orientation (CS+) was associated with sugar water, while another orientation (CS-) was not rewarded, the honeybees could discriminate CS+ from CS-. Bees whose DRA was inactivated by painting did not learn CS+. When ultraviolet (UV) polarized light (350 nm) was used for CS, the bees discriminated CS+ from CS-, but no discrimination was observed in blue (442 nm) or green light (546 nm). Our data indicate that honeybees can learn and discriminate between different e-vector orientations, sensed by the UV receptors of the DRA, suggesting that bees can determine their flight direction from polarized UV skylight during foraging. Fixing the bees' heads during the experiments did not prevent learning, indicating that they use an 'instantaneous' algorithm of e-vector detection; that is, the bees do not need to actively scan the sky with their DRAs ('sequential' method) to determine e-vector orientation.

Honeybees as a model for the study of visually guided flight, navigation, and biologically inspired robotics

Research over the past century has revealed the impressive capacities of the honeybee, Apis mellifera, in relation to visual perception, flight guidance, navigation, and learning and memory. These observations, coupled with the relative ease with which these creatures can be trained, and the relative simplicity of their nervous systems, have made honeybees an attractive model in which to pursue general principles of sensorimotor function in a variety of contexts, many of which pertain not just to honeybees, but several other animal species, including humans. This review begins by describing the principles of visual guidance that underlie perception of the world in three dimensions, obstacle avoidance, control of flight speed, and orchestrating smooth landings. We then consider how navigation over long distances is accomplished, with particular reference to how bees use information from the celestial compass to determine their flight bearing, and information from the movement of the environment in their eyes to gauge how far they have flown. Finally, we illustrate how some of the principles gleaned from these studies are now being used to design novel, biologically inspired algorithms for the guidance of unmanned aerial vehicles.

Polarization-sensitive descending neurons in the locust: connecting the brain to thoracic ganglia

Many animal species, in particular insects, exploit the E-vector pattern of the blue sky for sun compass navigation. Like other insects, locusts detect dorsal polarized light via photoreceptors in a specialized dorsal rim area of the compound eye. Polarized light information is transmitted through several processing stages to the central complex, a brain area involved in the control of goal-directed orientation behavior. To investigate how polarized light information is transmitted to thoracic motor circuits, we studied the responses of locust descending neurons to polarized light. Three sets of polarization-sensitive descending neurons were characterized through intracellular recordings from axonal fibers in the neck connectives combined with single-cell dye injections. Two descending neurons from the brain, one with ipsilaterally and the second with contralaterally descending axon, are likely to bridge the gap between polarization-sensitive neurons in the brain and thoracic motor centers. In both neurons, E-vector tuning changed linearly with daytime, suggesting that they signal time-compensated spatial directions, an important prerequisite for navigation using celestial signals. The third type connects the suboesophageal ganglion with the prothoracic ganglion. It showed no evidence for time compensation in E-vector tuning and might play a role in flight stabilization and control of head movements.

Sun compass integration of skylight cues in migratory monarch butterflies

Migrating monarch butterflies (Danaus plexippus) use a time-compensated sun compass to navigate from eastern North America to their overwintering grounds in central Mexico. Here we describe the neuronal layout of those aspects of the butterfly's central complex likely to establish part of the internal sun compass and find them highly homologous to those of the desert locust. Intracellular recordings from neurons in the monarch sun compass network reveal responses tuned to specific E-vector angles of polarized light, as well as azimuth-dependent responses to unpolarized light, independent of spectral composition. The neural responses to these two stimuli in individual neurons are mediated through different regions of the compound eye. Moreover, these dual responses are integrated to create a consistent representation of skylight cues in the sun compass throughout the day. The results advance our understanding of how ambiguous sensory signals are processed by the brain to elicit a robust behavioral response.

Always on the bright side of life: anti-adhesive properties of insect ommatidia grating

The surface of some insect eyes consists of arrays of cuticular protuberances, which are 50-300 nm in diameter, and are termed corneal nipples or ommatidia gratings. They were widely reported to reduce the reflectance for normally incident light, contributing to camouflage by reducing glare to predators, while furthermore enhancing the intake of light, which is especially important for nocturnal insects. Our preliminary observations suggest a third function: in contrast to the rest of the body, ommatidia of various insects remain clean, even in a heavy contaminated environment. In order to prove such an anti-contamination hypothesis of these structures, we measured the adhesive properties of polymer moulds of insect ommatidia, and compared these data with control surfaces having the same curvature radii but lacking such a nanostructure. A scanning electron microscope (SEM) study and force measurements using an atomic force microscope (AFM) on the eye surfaces of three different insect species, dragonfly Aeshna mixta (Odonata), moth Laothoe populi (Lepidoptera) and fly Volucella pellucens (Diptera), were undertaken. We revealed that adhesion is greatly reduced by corneal grating in L. populi and V. pellucens when compared with their smooth controls. The smooth cornea of A. mixta showed no statistically significant difference to its control. We assume that this anti-adhesive phenomenon is due to a decrease in the real contact area between contaminating particles and the eye's surface. Such a combination of three functions in one nanostructure can be interesting for the development of industrial multifunctional surfaces capable of enhancing light harvesting while reducing light reflection and adhesion.

Conditional Perception Under Stimulus Ambiguity: Polarization- and Azimuth-Sensitive Neurons in the Locust Brain Are Inhibited by Low Degrees of Polarization

Sensory perception often relies on the integration and matching of multisensory inputs. In the brain of desert locusts, identified neurons that signal the sun's direction relative to the animal's head integrate information about the polarization pattern of the sky with information on the color and intensity contrast of the sky. The cloudless blue sky exhibits a gradient from unpolarized sunlight to strongly polarized light at 90° from the sun. Therefore the percentage of polarized light in the sky is highest at dusk and dawn and lowest when the sun is in the zenith. We investigated the effect of different degrees of polarization on neurons of the anterior optic tubercle of the desert locust through intracellular recordings. Whereas dorsal presentation of strongly polarized light largely excited the neurons, weakly polarized light, i.e., a blend of polarized light of many orientations, led to inhibition. The data suggest that the polarization input to these neurons is inhibited within a radius of 50° around the sun, thereby avoiding conflicting input from the polarization and direct sunlight channels. These properties can be regarded as sensory filters to avoid ambiguous signaling during sky compass orientation.

Caste-specific visual adaptations to distinct daily activity schedules in Australian Myrmecia ants

Animals are active at different times of the day and their activity schedules are shaped by competition, time-limited food resources and predators. Different temporal niches provide different light conditions, which affect the quality of visual information available to animals, in particular for navigation. We analysed caste-specific differences in compound eyes and ocelli in four congeneric sympatric species of Myrmecia ants, with emphasis on within-species adaptive flexibility and daily activity rhythms. Each caste has its own lifestyle: workers are exclusively pedestrian; alate females lead a brief life on the wing before becoming pedestrian; alate males lead a life exclusively on the wing. While workers of the four species range from diurnal, diurnal-crepuscular, crepuscular-nocturnal to nocturnal, the activity times of conspecific alates do not match in all cases. Even within a single species, we found eye area, facet numbers, facet sizes, rhabdom diameters and ocelli size to be tuned to the distinct temporal niche each caste occupies. We discuss these visual adaptations in relation to ambient light levels, visual tasks and mode of locomotion.

Evidence for the possible existence of a second polarization-vision pathway in the locust brain

For spatial orientation and navigation, many insects derive compass information from the polarization pattern of the blue sky. The desert locust Schistocerca gregaria detects polarized light with a specialized dorsal rim area of its compound eye. In the locust brain, polarized-light signals are passed through the anterior optic tract and tubercle to the central complex which most likely serves as an internal sky compass. Here, we suggest that neurons of a second visual pathway, via the accessory medulla and posterior optic tubercle, also provide polarization information to the central complex. Intracellular recordings show that two types of neuron in this posterior pathway are sensitive to polarized light. One cell type connects the dorsal rim area of the medulla with the medulla and accessory medulla, and a second type connects the bilaterally paired posterior optic tubercles. Given the evidence for a role of the accessory medulla as the master clock controlling circadian changes in behavioral activity in flies and cockroaches, our data open the possibility that time-compensated polarized-light signals may reach the central complex via this pathway for time-compensated sky-compass navigation.

Polarizing optics in a spider eye

Many arthropods including insects and spiders exploit skylight polarization for navigation. One of the four eye pairs of the spider Drassodes cupreus is dedicated to detect skylight polarization. These eyes are equipped with a tapetum that strongly plane-polarizes reflected light. This effectively enhances the polarization-sensitivity of the photoreceptors, improving orientation performance. With a multidisciplinary approach, we demonstrate that D. cupreus exploits reflective elements also present in non-polarizing tapetal eyes of other species such as Agelena labyrinthica. By approximately orthogonal arrangement of two multilayer reflectors consisting of reflecting guanine platelets, the tapetum uses the mechanism of polarization by reflection for polarizing reflected light.