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

A Bio-Inspired Polarization Sensor with High Outdoor Accuracy and Central-Symmetry Calibration Method with Integrating Sphere

A bio-inspired polarization sensor with lenses for navigation was evaluated in this study. Two new calibration methods are introduced, referred to as "central-symmetry calibration" (with an integrating sphere) and "noncontinuous calibration". A comparison between the indoor calibration results obtained from different calibration methods shows that the two proposed calibration methods are more effective. The central-symmetry calibration method optimized the nonconstant calibration voltage deviations, caused by the off-axis feature of the integrating sphere, to be constant values which can be calibrated easily. The section algorithm proposed previously showed no experimental advantages until the central-symmetry calibration method was proposed. The outdoor experimental results indicated that the indoor calibration parameters did not perform very well in practice outdoor conditions. To establish the reason, four types of calibration parameters were analyzed using the replacement method. It can be concluded that three types can be easily calibrated or affect the sensor accuracy slightly. However, before the sensor is used outdoors every time, the last type must be replaced with the corresponding outdoor parameter, and the calculation needs a precise rotary table. This parameter, which is mainly affected by the spectrum of incident light, is the main factor determining the sensor accuracy. After calibration, the sensor reaches an indoor accuracy of ±0.009° and a static outdoor accuracy of ±0.05° under clear sky conditions. The dynamic outdoor experiment shows a ±0.5° heading deviation between the polarization sensor and the inertial navigation system with a ±0.06° angular accuracy.

Ocellar structure of African and Australian desert ants

Few walking insects possess simple eyes known as the ocelli. The role of the ocelli in walking insects such as ants has been less explored. Physiological and behavioural evidence in the desert ant, Cataglyphis bicolor, indicates that ocellar receptors are polarisation sensitive and are used to derive compass information from the pattern of polarised skylight. The ability to detect polarised skylight can also be inferred from the structure and the organisation of the ocellar retina. However, the functional anatomy of the desert ant ocelli has not been investigated. Here we characterised the anatomical organisation of the ocelli in three species of desert ants. The two congeneric species of Cataglyphis we studied had a fused rhabdom, but differed in their organisation of the retina. In Cataglyphis bicolor, each retinula cell contributed microvilli in one orientation enabling them to compare e-vector intensities. In Cataglyphis fortis, some retinula cells contributed microvilli in more than one orientation, indicating that not all cells are polarisation sensitive. The desert ant Melophorus bagoti had an unusual ocellar retina with a hexagonal or pentagonal rhabdomere arrangement forming an open rhabdom. Each retinula cell contributed microvilli in more than one orientation, making them unlikely to be polarisation detectors.

From skylight input to behavioural output: A computational model of the insect polarised light compass

Many insects navigate by integrating the distances and directions travelled on an outward path, allowing direct return to the starting point. Fundamental to the reliability of this process is the use of a neural compass based on external celestial cues. Here we examine how such compass information could be reliably computed by the insect brain, given realistic constraints on the sky polarisation pattern and the insect eye sensor array. By processing the degree of polarisation in different directions for different parts of the sky, our model can directly estimate the solar azimuth and also infer the confidence of the estimate. We introduce a method to correct for tilting of the sensor array, as might be caused by travel over uneven terrain. We also show that the confidence can be used to approximate the change in sun position over time, allowing the compass to remain fixed with respect to 'true north' during long excursions. We demonstrate that the compass is robust to disturbances and can be effectively used as input to an existing neural model of insect path integration. We discuss the plausibility of our model to be mapped to known neural circuits, and to be implemented for robot navigation.

Bio-inspired orientation using the polarization pattern in the sky based on artificial neural networks

Many insects use the pattern of polarized light in the sky as a navigational cue. In this study, we use this sensory ability as a source of inspiration to create a computational orientation model based on an artificial neural network (POL-ANN). After a training phase using numerically generated sky polarization patterns, stable and convergent networks are obtained. We undertook a series of verification tests using four typical but different sky conditions and showed that the post-trained networks were able to make an accurate prediction of the direction of the sun. Comparisons between the proposed models and models based on the convolutional neural network (CNN) structure revealed the merits of the bio-inspired architecture. We further investigated the accuracy of the models based on two different (locust-like, broader; Drosophila-like, narrower) visual fields of the sky. We find that the accuracy of the computations depends on the overhead visual scene, specifically that wider fields of view perform better when information about the overhead polarization pattern is missing.

Two Compasses in the Central Complex of the Locust Brain

Many migratory insects rely on a celestial compass for spatial orientation. Several features of the daytime sky, all generated by the sun, can be exploited for navigation. Two of these are the position of the sun and the pattern of polarized skylight. Neurons of the central complex (CX), a group of neuropils in the central brain of insects, have been shown to encode sky compass cues. In desert locusts, the CX holds a topographic, compass-like representation of the plane of polarized light (E-vector) presented from dorsal direction. In addition, these neurons also encode the azimuth of an unpolarized light spot, likely representing the sun. Here, we investigate whether, in addition to E-vector orientation, the solar azimuth is represented topographically in the CX. We recorded intracellularly from eight types of CX neuron while stimulating animals of either sex with polarized blue light from zenithal direction and an unpolarized green light spot rotating around the animal's head at different elevations. CX neurons did not code for elevation of the unpolarized light spot. However, two types of columnar neuron showed a linear correlation between innervated slice in the CX and azimuth tuning to the unpolarized green light spot, consistent with an internal compass representation of solar azimuth. Columnar outputs of the CX also showed a topographic representation of zenithal E-vector orientation, but the two compasses were not linked to each other. Combined stimulation with unpolarized green and polarized blue light suggested that the two compasses interact in a nonlinear way.SIGNIFICANCE STATEMENT In the brain of the desert locust, neurons sensitive to the plane of celestial polarization are arranged like a compass in the slices of the central complex (CX). These neurons, in addition, code for the horizontal direction of an unpolarized light cue possibly representing the sun. We show here that horizontal directions are, in addition to E-vector orientations from the dorsal direction, represented in a compass-like manner across the slices of the CX. However, the two compasses are not linked to each other, but rather seem to interact in a cell-specific, nonlinear way. Our study confirms the role of the CX in signaling heading directions and shows that different cues are used for this task.

Bumblebee visual allometry results in locally improved resolution and globally improved sensitivity

The quality of visual information that is available to an animal is limited by the size of its eyes. Differences in eye size can be observed even between closely related individuals, yet we understand little about how this affects vision. Insects are good models for exploring the effects of size on visual systems because many insect species exhibit size polymorphism. Previous work has been limited by difficulties in determining the 3D structure of eyes. We have developed a novel method based on x-ray microtomography to measure the 3D structure of insect eyes and to calculate predictions of their visual capabilities. We used our method to investigate visual allometry in the bumblebee Bombus terrestris and found that size affects specific aspects of vision, including binocular overlap, optical sensitivity, and dorsofrontal visual resolution. This reveals that differential scaling between eye areas provides flexibility that improves the visual capabilities of larger bumblebees.

Bees fly through complex environments in search of nectar from flowers. They are aided in this quest by excellent eyesight. Scientists have extensively studied the eyesight of honeybees to learn more about how such tiny eyes work and how they process and learn visual information. Less is known about the honeybee’s larger cousins, the bumblebees, which are also important pollinators. Bumblebees come in different sizes and one question scientists have is how eye size affects vision.

Bigger bumblebees are known to have bigger eyes, and bigger eyes are usually better. But which aspects of vision are improved in larger eyes is not clear. For example, does the size of a bee’s eyes affect how large their field of view is, or how sensitive they are to light? Or does it impact their visual acuity, a measurement of the smallest objects the eye can see? Scaling up an eye would likely improve all these aspects of sight slightly, but changes in a small area of the eye might more drastically improve some parts of vision.

Now, Taylor et al. show that larger bumblebees with bigger eyes have better vision than their smaller counterparts. In the experiments, a technique called microtomography was used to measure the 3D structure of bumblebee eyes. The measurements were then applied to build 3D models of the bumblebee eyes, and computational geometry was used to calculate the sensitivity, acuity, and viewing direction across the entire surface of each model eye. Taylor et al. found that larger bees had improved ability to see small objects in front or slightly above them. They had a bigger area of overlap between the sight in both eyes when they looked forward and up. They were also more sensitive to light across the eye. The experiments show that improvements in eyesight with larger size are very specific and likely help larger bees to adapt to their environment.

Behavioral studies could help scientists better understand how these changes help bigger bees and how the traits evolved. These findings might also help engineers trying to design miniature cameras to help small, flying autonomous vehicles navigate. Bees fly through complex environments and face challenges similar to those small flying vehicles would face. Emulating the design of bee eyes and how they change with size might lead to the development of better cameras for these vehicles.

Celestial navigation in Drosophila

Many casual observers typecast Drosophila melanogaster as a stationary pest that lurks around fruit and wine. However, the omnipresent fruit fly, which thrives even in desert habitats, likely established and maintained its cosmopolitan status via migration over large spatial scales. To perform long-distance dispersal, flies must actively maintain a straight compass heading through the use of external orientation cues, such as those derived from the sky. In this Review, we address how D. melanogaster accomplishes long-distance navigation using celestial cues. We focus on behavioral and physiological studies indicating that fruit flies can navigate both to a pattern of linearly polarized light and to the position of the sun - the same cues utilized by more heralded insect navigators such as monarch butterflies and desert ants. In both cases, fruit flies perform menotaxis, selecting seemingly arbitrary headings that they then maintain over time. We discuss how the fly's nervous system detects and processes this sensory information to direct the steering maneuvers that underlie navigation. In particular, we highlight recent findings that compass neurons in the central complex, a set of midline neuropils, are essential for navigation. Taken together, these results suggest that fruit flies share an ancient, latent capacity for celestial navigation with other insects. Furthermore, they illustrate the potential of D. melanogaster to help us to elucidate both the cellular basis of navigation and mechanisms of directed dispersal on a landscape scale.

Polarized skylight-based heading measurements: a bio-inspired approach

Many insects such as desert ants, crickets, locusts, dung beetles, bees and monarch butterflies have been found to extract their navigation cues from the regular pattern of the linearly polarized skylight. These species are equipped with ommatidia in the dorsal rim area of their compound eyes, which are sensitive to the angle of polarization of the skylight. In the polarization-based robotic vision, most of the sensors used so far comprise high-definition CCD or CMOS cameras topped with linear polarizers. Here, we present a 2-pixel polarization-sensitive visual sensor, which was strongly inspired by the dorsal rim area of desert ants' compound eyes, designed to determine the direction of polarization of the skylight. The spectral sensitivity of this minimalistic sensor, which requires no lenses, is in the ultraviolet range. Five different methods of computing the direction of polarization were implemented and tested here. Our own methods, the extended and AntBot method, outperformed the other three, giving a mean angular error of only 0.62° ± 0.40° (median: 0.24°) and 0.69° ± 0.52° (median: 0.39°), respectively (mean ± standard deviation). The results obtained in outdoor field studies show that our celestial compass gives excellent results at a very low computational cost, which makes it highly suitable for autonomous outdoor navigation purposes.

Anatomy of the stemmata in the Photuris firefly larva

Fireflies (Coleoptera: Lampyridae) have distinct visual systems at different stages of development. Larvae have stemmata and adults have compound eyes. Adults use compound eyes to mediate photic communication during courtship. Larvae do not manifest this behavior, yet they are bioluminescent. We investigated the structure of stemmata in Photuris firefly larvae to identify anatomical substrates (i.e., rhabdomeres) conferring visual function. Stemmata were located bilaterally on the antero-lateral surfaces of the head. Beneath the ~ 130 µm diameter lens, we identified a pigmented eye-cup. At its widest point, the eye-cup was ~ 150 µm in diameter. The optic nerve exited the eye-cup opposite the lens. Two distinct regions, asymmetric in size and devoid of pigmentation, were characterized in stemmata cross-sections. We refer to these regions as lobes. Each lobe contained a rhabdom of a radial network of rhabdomeres. Pairs of rhabdomeres formed interdigitating microvilli contributed from neighboring photoreceptor cell bodies. The optic nerve contained 88 axons separable into two populations based on size. The number of axons in the optic nerve together with distinct rhabdoms suggests these structures were formed from ‘fusion stemmata.’ This structural specialization provides an anatomical substrate for future studies of visually mediated behaviors in Photuris larvae.

Three-dimensional ultrastructural organization of the ommatidium of the minute parasitoid wasp Trichogramma evanescens

Existing information on insect compound eyes is mainly limited to two-dimensional information derived from histological or ultrathin sections. These allow a basic description of eye morphology, but are limited in z-axis resolution because of the section thickness or intervals between sections, so that accurate volumetric information cannot be generated. Here we use serial-sectioning transmission electron microscopy to present a 3-D reconstruction at ultrastructural level of a complete ommatidium of a miniaturized insect compound eye. Besides the general presentation of the three dimensional arrangement of the different cell types within the ommatidium, the reconstruction allowed volumetric measurements and numerical analyses to be undertaken, revealing new insights into the number, size and distribution of cell organelles in insect ommatidia. Morphological features that can be related to miniaturization, namely the dimensions and displacement of nuclei, reduction of average pigment granule volume and loss of pigment granules in the terminals of the cone cells, the impact of metabolic activity of cell types on miniaturization, as well as maintenance of rhabdomere volume and limits to its miniaturization, are all discussed.

Ultrastructure of the single-chamber stemmata of Arge pagana (Panzer, 1798) (Hymenoptera: Argidae)

Stemmata are peculiar visual organs of most larvae in holometabolous insects. In Hymenoptera, Symphyta larvae exclusively possess a pair of stemmata, whose cellular organizations have not been thoroughly elucidated to date. In this paper, the morphology and fine structure of stemmata were investigated in the large rose sawfly Arge pagana (Panzer, 1798) using light and electron microscopy. The larvae possess a pair of stemmata, which belong to the "unicorneal composite eye" or single-chamber stemmata. Each stemma is composed of a biconvex cornea lens, a layer of corneagenous cells, numerous pigment cells, and hundreds of retinula cells. According to the number of retinula cells forming a rhabdom, the stemma can be divided into two regions, the larger Region I and the smaller Region II. The former occupies the largest area of the stemma and contains the majority of rhabdoms, each of which is formed by the rhabdomeres of eight retinula cells. The latter occupies a narrow posterior margin, where each rhabdom consists of nine retinula cells. Based on the different cellular organizations of rhabdoms, the stemma of Argidae is likely developed by the fusion of two types of ommatidial units.

The giant butterfly-moth Paysandisia archon has spectrally rich apposition eyes with unique light-dependent photoreceptor dynamics

The palm borer moth Paysandisia archon (Burmeister, 1880) (fam. Castniidae) is a large, diurnally active palm pest. Its compound eyes consist of ~ 20,000 ommatidia and have apposition optics with interommatidial angles below 1°. The ommatidia contain nine photoreceptor cells and appear structurally similar to those in nymphalid butterflies. Two morphological ommatidial types were identified. Using the butterfly numbering scheme, in type I ommatidia, the distal rhabdom consists exclusively of the rhabdomeres of photoreceptors R1–2; the medial rhabdom has contributions from R1–8. The rhabdom in type II ommatidia is distally split into two sub-rhabdoms, with contributions from photoreceptors R2, R3, R5, R6 and R1, R4, R7, R8, respectively; medially, only R3–8 and not R1–2 contribute to the fused rhabdom. In both types, the pigmented bilobed photoreceptors R9 contribute to the rhabdom basally. Their nuclei reside in one of the lobes. Upon light adaptation, in both ommatidial types, the rhabdoms secede from the crystalline cones and pigment granules invade the gap. Intracellular recordings identified four photoreceptor classes with peak sensitivities in the ultraviolet, blue, green and orange wavelength regions (at 360, 465, 550, 580 nm, respectively). We discuss the eye morphology and optics, the photoreceptor spectral sensitivities, and the adaptation to daytime activity from a phylogenetic perspective.

How to Navigate in Different Environments and Situations: Lessons From Ants

Ants are a globally distributed insect family whose members have adapted to live in a wide range of different environments and ecological niches. Foraging ants everywhere face the recurring challenge of navigating to find food and to bring it back to the nest. More than a century of research has led to the identification of some key navigational strategies, such as compass navigation, path integration, and route following. Ants have been shown to rely on visual, olfactory, and idiothetic cues for navigational guidance. Here, we summarize recent behavioral work, focusing on how these cues are learned and stored as well as how different navigational cues are integrated, often between strategies and even across sensory modalities. Information can also be communicated between different navigational routines. In this way, a shared toolkit of fundamental navigational strategies can lead to substantial flexibility in behavioral outcomes. This allows individual ants to tune their behavioral repertoire to different tasks (e.g., foraging and homing), lifestyles (e.g., diurnal and nocturnal), or environments, depending on the availability and reliability of different guidance cues. We also review recent anatomical and physiological studies in ants and other insects that have started to reveal neural correlates for specific navigational strategies, and which may provide the beginnings of a truly mechanistic understanding of navigation behavior.

A light-dependent magnetoreception mechanism insensitive to light intensity and polarization

Billions of migratory birds navigate thousands of kilometres every year aided by a magnetic compass sense, the biophysical mechanism of which is unclear. One leading hypothesis is that absorption of light by specialized photoreceptors in the retina produces short-lived chemical intermediates known as radical pairs whose chemistry is sensitive to tiny magnetic interactions. A potentially serious but largely ignored obstacle to this theory is how directional information derived from the Earth's magnetic field can be separated from the much stronger variations in the intensity and polarization of the incident light. Here we propose a simple solution in which these extraneous effects are cancelled by taking the ratio of the signals from two neighbouring populations of magnetoreceptors. Geometric and biological arguments are used to derive a set of conditions that make this possible. We argue that one likely location of the magnetoreceptor molecules would be in association with ordered opsin dimers in the membrane discs of the outer segments of double-cone photoreceptor cells.

Bioinspired polarization vision enables underwater geolocalization

With its never-ending blue color, the underwater environment often seems monotonic and featureless. However, to an animal with polarization-sensitive vision, it is anything but bland. The rich repertoire of underwater polarization patterns-a consequence of light's air-to-water transmission and in-water scattering-can be exploited both as a compass and for geolocalization purposes. We demonstrate that, by using a bioinspired polarization-sensitive imager, we can determine the geolocation of an observer based on radial underwater polarization patterns. Our experimental data, recorded at various locations around the world, at different depths and times of day, indicate that the average accuracy of our geolocalization is 61 km, or 6 m of error for every 1 km traveled. This proof-of-concept study of our bioinspired technique opens new possibilities in long-distance underwater navigation and suggests additional mechanisms by which marine animals with polarization-sensitive vision might perform both local and long-distance navigation.

Diversity and common themes in the organization of ocelli in Hymenoptera, Odonata and Diptera

We show in a comparative analysis that distinct retinal specializations in insect ocelli are much more common than previously realized and that the rhabdom organization of ocellar photoreceptors is extremely diverse. Hymenoptera, Odonata and Diptera show prominent equatorial fovea-like indentations of the ocellar retinae, where distal receptor endings are furthest removed from the lens surface and receptor densities are highest. In contrast, rhabdomere arrangements are very diverse across insect groups: in Hymenoptera, with some exceptions, pairs of ocellar retinular cells form sheet-like rhabdoms that form elongated rectangular shapes in cross-section, with highly aligned microvilli directions perpendicular to the long axis of cross-sections. This arrangement makes most ocellar retinular cells in Hymenoptera sensitive to the direction of polarized light. In dragonflies, triplets of retinular cells form a y-shaped fused rhabdom with microvilli directions oriented at 60° to each other. In Dipteran ocellar retinular cells microvilli directions are randomised, which destroys polarization sensitivity. We suggest that the differences in ocellar organization between insect groups may reflect the different head attitude control systems that have evolved in these insect groups, but possibly also differences in the mode of locomotion and in the need for celestial compass information.

Polarisation vision: overcoming challenges of working with a property of light we barely see

In recent years, the study of polarisation vision in animals has seen numerous breakthroughs, not just in terms of what is known about the function of this sensory ability, but also in the experimental methods by which polarisation can be controlled, presented and measured. Once thought to be limited to only a few animal species, polarisation sensitivity is now known to be widespread across many taxonomic groups, and advances in experimental techniques are, in part, responsible for these discoveries. Nevertheless, its study remains challenging, perhaps because of our own poor sensitivity to the polarisation of light, but equally as a result of the slow spread of new practices and methodological innovations within the field. In this review, we introduce the most important steps in designing and calibrating polarised stimuli, within the broader context of areas of current research and the applications of new techniques to key questions. Our aim is to provide a constructive guide to help researchers, particularly those with no background in the physics of polarisation, to design robust experiments that are free from confounding factors.

Insect Responses to Linearly Polarized Reflections: Orphan Behaviors Without Neural Circuits

The e-vector orientation of linearly polarized light represents an important visual stimulus for many insects. Especially the detection of polarized skylight by many navigating insect species is known to improve their orientation skills. While great progress has been made towards describing both the anatomy and function of neural circuit elements mediating behaviors related to navigation, relatively little is known about how insects perceive non-celestial polarized light stimuli, like reflections off water, leaves, or shiny body surfaces. Work on different species suggests that these behaviors are not mediated by the “Dorsal Rim Area” (DRA), a specialized region in the dorsal periphery of the adult compound eye, where ommatidia contain highly polarization-sensitive photoreceptor cells whose receptive fields point towards the sky. So far, only few cases of polarization-sensitive photoreceptors have been described in the ventral periphery of the insect retina. Furthermore, both the structure and function of those neural circuits connecting to these photoreceptor inputs remain largely uncharacterized. Here we review the known data on non-celestial polarization vision from different insect species (dragonflies, butterflies, beetles, bugs and flies) and present three well-characterized examples for functionally specialized non-DRA detectors from different insects that seem perfectly suited for mediating such behaviors. Finally, using recent advances from circuit dissection in Drosophila melanogaster, we discuss what types of potential candidate neurons could be involved in forming the underlying neural circuitry mediating non-celestial polarization vision.

The Fly Sensitizing Pigment Enhances UV Spectral Sensitivity While Preventing Polarization-Induced Artifacts

Microvillar photoreceptors are intrinsically capable of detecting the orientation of e-vector of linearly polarized light. They provide most invertebrates with an additional sensory channel to detect important features of their visual environment. However, polarization sensitivity (PS) of photoreceptors may lead to the detection of polarization-induced false colors and intensity contrasts. Most insect photoreceptors are thus adapted to have minimal PS. Flies have twisted rhabdomeres with microvilli rotated along the length of the ommatidia to reduce PS. The additional UV-absorbing sensitizing pigment on their opsin minimizes PS in the ultraviolet. We recorded voltage from Drosophila photoreceptors R1-6 to measure the spectral dependence of PS and found that PS in the UV is invariably negligible but can be substantial above 400 nm. Using modeling, we demonstrate that in R1-6 without the sensitizing pigment, PS in the UV (PS UV ) would exceed PS in the visible part of the spectrum (PS VIS ) by a factor PS UV /PS VIS = 1.2-1.8, as lower absorption of Rh1 rhodopsin reduces self-screening. We use polarimetric imaging of objects relevant to fly polarization vision to show that their degree of polarization outdoors is highest in the short-wavelength part of the spectrum. Thus, under natural illumination, the sensitizing pigment in R1-6 renders even those cells with high PS in the visible part unsuitable for proper polarization vision. We assume that fly ventral polarization vision can be mediated by R7 alone, with R1-6 serving as an unpolarized reference channel.

Orthogonal vector algorithm to obtain the solar vector using the single-scattering Rayleigh model

Information obtained from a polarization pattern in the sky provides many animals like insects and birds with vital long-distance navigation cues. The solar vector can be derived from the polarization pattern using the single-scattering Rayleigh model. In this paper, an orthogonal vector algorithm, which utilizes the redundancy of the single-scattering Rayleigh model, is proposed. We use the intersection angles between the polarization vectors as the main criteria in our algorithm. The assumption that all polarization vectors can be considered coplanar is used to simplify the three-dimensional (3D) problem with respect to the polarization vectors in our simulation. The surface-normal vector of the plane, which is determined by the polarization vectors after translation, represents the solar vector. Unfortunately, the two-directionality of the polarization vectors makes the resulting solar vector ambiguous. One important result of this study is, however, that this apparent disadvantage has no effect on the complexity of the algorithm. Furthermore, two other universal least-squares algorithms were investigated and compared. A device was then constructed, which consists of five polarized-light sensors as well as a 3D attitude sensor. Both the simulation and experimental data indicate that the orthogonal vector algorithms, if used with a suitable threshold, perform equally well or better than the other two algorithms. Our experimental data reveal that if the intersection angles between the polarization vectors are close to 90°, the solar-vector angle deviations are small. The data also support the assumption of coplanarity. During the 51 min experiment, the mean of the measured solar-vector angle deviations was about 0.242°, as predicted by our theoretical model.

The Role of Celestial Compass Information in Cataglyphis Ants during Learning Walks and for Neuroplasticity in the Central Complex and Mushroom Bodies

Central place foragers are faced with the challenge to learn the position of their nest entrance in its surroundings, in order to find their way back home every time they go out to search for food. To acquire navigational information at the beginning of their foraging career, Cataglyphis noda performs learning walks during the transition from interior worker to forager. These small loops around the nest entrance are repeatedly interrupted by strikingly accurate back turns during which the ants stop and precisely gaze back to the nest entrance—presumably to learn the landmark panorama of the nest surroundings. However, as at this point the complete navigational toolkit is not yet available, the ants are in need of a reference system for the compass component of the path integrator to align their nest entrance-directed gazes. In order to find this directional reference system, we systematically manipulated the skylight information received by ants during learning walks in their natural habitat, as it has been previously suggested that the celestial compass, as part of the path integrator, might provide such a reference system. High-speed video analyses of distinct learning walk elements revealed that even exclusion from the skylight polarization pattern, UV-light spectrum and the position of the sun did not alter the accuracy of the look back to the nest behavior. We therefore conclude that C. noda uses a different reference system to initially align their gaze directions. However, a comparison of neuroanatomical changes in the central complex and the mushroom bodies before and after learning walks revealed that exposure to UV light together with a naturally changing polarization pattern was essential to induce neuroplasticity in these high-order sensory integration centers of the ant brain. This suggests a crucial role of celestial information, in particular a changing polarization pattern, in initially calibrating the celestial compass system.

Polarized light use in the nocturnal bull ant, Myrmecia midas

Solitary foraging ants have a navigational toolkit, which includes the use of both terrestrial and celestial visual cues, allowing individuals to successfully pilot between food sources and their nest. One such celestial cue is the polarization pattern in the overhead sky. Here, we explore the use of polarized light during outbound and inbound journeys and with different home vectors in the nocturnal bull ant, Myrmecia midas. We tested foragers on both portions of the foraging trip by rotating the overhead polarization pattern by ±45°. Both outbound and inbound foragers responded to the polarized light change, but the extent to which they responded to the rotation varied. Outbound ants, both close to and further from the nest, compensated for the change in the overhead e-vector by about half of the manipulation, suggesting that outbound ants choose a compromise heading between the celestial and terrestrial compass cues. However, ants returning home compensated for the change in the e-vector by about half of the manipulation when the remaining home vector was short (1-2 m) and by more than half of the manipulation when the remaining vector was long (more than 4 m). We report these findings and discuss why weighting on polarization cues change in different contexts.

Anatomical Reconstruction and Functional Imaging Reveal an Ordered Array of Skylight Polarization Detectors in Drosophila

Many insects exploit skylight polarization as a compass cue for orientation and navigation. In the fruit fly, Drosophila melanogaster, photoreceptors R7 and R8 in the dorsal rim area (DRA) of the compound eye are specialized to detect the electric vector (e-vector) of linearly polarized light. These photoreceptors are arranged in stacked pairs with identical fields of view and spectral sensitivities, but mutually orthogonal microvillar orientations. As in larger flies, we found that the microvillar orientation of the distal photoreceptor R7 changes in a fan-like fashion along the DRA. This anatomical arrangement suggests that the DRA constitutes a detector for skylight polarization, in which different e-vectors maximally excite different positions in the array. To test our hypothesis, we measured responses to polarized light of varying e-vector angles in the terminals of R7/8 cells using genetically encoded calcium indicators. Our data confirm a progression of preferred e-vector angles from anterior to posterior in the DRA, and a strict orthogonality between the e-vector preferences of paired R7/8 cells. We observed decreased activity in photoreceptors in response to flashes of light polarized orthogonally to their preferred e-vector angle, suggesting reciprocal inhibition between photoreceptors in the same medullar column, which may serve to increase polarization contrast. Together, our results indicate that the polarization-vision system relies on a spatial map of preferred e-vector angles at the earliest stage of sensory processing.

SIGNIFICANCE STATEMENT

The fly's visual system is an influential model system for studying neural computation, and much is known about its anatomy, physiology, and development. The circuits underlying motion processing have received the most attention, but researchers are increasingly investigating other functions, such as color perception and object recognition. In this work, we investigate the early neural processing of a somewhat exotic sense, called polarization vision. Because skylight is polarized in an orientation that is rigidly determined by the position of the sun, this cue provides compass information. Behavioral experiments have shown that many species use the polarization pattern in the sky to direct locomotion. Here we describe the input stage of the fly's polarization-vision system.

Interaction of compass sensing and object-motion detection in the locust central complex

Goal-directed behavior is often complicated by unpredictable events, such as the appearance of a predator during directed locomotion. This situation requires adaptive responses like evasive maneuvers followed by subsequent reorientation and course correction. Here we study the possible neural underpinnings of such a situation in an insect, the desert locust. As in other insects, its sense of spatial orientation strongly relies on the central complex, a group of midline brain neuropils. The central complex houses sky compass cells that signal the polarization plane of skylight and thus indicate the animal's steering direction relative to the sun. Most of these cells additionally respond to small moving objects that drive fast sensory-motor circuits for escape. Here we investigate how the presentation of a moving object influences activity of the neurons during compass signaling. Cells responded in one of two ways: in some neurons, responses to the moving object were simply added to the compass response that had adapted during continuous stimulation by stationary polarized light. By contrast, other neurons disadapted, i.e., regained their full compass response to polarized light, when a moving object was presented. We propose that the latter case could help to prepare for reorientation of the animal after escape. A neuronal network based on central-complex architecture can explain both responses by slight changes in the dynamics and amplitudes of adaptation to polarized light in CL columnar input neurons of the system.NEW & NOTEWORTHY Neurons of the central complex in several insects signal compass directions through sensitivity to the sky polarization pattern. In locusts, these neurons also respond to moving objects. We show here that during polarized-light presentation, responses to moving objects override their compass signaling or restore adapted inhibitory as well as excitatory compass responses. A network model is presented to explain the variations of these responses that likely serve to redirect flight or walking following evasive maneuvers.

Optimizing the use of a sensor resource for opponent polarization coding

Flies use specialized photoreceptors R7 and R8 in the dorsal rim area (DRA) to detect skylight polarization. R7 and R8 form a tiered waveguide (central rhabdomere pair, CRP) with R7 on top, filtering light delivered to R8. We examine how the division of a given resource, CRP length, between R7 and R8 affects their ability to code polarization angle. We model optical absorption to show how the length fractions allotted to R7 and R8 determine the rates at which they transduce photons, and correct these rates for transduction unit saturation. The rates give polarization signal and photon noise in R7, and in R8. Their signals are combined in an opponent unit, intrinsic noise added, and the unit's output analysed to extract two measures of coding ability, number of discriminable polarization angles and mutual information. A very long R7 maximizes opponent signal amplitude, but codes inefficiently due to photon noise in the very short R8. Discriminability and mutual information are optimized by maximizing signal to noise ratio, SNR. At lower light levels approximately equal lengths of R7 and R8 are optimal because photon noise dominates. At higher light levels intrinsic noise comes to dominate and a shorter R8 is optimum. The optimum R8 length fractions falls to one third. This intensity dependent range of optimal length fractions corresponds to the range observed in different fly species and is not affected by transduction unit saturation. We conclude that a limited resource, rhabdom length, can be divided between two polarization sensors, R7 and R8, to optimize opponent coding. We also find that coding ability increases sub-linearly with total rhabdom length, according to the law of diminishing returns. Consequently, the specialized shorter central rhabdom in the DRA codes polarization twice as efficiently with respect to rhabdom length than the longer rhabdom used in the rest of the eye.

Anatomical organization of the brain of a diurnal and a nocturnal dung beetle

To avoid the fierce competition for food, South African ball-rolling dung beetles carve a piece of dung off a dung-pile, shape it into a ball and roll it away along a straight line path. For this unidirectional exit from the busy dung pile, at night and day, the beetles use a wide repertoire of celestial compass cues. This robust and relatively easily measurable orientation behavior has made ball-rolling dung beetles an attractive model organism for the study of the neuroethology behind insect orientation and sensory ecology. Although there is already some knowledge emerging concerning how celestial cues are processed in the dung beetle brain, little is known about its general neural layout. Mapping the neuropils of the dung beetle brain is thus a prerequisite to understand the neuronal network that underlies celestial compass orientation. Here, we describe and compare the brains of a day-active and a night-active dung beetle species based on immunostainings against synapsin and serotonin. We also provide 3D reconstructions for all brain areas and many of the fiber bundles in the brain of the day-active dung beetle. Comparison of neuropil structures between the two dung beetle species revealed differences that reflect adaptations to different light conditions. Altogether, our results provide a reference framework for future studies on the neuroethology of insects in general and dung beetles in particular.

Fine structure of the anterior median eyes of the funnel-web spider Agelena labyrinthica (Araneae: Agelenidae)

Only few electron microscopic studies exist on the structure of the main eyes (anterior median eyes, AME) of web spiders. The present paper provides details on the anatomy of the AME in the funnel-web spider Agelena labyrinthica. The retina consists of two separate regions with differently arranged photoreceptor cells. Its central part has sensory cells with rhabdomeres on 2, 3, or 4 sides, whereas those of the ventral retina have only two rhabdomeres on opposite sides. In addition, the rhabdomeres of the ventral retina are arranged in a specific way: Whereas in the most ventral part they form long tangential rows, those towards the center are detached and are arranged radially. All sensory cells are wrapped by unpigmented pigment cell processes. In agelenid spiders the axons of the sensory cells exit from the middle of the cell body; their fine structure and course through the eye cup is described in detail. In the central part of the retina efferent nerve fibres were found forming synapses along the distal region of the receptor cells. A muscle is attached laterally to each eye cup that allows mainly rotational movements of the eyes. The optical performance (image resolution) of these main eyes with relatively few visual cells is discussed.

Social Life in Arid Environments: The Case Study of Cataglyphis Ants

Unlike most desert-dwelling animals, Cataglyphis ants do not attempt to escape the heat; rather, they apply their impressive heat tolerance to avoid competitors and predators. This thermally defined niche has promoted a range of adaptations both at the individual and colony levels. We have also recently discovered that within the genus Cataglyphis there are incredibly diverse social systems, modes of reproduction, and dispersal, prompting the tantalizing question of whether social diversity may also be a consequence of the harsh environment within which we find these charismatic ants. Here we review recent advances regarding the physiological, behavioral, life-history, colony, and ecological characteristics of Cataglyphis and consider perspectives on future research that will build our understanding of organic adaptive responses to desertification.

Can invertebrates see the e-vector of polarization as a separate modality of light?

The visual world is rich in linearly polarized light stimuli, which are hidden from the human eye. But many invertebrate species make use of polarized light as a source of valuable visual information. However, exploiting light polarization does not necessarily imply that the electric (e)-vector orientation of polarized light can be perceived as a separate modality of light. In this Review, I address the question of whether invertebrates can detect specific e-vector orientations in a manner similar to that of humans perceiving spectral stimuli as specific hues. To analyze e-vector orientation, the signals of at least three polarization-sensitive sensors (analyzer channels) with different e-vector tuning axes must be compared. The object-based, imaging polarization vision systems of cephalopods and crustaceans, as well as the water-surface detectors of flying backswimmers, use just two analyzer channels. Although this excludes the perception of specific e-vector orientations, a two-channel system does provide a coarse, categoric analysis of polarized light stimuli, comparable to the limited color sense of dichromatic, 'color-blind' humans. The celestial compass of insects employs three or more analyzer channels. However, that compass is multimodal, i.e. e-vector information merges with directional information from other celestial cues, such as the solar azimuth and the spectral gradient in the sky, masking e-vector information. It seems that invertebrate organisms take no interest in the polarization details of visual stimuli, but polarization vision grants more practical benefits, such as improved object detection and visual communication for cephalopods and crustaceans, compass readings to traveling insects, or the alert 'water below!' to water-seeking bugs.

Ultrastructure of GABA- and Tachykinin-Immunoreactive Neurons in the Lower Division of the Central Body of the Desert Locust

The central complex, a group of neuropils spanning the midline of the insect brain, plays a key role in spatial orientation and navigation. In the desert locust and other species, many neurons of the central complex are sensitive to the oscillation plane of polarized light above the animal and are likely involved in the coding of compass directions derived from the polarization pattern of the sky. Polarized light signals enter the locust central complex primarily through two types of γ-aminobutyric acid (GABA)-immunoreactive tangential neurons, termed TL2 and TL3 that innervate specific layers of the lower division of the central body (CBL). Candidate postsynaptic partners are columnar neurons (CL1) connecting the CBL to the protocerebral bridge (PB). Subsets of CL1 neurons are immunoreactive to antisera against locustatachykinin (LomTK). To better understand the synaptic connectivities of tangential and columnar neurons in the CBL, we studied its ultrastructural organization in the desert locust, both with conventional electron microscopy and in preparations immunolabeled for GABA or LomTK. Neuronal profiles in the CBL were rich in mitochondria and vesicles. Three types of vesicles were distinguished: small clear vesicles with diameters of 20–40 nm, dark dense-core vesicles (diameter 70–120 nm), and granular dense-core vesicles (diameter 70–80 nm). Neurons were connected via divergent dyads and, less frequently, through convergent dyads. GABA-immunoreactive neurons contained small clear vesicles and small numbers of dark dense core vesicles. They had both pre- and postsynaptic contacts but output synapses were observed more frequently than input synapses. LomTK immunostaining was concentrated on large granular vesicles; neurons had pre- and postsynaptic connections often with neurons assumed to be GABAergic. The data suggest that GABA-immunoreactive tangential neurons provide signals to postsynaptic neurons in the CBL, including LomTK-immunolabeled CL1 neurons, but in addition also receive input from LomTK-labeled neurons. Both types of neuron are additionally involved in local circuits with other constituents of the CBL.

Neurons in the brain of the desert locust Schistocerca gregaria sensitive to polarized light at low stimulus elevations

Desert locusts (Schistocerca gregaria) sense the plane of dorsally presented polarized light through specialized dorsal eye regions that are likely adapted to exploit the polarization pattern of the blue sky for spatial orientation. Receptive fields of these dorsal rim photoreceptors and polarization-sensitive interneurons are directed toward the upper sky but may extend to elevations below 30°. Behavioral data, however, suggests that S. gregaria is even able to detect polarized light from ventral directions but physiological evidence for this is still lacking. In this study we characterized neurons in the locust brain showing polarization sensitivity at low elevations down to the horizon. In most neurons polarization sensitivity was absent or weak when stimulating from the zenith. All neurons, including projection and commissural neurons of the optic lobe and local interneurons of the central brain, are novel cell types, distinct from polarization-sensitive neurons studied so far. Painting dorsal rim areas in both eyes black to block visual input had no effect on the polarization sensitivity of these neurons, suggesting that they receive polarized light input from the main eye. A possible role of these neurons in flight stabilization or the perception of polarized light reflected from bodies of water or vegetation is discussed.

Neural mechanisms of insect navigation

We know more about the ethology of insect navigation than the neural substrates. Few studies have shown direct effects of brain manipulation on navigational behaviour; or measure brain responses that clearly relate to the animal's current location or spatial target, independently of specific sensory cues. This is partly due to the methodological problems of obtaining neural data in a naturally behaving animal. However, substantial indirect evidence, such as comparative anatomy and knowledge of the neural circuits that provide relevant sensory inputs provide converging arguments for the role of some specific brain areas: the mushroom bodies; and the central complex. Finally, modelling can help bridge the gap by relating the computational requirements of a given navigational task to the type of computation offered by different brain areas.

Compound eye and ocellar structure for walking and flying modes of locomotion in the Australian ant, Camponotus consobrinus

Ants are unusual among insects in that individuals of the same species within a single colony have different modes of locomotion and tasks. We know from walking ants that vision plays a significant role in guiding this behaviour, but we know surprisingly little about the potential contribution of visual sensory structures for a flying mode of locomotion. Here we investigate the structure of the compound eye and ocelli in pedestrian workers, alate females and alate males of an Australian ant, Camponotus consobrinus, and discuss the trade-offs involved in optical sensitivity and spatial resolution. Male ants have more but smaller ommatidia and the smallest interommatidial angles, which is most likely an adaptation to visually track individual flying females. Both walking and flying forms of ants have a similar proportion of specialized receptors sensitive to polarized skylight, but the absolute number of these receptors varies, being greatest in males. Ocelli are present only in the flying forms. Each ocellus consists of a bipartite retina with a horizon-facing dorsal retina, which contains retinula cells with long rhabdoms, and a sky-facing ventral retina with shorter rhabdoms. We discuss the implications of these and their potential for sensing the pattern of polarized skylight.

Sky Compass Orientation in Desert Locusts-Evidence from Field and Laboratory Studies

Locusts are long-range migratory insects. At high population density, immature animals form marching hopper bands while adults take off and form huge swarms of millions of animals. At low population densities animals are solitarious, but likewise migrate, mostly during the night. Numerous studies aimed at predicting locust infestations showed that migrations both as hopper bands and as adults are largely downwind following seasonal shifts of the tropical convergence zone taking the animals to areas of rainfall. Only a few studies provided evidence for active orientation mechanisms, including the involvement of a sun compass. This scarcity of evidence stands in contrast to recent neurobiological data showing sophisticated neuronal adaptations suited for sky compass navigation. These include a special dorsal eye region with photoreceptors suited to analyze the polarization pattern of the sky and a system of topographically arranged sky compass neurons in the central complex of the brain. Laboratory experiments, moreover, demonstrated polarotaxis in tethered flying animals. The discrepancy of these findings call for more rigorous field studies on active orientation mechanisms in locusts. It remains to be shown how locusts use their internal sky compass during mass migrations and what role it plays to guide solitarious locusts in their natural habitat.

Transmedulla Neurons in the Sky Compass Network of the Honeybee (Apis mellifera) Are a Possible Site of Circadian Input

Honeybees are known for their ability to use the sun's azimuth and the sky's polarization pattern for spatial orientation. Sky compass orientation in bees has been extensively studied at the behavioral level but our knowledge about the underlying neuronal systems and mechanisms is very limited. Electrophysiological studies in other insect species suggest that neurons of the sky compass system integrate information about the polarization pattern of the sky, its chromatic gradient, and the azimuth of the sun. In order to obtain a stable directional signal throughout the day, circadian changes between the sky polarization pattern and the solar azimuth must be compensated. Likewise, the system must be modulated in a context specific way to compensate for changes in intensity, polarization and chromatic properties of light caused by clouds, vegetation and landscape. The goal of this study was to identify neurons of the sky compass pathway in the honeybee brain and to find potential sites of circadian and neuromodulatory input into this pathway. To this end we first traced the sky compass pathway from the polarization-sensitive dorsal rim area of the compound eye via the medulla and the anterior optic tubercle to the lateral complex using dye injections. Neurons forming this pathway strongly resembled neurons of the sky compass pathway in other insect species. Next we combined tracer injections with immunocytochemistry against the circadian neuropeptide pigment dispersing factor and the neuromodulators serotonin, and γ-aminobutyric acid. We identified neurons, connecting the dorsal rim area of the medulla to the anterior optic tubercle, as a possible site of neuromodulation and interaction with the circadian system. These neurons have conspicuous spines in close proximity to pigment dispersing factor-, serotonin-, and GABA-immunoreactive neurons. Our data therefore show for the first time a potential interaction site between the sky compass pathway and the circadian clock.

The visual system of the Australian 'Redeye' cicada (Psaltoda moerens)

We investigated the functional anatomy of the visual system in the Australian 'Redeye' cicada Psaltoda moerens, including compound eyes and ocelli. The compound eyes have large visual fields, about 7500 ommatidia per eye and binocular overlaps of 10-15° in the frontal and of 50-60° in the dorsal visual field. The diameters of corneal facet lenses range between 22 and 34 μm and the lenses are unusually long with up to 100 μm in some eye regions. In the posterior part of the eyes, the hexagonal facet array changes to a square lattice. The compound eyes are of the eucone apposition type with 8 retinular cells contributing to a fused rhabdom in each ommatidium. The red eye colour is due to the pigment granules in the secondary pigment cells. We found a small Dorsal Rim Area (DRA), in which rhabdom cross-sections are rectangular rather than round. The cross-sections of DRA rhabdoms do not systematically change orientation along the length of the rhabdom, indicating that microvilli directions do not twist, which would make retinular cells in the DRA polarization sensitive. The three ocelli have unusual lenses with a champagne-cork shape in longitudinal sections. Retinular cells are short in the dorsal and ventral part of the retinae, and long in their equatorial part. Ocellar rhabdoms are short (<10 μm), positioned close to the corneagenous layer and are formed by pairs of retinular cells. In cross-section, the rhabdomeres are 2-5 μm long and straight. The red colour of ocelli is produced by screening pigments that form an iris around the base of the ocellar lens and by screening pigments between the ocellar retinular cells. We discuss the organization of the compound eye rhabdom, the organization of the ocelli and the presence of a DRA in the light of what is known about Hemipteran compound eyes. We note in particular that cicadas are the only Hemipteran group with fused rhabdoms, thus making Hemiptera an interesting case to study the evolution of open rhabdoms and neural superposition.

Sky light polarization detection with linear polarizer triplet in light field camera inspired by insect vision

Stable information of a sky light polarization pattern can be used for navigation with various advantages such as better performance of anti-interference, no "error cumulative effect," and so on. But the existing method of sky light polarization measurement is weak in real-time performance or with a complex system. Inspired by the navigational capability of a Cataglyphis with its compound eyes, we introduce a new approach to acquire the all-sky image under different polarization directions with one camera and without a rotating polarizer, so as to detect the polarization pattern across the full sky in a single snapshot. Our system is based on a handheld light field camera with a wide-angle lens and a triplet linear polarizer placed over its aperture stop. Experimental results agree with the theoretical predictions. Not only real-time detection but simple and costless architecture demonstrates the superiority of the approach proposed in this paper.

Neural coding underlying the cue preference for celestial orientation

Diurnal and nocturnal African dung beetles use celestial cues, such as the sun, the moon, and the polarization pattern, to roll dung balls along straight paths across the savanna. Although nocturnal beetles move in the same manner through the same environment as their diurnal relatives, they do so when light conditions are at least 1 million-fold dimmer. Here, we show, for the first time to our knowledge, that the celestial cue preference differs between nocturnal and diurnal beetles in a manner that reflects their contrasting visual ecologies. We also demonstrate how these cue preferences are reflected in the activity of compass neurons in the brain. At night, polarized skylight is the dominant orientation cue for nocturnal beetles. However, if we coerce them to roll during the day, they instead use a celestial body (the sun) as their primary orientation cue. Diurnal beetles, however, persist in using a celestial body for their compass, day or night. Compass neurons in the central complex of diurnal beetles are tuned only to the sun, whereas the same neurons in the nocturnal species switch exclusively to polarized light at lunar light intensities. Thus, these neurons encode the preferences for particular celestial cues and alter their weighting according to ambient light conditions. This flexible encoding of celestial cue preferences relative to the prevailing visual scenery provides a simple, yet effective, mechanism for enabling visual orientation at any light intensity.

The evolutionary diversity of insect retinal mosaics: common design principles and emerging molecular logic

Independent evolution has resulted in a vast diversity of eyes. Despite the lack of a common Bauplan or ancestral structure, similar developmental strategies are used. For instance, different classes of photoreceptor cells (PRs) are distributed stochastically and/or localized in different regions of the retina. Here, we focus on recent progress made towards understanding the molecular principles behind patterning retinal mosaics of insects, one of the most diverse groups of animals adapted to life on land, in the air, under water, or on the water surface. Morphological, physiological, and behavioral studies from many species provide detailed descriptions of the vast variation in retinal design and function. By integrating this knowledge with recent progress in the characterization of insect Rhodopsins as well as insight from the model organism Drosophila melanogaster, we seek to identify the molecular logic behind the adaptation of retinal mosaics to the habitat and way of life of an animal.

Photoreceptor projections and receptive fields in the dorsal rim area and main retina of the locust eye

In many insect species, photoreceptors of a small dorsal rim area of the eye are specialized for sensitivity to the oscillation plane of polarized skylight and, thus, serve a role in sky compass orientation. To further understand peripheral mechanisms of polarized-light processing in the optic lobe, we have studied the projections of photoreceptors and their receptive fields in the main eye and dorsal rim area of the desert locust, a model system for polarization vision analysis. In both eye regions, one photoreceptor per ommatidium, R7, has a long visual fiber projecting through the lamina to the medulla. Axonal fibers from R7 receptors of the dorsal rim area have short side branches throughout the depth of the dorsal lamina and maintain retinotopic projections to the dorsal medulla following the first optic chiasma. Receptive fields of dorsal rim photoreceptors are considerably larger (average acceptance angle 33°) than those of the main eye (average acceptance angle 2.04°) and, taken together, cover almost the entire sky. The data challenge previous reports of two long visual fibers per ommatidium in the main eye of the locust and provide data for future analysis of peripheral networks underlying polarization opponency in the locust brain.

Measurements of skylight polarization: a case study in urban region with high-loading aerosol

We investigate skylight polarization patterns in an urban region using our developed full-Stokes imaging polarimeter. A detailed description of our imaging polarimeter and its calibration are given, then, we measure skylight polarization patterns at wavelength λ=488  nm and at solar elevation between -05°10' and +35°42' in the city of Hefei, China. We show that in an urban region with high-loading aerosols: (1) the measured degree of linear polarization reaches the maximum near sunset, and large areas of unpolarized sky exist in the forward sunlight direction close to the Sun; (2) the position of neural points shifts from the local meridian plane and, if compared with a clear sky, alters the symmetrical characteristics of celestial polarization pattern; and (3) the observed circular polarization component is negligible.

Polarization signaling in swordtails alters female mate preference

Polarization of light, and visual sensitivity to it, is pervasive across aquatic and terrestrial environments. Documentation of invertebrate use of polarized light is widespread from navigation and foraging to species recognition. However, studies demonstrating that polarization body patterning serves as a communication signal (e.g., with evidence of changes in receiver behavior) are rare among invertebrate taxa and conspicuously absent among vertebrates. Here, we investigate polarization-mediated communication by northern swordtails, Xiphophorus nigrensis, using a custom-built videopolarimeter to measure polarization signals and an experimental paradigm that manipulates polarization signals without modifying their brightness or color. We conducted mate choice trials in an experimental tank that illuminates a pair of males with light passed through a polarization filter and a diffusion filter. By alternating the order of these filters between males, we presented females with live males that differed in polarization reflectance by >200% but with intensity and color differences below detection thresholds (∼5%). Combining videopolarimetry and polarization-manipulated mate choice trials, we found sexually dimorphic polarized reflectance and polarization-dependent female mate choice behavior with no polarization-dependent courtship behavior by males. Male swordtails exhibit greater within-body and body-to-background polarization contrast than females, and females preferentially associate with high-polarization-reflecting males. We also found limited support that males increase polarization contrast in social conditions over asocial conditions. Polarization cues in mate choice contexts may provide aquatic vertebrates with enhanced detection of specific display features (e.g., movements, angular information), as well as a signaling mechanism that may enhance detection by intended viewers while minimizing detection by others.

Polarization transition between sunlit and moonlit skies with possible implications for animal orientation and Viking navigation: anomalous celestial twilight polarization at partial moon

Using full-sky imaging polarimetry, we measured the celestial distribution of polarization during sunset and sunrise at partial (78% and 72%) and full (100%) moon in the red (650 nm), green (550 nm), and blue (450 nm) parts of the spectrum. We investigated the temporal change of the patterns of degree p and angle α of linear polarization of sunlit and moonlit skies at dusk and dawn. We describe here the position change of the neutral points of sky polarization, and present video clips about the celestial polarization transition at moonlit twilight. We found that at partial moon and at a medium latitude (47° 15.481' N) during this transition there is a relatively short (10-20 min) period when (i) the maximum of p of skylight decreases, and (ii) from the celestial α pattern neither the solar-antisolar nor the lunar-antilunar meridian can be unambiguously determined. These meridians can serve as reference directions of animal orientation and Viking navigation based on sky polarization. The possible influence of these atmospheric optical phenomena during the polarization transition between sunlit and moonlit skies on the orientation of polarization-sensitive crepuscular/nocturnal animals and the hypothesized navigation of sunstone-aided Viking seafarers is discussed.

Out of the blue: the evolution of horizontally polarized signals in Haptosquilla (Crustacea, Stomatopoda, Protosquillidae)

The polarization of light provides information that is used by many animals for a number of different visually guided behaviours. Several marine species, such as stomatopod crustaceans and cephalopod molluscs, communicate using visual signals that contain polarized information, content that is often part of a more complex multi-dimensional visual signal. In this work, we investigate the evolution of polarized signals in species of Haptosquilla, a widespread genus of stomatopod, as well as related protosquillids. We present evidence for a pre-existing bias towards horizontally polarized signal content and demonstrate that the properties of the polarization vision system in these animals increase the signal-to-noise ratio of the signal. Combining these results with the increase in efficacy that polarization provides over intensity and hue in a shallow marine environment, we propose a joint framework for the evolution of the polarized form of these complex signals based on both efficacy-driven (proximate) and content-driven (ultimate) selection pressures.