Search

Insect attraction to artificial light can potentially facilitate disease transmission by increasing contact between humans and vectors. Previous research has identified specific wavelength bands, such as yellow and red, that are unattractive to biting flies. However, narrow-band, non-white lights are unsuitable for home lighting use as their very poor color rendering is often considered aesthetically undesirable. The creation of a white light that is unattractive to insects has so far remained elusive. White light can be created by combining a number of narrow-band light-emitting diodes (LEDs). Through choice chamber experiments on Culex pipiens (Cx. pipiens) mosquitoes, we examine whether combining specific wavelength bands has an additive, subtractive or synergistic effect on insect attraction. We show that a white light created by combining narrow-band red, green and blue (RGB) LEDs is less attractive to Cx. pipiens than a broad-spectrum white light; and that a white light created by combining narrow-band blue and yellow LEDs is more attractive than a broad-spectrum white light. White light produced by RGB combinations could therefore serve as a safer and cheaper light in countries where phototactic vectors and vector-borne disease are endemic.

Insect attraction to artificial light can potentially facilitate disease transmission by increasing contact between humans and vectors. Previous research has identified specific wavelength bands, such as yellow and red, that are unattractive to biting flies. However, narrow-band, non-white lights are unsuitable for home lighting use as their very poor color rendering is often considered aesthetically undesirable. The creation of a white light that is unattractive to insects has so far remained elusive. White light can be created by combining a number of narrow-band light-emitting diodes (LEDs). Through choice chamber experiments on

Collective movement is critical to the survival of some animals. Despite substantial progress in understanding animal collectives such as fish shoals and bird flocks, it is unknown how collective behaviour is affected by changes in multiple environmental conditions that can interact as stressors. Using a fully factorial repeated-measures design, we tested the independent and combined effects of darkness and acoustic noise on the collective motion of three-spined sticklebacks, Gasterosteus aculeatus, quantified using high-resolution tracking data from video. Corresponding to the importance of vision in shoaling behaviour, darkness increased nearest-neighbour distances and reduced coordination, measured as speed correlations and differences in directional heading between nearest neighbours. Although individual swimming speeds were not impacted by darkness, the group's centre of mass was slower, an emergent effect of reduced polarisation (i.e. greater group disorder) in darkness. While additional acoustic noise had no detectable effect on these variables, it altered group structure, with fish being more likely to be found side by side one another. Fish were also further from the arena wall (i.e. showed reduced wall following) when there was additional acoustic noise. There was only weak evidence for additive or interactive effects of the two stressors. Across the different environmental contexts, there were consistent, repeatable differences between groups (i.e. group personality variation) in the speed, turning angle and distance from the arena wall of individuals, but the only collective behaviour that was repeatable was group polarization. Our study demonstrates that multiple stressors can have independent effects that impact different aspects of behaviour and highlights the need for empirical studies on multiple stressors as their effects can be unpredictable.

The sun is the most reliable celestial cue for orientation available to daytime migrants. It is widely assumed that diurnal migratory insects use a ‘time-compensated sun compass’ to adjust for the changing position of the sun throughout the day, as demonstrated in some butterfly species. The mechanisms used by other groups of diurnal insect migrants remain to be elucidated. Migratory species of hoverflies (Diptera: Syrphidae) are one of the most abundant and beneficial groups of diurnal migrants, providing multiple ecosystem services and undergoing directed seasonal movements throughout much of the temperate zone. To identify the hoverfly navigational strategy, a flight simulator was used to measure orientation responses of the hoverfliesScaeva pyrastriandScaeva seleniticato celestial cues during their autumn migration. Hoverflies oriented southwards when they could see the sun and shifted this orientation westward following a 6 h advance of their circadian clocks. Our results demonstrate the use of a time-compensated sun compass as the primary navigational mechanism, consistent with field observations that hoverfly migration occurs predominately under clear and sunny conditions.

Light trapping is an important tool for monitoring insect populations. This is especially true for biting Diptera, where light traps play a crucial role in disease surveillance by tracking the presence and abundance of vector species. Physiological and behavioural data have been instrumental in identifying factors that influence dipteran phototaxis and have spurred the development of more effective light traps. However, the development of less attractive domestic lights has received comparatively little interest but could be important for reducing interactions between humans and vector insects, with consequences for reducing disease transmission. Here, we discuss how dipteran eyes respond to light and the factors influencing positive phototaxis, and conclude by identifying key areas for further research. In addition, we include a synthesis of attractive and unattractive wavelengths for a number of vector species. A more comprehensive understanding of how Diptera perceive and respond to light would allow for more efficient vector sampling as well as potentially limiting the risk posed by domestic lighting. Graphical Abstract

Polarization vision is widespread in nature, mainly among invertebrates, and is used for a range of tasks including navigation, habitat localization and communication. In marine environments, some species such as those from the Crustacea and Cephalopoda that are principally monochromatic, have evolved to use this adaptation to discriminate objects across the whole visual field, an ability similar to our own use of colour vision. The performance of these polarization vision systems varies, and the few cephalopod species tested so far have notably acute thresholds of discrimination. However, most studies to date have used artificial sources of polarized light that produce levels of polarization much higher than found in nature. In this study, the ability of octopuses to detect polarization contrasts varying in angle of polarization (AoP) was investigated over a range of different degrees of linear polarization (DoLP) to better judge their visual ability in more ecologically relevant conditions. The ‘just-noticeable-differences’ (JND) of AoP contrasts varied consistently with DoLP. These JND thresholds could be largely explained by their ‘polarization distance’, a neurophysical model that effectively calculates the level of activity in opposing horizontally and vertically oriented polarization channels in the cephalopod visual system. Imaging polarimetry from the animals’ natural environment was then used to illustrate the functional advantage that these polarization thresholds may confer in behaviourally relevant contexts., Summary: Octopuses are highly sensitive to small changes in the angle of polarization (

A great diversity of adaptations is found among animals with compound eyes and even closely related taxa can show large variation in their light-adaptation strategies. A prime example of a visual system evolved to function in specific light environments is the fiddler crab, used widely as a model to research aspects of crustacean vision and neural pathways. However, questions remain regarding how their eyes respond to the changes in brightness spanning many orders of magnitude, associated with their habitat and ecology. The fiddler crab Afruca tangeri forages at low tide on tropical and semi-tropical mudflats, under bright sunlight and on moonless nights, suggesting that their eyes undergo effective light adaptation. Using synchrotron X-ray tomography, light and transmission electron microscopy and in vivo ophthalmoscopy, we describe the ultrastructural changes in the eye between day and night. Dark adaptation at dusk triggered extensive widening of the rhabdoms and crystalline cone tips. This doubled the ommatidial acceptance angles and increased microvillar surface area for light capture in the rhabdom, theoretically boosting optical sensitivity 7.4 times. During daytime, only partial dark-adaptation was achieved and rhabdoms remained narrow, indicating strong circadian control on the process. Bright light did not evoke changes in screening pigment distributions, suggesting a structural inability to adapt rapidly to the light level fluctuations frequently experienced when entering their burrow to escape predators. This should enable fiddler crabs to shelter for several minutes without undergoing significant dark-adaptation, their vision remaining effectively adapted for predator detection when surfacing again in bright light.

Stomatopods engage in a highly dynamic lifestyle that includes ritual combat, territoriality, and active predation of live prey. Adaptations to this lifestyle include powerful raptorial appendages and an extraordinarily complex visual system, which includes 12-channel colour vision and the ability to discriminate the polarization of light. The neural processing underlying their colour vision has yet to be determined, though there is some evidence that the stomatopod colour vision system is based on wavelength recognition rather than spectral discrimination. We show that Odontodactylus scyllarus has an innate preference for objects that reflect wavelengths between 575 nm and 600 nm (corresponding to the human ‘yellow’). Comparatively, they show a reduced preference for objects that reflect both at shorter wavelengths, between 525 nm and 575 nm (human ‘green’) and at longer wavelengths, between 600 nm and 650 nm (human ‘red’). Within the wavelength preference, decisions are affected by the contrast of the object, with choice directed towards the version of the object that exhibited a greater Weber contrast against the background, despite reflecting in the same wavelength interval. As for other animals, the innate preference for objects reflecting particular wavelengths may act to increase the fitness of naive animals.

Both vertebrates and invertebrates commonly exploit photonic structures adjacent to their photoreceptors for visual benefits. For example, use of a reflecting structure (tapetum) behind the retina increases photon capture, enhancing vision in dim light [1–5]. Colored filters positioned lateral or distal to a photoreceptive unit may also be used to tune spectral sensitivity by selective transmission of wavelengths not absorbed or scattered by the filters [6–8]. Here we describe a new category of biological optical filter that acts simultaneously as both a transmissive spectral filter and narrowband reflector. Discovered in the larval eyes of only one family of mantis shrimp (stomatopod) crustaceans (Nannosquillidae), each crystalline structure bisects the photoreceptive rhabdom into two tiers and contains an ordered array of membrane-bound vesicles with sub-wavelength diameters of 153 ± 5 nm. Axial illumination of the intrarhabdomal structural reflector (ISR) in vivo produces a narrow band of yellow reflectance (mean peak reflectivity, 572 ± 18 nm). The ISR is similar to several synthetic devices, such as bandgap filters, laser mirrors, and (in particular) fiber Bragg gratings used in optical sensors for a wide range of industries. To our knowledge, the stomatopod larval ISR is the first example of a naturally occurring analog to these human-made devices. Considering what is known about these animals’ visual ecology, we propose that these reflecting filters may help improve the detection of pelagic bioluminescence in shallow water at night. Video Abstract: Feller et al. report how one family of mantis shrimp larvae uses visual filtering structures to selectively reflect and transmit light within photoreceptors. These reflective filters likely enhance bioluminescence detection underwater at night and are similar to human-made optical devices such as fiber Bragg gratings or band gap filters.

Photoreceptors have high metabolic demands and age rapidly, undermining visual function. We base our understanding mainly on ageing mice where elevated inflammation, extracellular deposition, including that of amyloid beta, and rod and cone photoreceptor loss occur, but cones are not lost in ageing primate although their function declines, revealing that primate and mouse age differently. We examine ageing primate retinae and show elevated stress but low inflammation. However, aged primates have a >70% reduction in adenosine triphosphate (ATP) and a decrease in cytochrome c oxidase. There is a shift in cone mitochondrial positioning and glycolytic activity increases. Bruch’s membrane thickens but unlike in mice, amyloid beta is absent. Hence, reduced ATP may explain cone functional decline in ageing but their retained presence offers the possibility of functional restoration if they can be fuelled appropriately to restore cellular function. This is important because as humans we largely depend on cone function to see and are rarely fully dark adapted. Presence of limited aged inflammation and amyloid beta deposition question some of the therapeutic approaches taken to resolve problems of retinal ageing in humans and the possible lack of success in clinical trials in macular degeneration that have targeted inflammatory agents.

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

Macular pigments (MPs), by absorbing potentially toxic short-wavelength (400–500 nm) visible light, provide protection against photo-chemical damage thought to be relevant in the pathogenesis of age-related macular degeneration (AMD). A method of screening for low levels of MPs could be part of a prevention strategy for helping people to delay the onset of AMD. We introduce a new method for assessing MP density that takes advantage of the polarization-dependent absorption of blue light by MPs, which results in the entoptic phenomenon called Haidinger’s brushes (HB). Subjects were asked to identify the direction of rotation of HB when presented with a circular stimulus illuminated with an even intensity of polarized white light in which the electric field vector was rotating either clockwise or anti-clockwise. By reducing the degree of polarization of the stimulus light, a threshold for perceiving HB (degree of polarization threshold) was determined and correlated (r 2 0.66) to macular pigment optical density assessed using dual-wavelength fundus autofluoresence. The speed and ease of measurement of degree of polarization threshold makes it well suited for large-scale screening of macular pigmentation.

Gaze stabilization is an almost ubiquitous animal behaviour, one that is required to see the world clearly and without blur. Stomatopods, however, only fix their eyes on scenes or objects of interest occasionally. Almost uniquely among animals they explore their visual environment with a series pitch, yaw and torsional (roll) rotations of their eyes, where each eye may also move largely independently of the other. In this work, we demonstrate that the torsional rotations are used to actively enhance their ability to see the polarization of light. Both Gonodactylus smithii and Odontodactylus scyllarus rotate their eyes to align particular photoreceptors relative to the angle of polarization of a linearly polarized visual stimulus, thereby maximizing the polarization contrast between an object of interest and its background. This is the first documented example of any animal displaying dynamic polarization vision, in which the polarization information is actively maximized through rotational eye movements., Mantis shrimps are known to display large pitch, yaw and torsional eye rotations. Here, the authors show that these eye movements allow mantis shrimp to orientate particular photoreceptors in order to better discriminate the polarization of light.

Bioinspired but static optical devices such as lenses, retarders, and reflectors have had a significant impact on the designs of many man-made optical technologies. However, while numerous adaptive and flexible optical mechanisms are found throughout the animal kingdom, highly desirable biomimetic copies of these remarkable smart systems remain, in many cases, a distant dream. Many aquatic animals have evolved highly efficient reflectors based on multilayer stacks of the crystallized nucleic acid base guanine. With exceptional levels of spectral and intensity control, these reflectors represent an interesting design pathway towards controllable micromirror structures. Here we show that individual guanine crystals, with dimensions of 5 μm × 20 μm × 70 nm, can be magnetically controlled to act as individual micromirrors. By applying magnetic fields of 500 mT, the reflectivity of these crystals can be switched off and on for the change in reflectivity. Overall, the use of guanine represents a novel design scheme for a highly efficient and controllable synthetic organic micromirror array.

Some vertebrate species have evolved means of extending their visual sensitivity beyond the range of human vision. One mechanism of enhancing sensitivity to long-wavelength light is to replace the 11-cis retinal chromophore in photopigments with 11-cis 3,4-didehydroretinal. Despite over a century of research on this topic, the enzymatic basis of this perceptual switch remains unknown. Here, we show that a cytochrome P450 family member, Cyp27c1, mediates this switch by converting vitamin A1 (the precursor of 11-cis retinal) into vitamin A2 (the precursor of 11-cis 3,4-didehydroretinal). Knockout of cyp27c1 in zebrafish abrogates production of vitamin A2, eliminating the animal's ability to red-shift its photoreceptor spectral sensitivity and reducing its ability to see and respond to near-infrared light. Thus, the expression of a single enzyme mediates dynamic spectral tuning of the entire visual system by controlling the balance of vitamin A1 and A2 in the eye.

Stomatopod crustaceans are renowned for their elaborate visual systems. Their eyes contain a plethora of photoreceptors specialized for chromatic and polarization detection, including several that are sensitive to varying wavelength ranges and angles of polarization within the ultraviolet (UV) range (less than 400 nm). Behavioural experiments have previously suggested that UV photoreception plays a role in stomatopod communication, but these experiments have only manipulated the entire UV range. Here, using a behavioural approach, we examine UV vision in the stomatopod Haptosquilla trispinosa . Using binary trained choice assays as well as innate burrow-choice experiments, we assessed the ability of H. trispinosa to detect and respond to narrow-band LED stimuli peaking near 314 nm (UVB) versus 379 nm (UVA) in wavelength. We find that H. trispinosa can discriminate these stimuli and appears to display an aversive reaction to UVB light, suggesting segregated behavioural responses to stimuli within the UV range. Furthermore, we find that H. trispinosa can discriminate stimuli peaking near 379 nm versus 351 nm in wavelength, suggesting that their wavelength discrimination in the UV is comparable to their performance in the human-visible range.

In addition to the visual information contained in intensity and color, imaging polarimetry allows visual information to be extracted from the polarization of light. However, a major challenge of imaging polarimetry is image degradation due to noise. This paper investigates the mitigation of noise through denoising algorithms and compares existing denoising algorithms with a new method, based on BM3D (Block Matching 3D). This algorithm, Polarization-BM3D (PBM3D), gives visual quality superior to the state of the art across all images and noise standard deviations tested. We show that denoising polarization images using PBM3D allows the degree of polarization to be more accurately calculated by comparing it with spectral polarimetry measurements.

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. Electronic supplementary material The online version of this article (10.1007/s00114-018-1551-3) contains supplementary material, which is available to authorized users.

Almost all animals, regardless of the anatomy of the eyes, require some level of gaze stabilization in order to see the world clearly and without blur. For the mantis shrimp, achieving gaze stabilization is unusually challenging as their eyes have an unprecedented scope for movement in all three rotational degrees of freedom: yaw, pitch and torsion. We demonstrate that the speciesOdontodactylus scyllarusperforms stereotypical gaze stabilization in the yaw degree of rotational freedom, which is accompanied by simultaneous changes in the pitch and torsion rotation of the eye. Surprisingly, yaw gaze stabilization performance is unaffected by both the torsional pose and the rate of torsional rotation of the eye. Further to this, we show, for the first time, a lack of a torsional gaze stabilization response in the stomatopod visual system. In the light of these findings, we suggest that the neural wide-field motion detection network in the stomatopod visual system may follow a radially symmetric organization to compensate for the potentially disorientating effects of torsional eye movements, a system likely to be unique to stomatopods.

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.

Biologically inspired solutions for modern-day sensory systems promise to deliver both higher capacity and faster, more efficient processing of information than current computational approaches. Many animals are able to perform remarkable sensing tasks despite only being able to process what would be considered modest data rates and bandwidths. The key biological innovations revolve around dedicated filter designs. By sacrificing some flexibility, specifically matched and hard-wired sensory systems, designed primarily for single roles, provide a blueprint for data and task-specific efficiency. In this paper, we examine several animal visual systems designed to use the polarization of light in spatial imaging. We investigate some implications for artificial optical processing based on models of polarization image processing in fiddler crabs, cuttlefish, octopus, and mantis shrimp.

In this paper, we present recent work on bioinspired polarization imaging sensors and their applications in biomedicine. In particular, we focus on three different aspects of these sensors. First, we describe the electro–optical challenges in realizing a bioinspired polarization imager, and in particular, we provide a detailed description of a recent low-power complementary metal–oxide–semiconductor (CMOS) polarization imager. Second, we focus on signal processing algorithms tailored for this new class of bioinspired polarization imaging sensors, such as calibration and interpolation. Third, the emergence of these sensors has enabled rapid progress in characterizing polarization signals and environmental parameters in nature, as well as several biomedical areas, such as label-free optical neural recording, dynamic tissue strength analysis, and early diagnosis of flat cancerous lesions in a murine colorectal tumor model. We highlight results obtained from these three areas and discuss future applications for these sensors.

Many animals use structural coloration to create bright and conspicuous visual signals. Selection of the size and shape of the optical structures animals use defines both the colour and intensity of the light reflected. The material used to create these reflectors is also important; however, animals are restricted to a limited number of materials: commonly chitin, guanine and the protein, reflectin. In this work we highlight that a particular set of material properties can also be under selection in order to increase the optical functionality of structural reflectors. Specifically, polarization properties, such as birefringence (the difference between the refractive indices of a material) and chirality (which relates to molecular asymmetry) are both under selection to create enhanced structural reflectivity. We demonstrate that the structural coloration of the gold beetle Chrysina resplendens and silvery reflective sides of the Atlantic herring, Clupea harengus are two examples of this phenomenon. Importantly, these polarization properties are not selected to control the polarization of the reflected light as a source of visual information per se. Instead, by creating higher levels of reflectivity than are otherwise possible, such internal polarization properties improve intensity-matching camouflage. This article is part of the themed issue ‘Animal coloration: production, perception, function and application’.

Stomatopods have an extraordinary visual system, incorporating independent movement of their eyes in all three degrees of rotational freedom. In this work, we demonstrate that in the peacock mantis shrimp, Odontodactylus scyllarus, the level of ocular independence is task dependent. During gaze stabilization in the context of optokinesis, there is weak but significant correlation between the left and right eyes in the yaw degree of rotational freedom, but not in pitch and torsion. When one eye is completely occluded, the uncovered eye does not drive the covered eye during gaze stabilization. However, occluding one eye does significantly affect the uncovered eye, lowering its gaze stabilization performance. There is a lateral asymmetry, with the magnitude of the effect depending on the eye (left or right) combined with the direction of motion of the visual field. In contrast, during a startle saccade, the uncovered eye does drive a covered eye. Such disparate levels of independence between the two eyes suggest that responses to individual visual tasks are likely to follow different neural pathways., Summary: The level of independence between the eyes of mantis shrimps (stomatopods) is task dependent, suggesting variability in neural processing of visual information.

The way in which light is polarized when reflected from leaves can be affected by infection with plant viruses. This has the potential to influence viral transmission by insect vectors due to altered visual attractiveness of infected plants. The optical and topological properties of cuticular waxes and trichomes are important determinants of how light is polarized upon reflection. Changes in expression of genes involved in the formation of surface structures have also been reported following viral infection. This paper investigates the role of altered surface structures in virus-induced changes to polarization reflection from leaves. The percentage polarization of reflections from Arabidopsis thaliana cer5, cer6 and cer8 wax synthesis mutants, and the gl1 leaf hair mutant, was compared to those from wild-type (WT) leaves. The cer5 mutant leaves were less polarizing than WT on the adaxial and abaxial surfaces; gl1 leaves were more polarizing than WT on the adaxial surfaces. The cer6 and cer8 mutations did not significantly affect polarization reflection. The impacts of Turnip vein clearing virus (TVCV) infection on the polarization of reflected light were significantly affected by cer5 mutation, with the reflections from cer5 mutants being higher than those from WT leaves, suggesting that changes in CER5 expression following infection could influence the polarization of the reflections. There was, however, no significant effect of the gl1 mutation on polarization following TVCV infection. The cer5 and gl1 mutations did not affect the changes in polarization following Cucumber mosaic virus (CMV) infection. The accumulation of TVCV and CMV did not differ significantly between mutant and WT leaves, suggesting that altered expression of surface structure genes does not significantly affect viral titres, raising the possibility that if such regulatory changes have any adaptive value it may possibly be through impacts on viral transmission.

In living color Animals live in a colorful world, but we rarely stop to think about how this color is produced and perceived, or how it evolved. Cuthill et al. review how color is used for social signals between individual animals and how it affects interactions with parasites, predators, and the physical environment. New approaches are elucidating aspects of animal coloration, from the requirements for complex cognition and perception mechanisms to the evolutionary dynamics surrounding its development and diversification. Science , this issue p. eaan0221

A combination of behavioural and electrophysiological experiments have previously shown that two species of stomatopod, Odontadactylus scyllarus and Gonodactylaceus falcatus, can differentiate between left and right handed circularly polarized light (CPL), and between CPL and linearly polarized light (LPL). It remains unknown if these visual abilities are common across all stomatopod species, and if so, how circular polarization sensitivity may vary between and within species. A sub-section of the midband, a specialized region of stomatopod eyes, contains distally placed photoreceptor cells, termed R8 (retinular cell number 8). These cells are specifically built with unidirectional microvilli and appear to be angled precisely to convert CPL into LPL. They are mostly quarter-wave retarders for human visible light (400-700nm) as well as being ultraviolet sensitive linear polarization detectors. The effectiveness of the R8 cells in this role is determined by their geometric and optical properties. In particular, the length and birefringence of the R8 cells are critical for retardation efficiency. Here, our comparative studies show that most species investigated have the theoretical ability to convert CPL into LPL, such that the handedness of an incoming circular reflection or signal could be discriminated. One species, Haptosquilla trispinosa, shows less than quarter-wave retardance. While some species are known to produce circularly polarized reflections (some Odontodactylus species and G. falcatus for example), others do not, so a variety of functions for this ability are worth considering.

Understanding the link between how proteins function in animals that live in extreme environments and selection on specific properties of amino acids has proved extremely challenging. Here we present the discovery of how the compressibility of opsin proteins in two evolutionarily distinct animal groups, teleosts and cephalopods, appears to be adapted to the high-pressure environment of the deep-sea. We report how in both groups, opsins in deeper living species are calculated to be less compressible. This is largely due to a common set of amino acid sites (bovRH# 159, 196, 213, 275) undergoing positive destabilizing selection in six of the twelve amino acid physiochemical properties that determine protein compressibility. This suggests a common evolutionary mechanism to reduce the adiabatic compressibility of opsin proteins. Intriguingly, the sites under selection are on the proteins’ outer faces at locations known to be involved in opsin-opsin dimer interactions.

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.

Dielectric multilayer reflectors that are non-polarizing are an important class of optical device and have numerous applications within optical fibres1, dielectric waveguides2 and light-emitting diodes3. Here, we report analyses of a biological non-polarizing optical mechanism found in the broadband guanine-cytoplasm ‘silver’ multilayer reflectors of three species of fish. Present in the fish stratum argenteum are two populations of birefringent guanine crystal, with their optical axes either parallel to the long axis of the crystal or perpendicular to the plane of the crystal, respectively. This arrangement neutralizes the polarization of reflection as a result of the different interfacial Brewster's angles of each population. The fish reflective mechanism is distinct from existing non-polarizing mirror designs4,5,6,7 in that, importantly, there is no refractive index contrast between the low-index layers in the reflector and the external environment. This mechanism could be readily manufactured and exploited in synthetic optical devices. The mechanism by which various species of silvery fish produce almost perfect broadband, polarization-neutral reflections is revealed. The answer lies with the use of multilayers composed of two types of birefringent guanine crystals, which each have their extraordinary and ordinary refractive indices orientated in different directions.

Alteration of leaf surface phenotypes due to virus infection has the potential to affect the likelihood of colonisation by insect vectors, or to affect their feeding activities. The aim of this study was to investigate whether viruses that rely on insects for their transmission, and which can be sensitive to the polarization of light, affect the percentage polarization of light reflected from leaves. We also set out to discover whether a correlation exists between the expression of ECERIFERUM (CER) genes involved in cuticular wax synthesis and the polarization of the light reflected from the leaf surfaces. It was found that the aphid-vectored viruses Potato virus Y and Cucumber mosaic virus (CMV) caused significant reductions in the percentage polarization of light reflected from the abaxial surfaces of leaves of Nicotiana tabacum, whereas the non-insect-vectored viruses Tobacco mosaic virus and Pepino mosaic virus did not induce this effect. In Arabidopsis thaliana, there was little difference in the impacts of CMV and the non-insect-vectored Turnip vein clearing virus on polarization reflection, with both viruses increasing the percentage polarization of light reflected from the abaxial surfaces of leaves. There was a trend towards increased accumulation of CER6 transcripts in N. tabacum and A. thaliana when infected with aphid-vectored viruses. No significant effect of infection on trichome densities was found in A. thaliana, suggesting that alterations to the formation of cuticular waxes may be the more likely phenotypic change on the leaf surface contributing to the changes in polarization reflection. The possible impacts and adaptive significance of these effects with regard to viral transmission by insects are discussed.

Many biophotonic structures have their spectral properties of reflection ‘tuned’ using the (zeroth-order) Bragg criteria for phase constructive interference. This is associated with a periodicity, or distribution of periodicities, parallel to the direction of illumination. The polarization properties of these reflections are, however, typically constrained by the dimensional symmetry and intrinsic dielectric properties of the biological materials. Here we report a linearly polarizing reflector in a stomatopod crustacean that consists of 6–8 layers of hollow, ovoid vesicles with principal axes of ~550 nm, ~250 nm and ~150 nm. The reflection of unpolarized normally incident light is blue/green in colour with maximum reflectance wavelength of 520 nm and a degree of polarization greater than 0.6 over most of the visible spectrum. We demonstrate that the polarizing reflection can be explained by a resonant coupling with the first-order, in-plane, Bragg harmonics. These harmonics are associated with a distribution of periodicities perpendicular to the direction of illumination, and, due to the shape-anisotropy of the vesicles, are different for each linear polarization mode. This control and tuning of the polarization of the reflection using shape-anisotropic hollow scatterers is unlike any optical structure previously described and could provide a new design pathway for polarization-tunability in man-made photonic devices.

In this paper, we present recent work on bioinspired polarization imaging sensors and their applications in biomedicine. In particular, we focus on three different aspects of these sensors. First, we describe the electro–optical challenges in realizing a bioinspired polarization imager, and in particular, we provide a detailed description of a recent low-power complementary metal–oxide–semiconductor (CMOS) polarization imager. Second, we focus on signal processing algorithms tailored for this new class of bioinspired polarization imaging sensors, such as calibration and interpolation. Third, the emergence of these sensors has enabled rapid progress in characterizing polarization signals and environmental parameters in nature, as well as several biomedical areas, such as label-free optical neural recording, dynamic tissue strength analysis, and early diagnosis of flat cancerous lesions in a murine colorectal tumor model. We highlight results obtained from these three areas and discuss future applications for these sensors.

Aquatic habitats are rich in polarized patterns that could provide valuable information about the environment to an animal with a visual system sensitive to polarization of light. Both cephalopods and fishes have been shown to behaviourally respond to polarized light cues, suggesting that polarization sensitivity (PS) may play a role in improving target detection and/or navigation/orientation. However, while there is general agreement concerning the presence of PS in cephalopods and some fish species, its functional significance remains uncertain. Testing the role of PS in predator or prey detection seems an excellent paradigm with which to study the contribution of PS to the sensory assets of both groups, because such behaviours are critical to survival. We developed a novel experimental set-up to deliver computer-generated, controllable, polarized stimuli to free-swimming cephalopods and fishes with which we tested the behavioural relevance of PS using stimuli that evoke innate responses (such as an escape response from a looming stimulus and a pursuing behaviour of a small prey-like stimulus). We report consistent responses of cephalopods to looming stimuli presented in polarization and luminance contrast; however, none of the fishes tested responded to either the looming or the prey-like stimuli when presented in polarization contrast.

Proc. R. Soc. B 285, 20180594 (Published online 2 May 2018) ([doi:10.1098/rspb.2018.0594][1]) We … [1]: http://dx.doi.org/10.1098/rspb.2018.0594

The accuracy of calculations of both the degree and angle of polarization depend strongly on the noise in the measurements used. The noise in the measurements recorded by both camera based systems and spectrometers can lead to significant artefacts and incorrect conclusions about high degrees of polarization when in fact none exist. Three approaches are taken in this work: Firstly, the absolute error introduced as a function of the signal to noise ratio for polarization measurements is quantified in detail. An important finding here is the reason for why several studies incorrectly suggest that black (low reflectivity) objects are highly polarized. The high degree of polarization is only an artefact of the noise in the calculation. Secondly, several simplesteps to avoid such errors are suggested. Thirdly, if these points can’t be followed, two methods are presented for mitigating the effects of noise: a maximum likelihood estimation method and a new denoising algorithm to best calculate the degree of polarization of natural polarization information.

Animals make use of a wealth of optical physics to control and manipulate light, for example, in creating reflective animal colouration1, 2, 3 and polarized light signals4. Their precise optics often surpass equivalent man-made optical devices in both sophistication and efficiency5. Here, we report a biophysical mechanism that creates a natural full-visible-range achromatic quarter-wave retarder in the eye of a stomatopod crustacean. Analogous, man-made retardation devices are important optical components, used in both scientific research and commercial applications for controlling polarized light. Typical synthetic retarders are not achromatic, and more elaborate designs, such as, multilayer subwavelength gratings or bicrystalline constructions, only achieve partial wavelength independence6. In this work, we use both experimental measurements and theoretical modelling of the photoreceptor structure to illustrate how a novel interplay of intrinsic and form birefringence results in a natural achromatic optic that significantly outperforms current man-made optical devices.

In our investigations of a family of achiral bent-core bis-(phenyl)oxadiazole derivatives, we observed some unusual properties of the nematic phase. These include evidence of segregation into domains of opposite handedness and the formation of clusters, nematic phase biaxiality, and a strongly kinetically governed phase behaviour, which leads to the display of unique filament structures at the onset of new order. We suggest that the combination of a deviant calamitic molecular shape with a considerable transverse dipole leads to an unusual strength and combination of molecular interactions, and as a consequence these nematic materials need to be considered as fluctuating and dynamically changing, multi-hierarchical fluids.

SummarySunlight is attenuated rapidly in the ocean, resulting in little visually useful light reaching deeper than ∼1000 m in even the clearest water [1]. To maximize sensitivity to the relatively brighter downwelling sunlight, to view the silhouette of animals above them, and to increase the binocular overlap of their eyes, many mesopelagic animals have developed upward-pointing tubular eyes [2–4]. However, these sacrifice the ability to detect bioluminescent [5] and reflective objects in other directions. Thus, some mesopelagic fish with tubular eyes extend their visual fields laterally and/or ventrally by lensless ocular diverticula, which are thought to provide unfocused images, allowing only simple detection of objects, with little spatial resolution [2–4]. Here, we show that a medial mirror within the ventrally facing ocular diverticulum of the spookfish, Dolichopteryx longipes, consisting of a multilayer stack derived from a retinal tapetum, is used to reflect light onto a lateral retina. The reflective plates are not orientated parallel to the surface of the mirror. Instead, plate angles change progressively around the mirror, and computer modeling indicates that this provides a well-focused image. This is the first report of an ocular image being formed in a vertebrate eye by a mirror.

The ability to control the position of a mesoscopic object with nanometric precision is important for the rapid progress of nanoscience. One of the most promising tools to achieve such control is optical tweezers, which trap objects near the focus of a laser beam. However, the drawbacks of conventional tweezers include a trapping volume that is diffraction-limited and significant brownian motion of trapped nanoobjects. Here, we report the first experimental realization of three-dimensional nanometric optical tweezers that are based on nanostructured substrates. Using electromagnetically coupled pairs of gold nanodots in a standard optical tweezers set-up, we create an array of subwavelength plasmonic optical traps that offer a significant increase in trapping efficiency. The nanodot optical near-fields reduce the trapping volume beyond the diffraction limit and quench brownian motion of the trapped nanoparticles by almost an order of magnitude as compared to conventional tweezers operating under the same trapping conditions. Our tweezers achieve nanoscale control of entities at significantly smaller laser powers and open new avenues for nanomanipulation of fragile biological objects.