Flower petal microtexture is a tactile cue for bees

Color-advertising strategies of invasive plants through the bee eye

Invasive plants represent a significant global challenge as they compete with native plants for limited resources such as space, nutrients and pollinators. Here, we focused on four invasive species that are widely spread in the French Pyrenees, Buddleja davidii, Reynoutria japonica, Spiraea japonica and Impatiens glandulifera, and analyzed their visual advertisement signals with respect to those displayed by their surrounding native species using a perceptual approach based on the neural mechanisms of bee vision given that bees are regular pollinators of these plants. We collected 543 spectral reflections from the 4 invasive species, and 66 native species and estimated achromatic and chromatic similarities to the bee eye. R. japonica, S. japonica and B. davidii were inconspicuous against the foliage background and could be hardly discriminated in terms of color from their surrounding native plants. These characteristics promote generalization, potentially attracting pollinators foraging on similar native species. Two morphs of I. glandulifera were both highly salient in chromatic and achromatic terms and different from their surrounding native species. This distinctive identity facilitates detection and learning in association with rich nectar. While visual signals are not the only sensory cue accounting for invasive-plant success, our study reveals new elements for understanding biological invasion processes from the perspective of pollinator perceptual processes.

Multimodal floral recognition by bumblebees

Flowers present information to their insect visitors in multiple simultaneous sensory modalities. Research has commonly focussed on information presented in visual and olfactory modalities. Recently, focus has shifted towards additional 'invisible' information, and whether information presented in multiple modalities enhances the interaction between flowers and their visitors. In this review, we highlight work that addresses how multimodality influences behaviour, focussing on work conducted on bumblebees (Bombus spp.), which are often used due to both their learning abilities and their ability to use multiple sensory modes to identify and differentiate between flowers. We review the evidence for bumblebees being able to use humidity, electrical potential, surface texture and temperature as additional modalities, and consider how multimodality enhances their performance. We consider mechanisms, including the cross-modal transfer of learning that occurs when bees are able to transfer patterns learnt in one modality to an additional modality without additional learning.

Osmophores and petal surface traits in Bignonieae species

This study characterizes the osmophores and corolla traits in 18 species of Bignonieae Dumort., a Bignoniaceae tribe occurring in the Cerrado, a neotropical savanna in Brazil. To detect osmophore distribution, whole, newly opened flowers were immersed in Neutral Red Solution. Samples from the corolla tube and lobes were also fixed and analyzed micromorphologically, anatomically, and histochemically. The osmophores showed six markedly different distribution patterns that were not clearly associated with histological features. In most species, osmophores comprised papillose secretory epidermises and a few layers of subepidermal parenchyma. Starch grains, lipid droplets, and terpenes were detected in osmophores. An ornamented cuticle, cuticular folds, glandular and non-glandular trichomes, raised stomata and epicuticular wax granules are common traits in the species studied and may be useful in determining the taxonomy of the group. We found that 94% of the species visited by bees had papillose epidermises while the single hummingbird-pollinated species presented a flattened epidermis. Variations in osmophore pattern among species visited by bees, including variations within the same plant genus, are novel finding. Additionally, the Bignonieae species visited by bees presented a textured corolla surface, which has been reported as facilitating bee attachment and movement towards the floral resource. Future studies with a greater number of Bignonieae species and more detailed pollinator behavioral assays may help in the interpretation of the variations in corolla traits and functional relationships between flowers and pollinators.

The Ability of Bumblebees Bombus terrestris (Hymenoptera: Apidae) to Detect Floral Humidity is Dependent Upon Environmental Humidity

Flowers produce local humidity that is often greater than that of the surrounding environment, and studies have shown that insect pollinators may be able to use this humidity difference to locate and identify suitable flowers. However, environmental humidity is highly heterogeneous, and is likely to affect the detectability of floral humidity, potentially constraining the contexts in which it can be used as a salient communication pathway between plants and their pollinators. In this study, we use differential conditioning techniques on bumblebees Bombus terrestris audax (Harris) to explore the detectability of an elevated floral humidity signal when presented against different levels of environmental noise. Artificial flowers were constructed that could be either dry or humid, and individual bumblebees were presented with consistent rewards in either the humid or dry flowers presented in an environment with four levels of constant humidity, ranging from low (~20% RH) to highly saturated (~95% RH). Ability to learn was dependent upon both the rewarding flower type and the environment: the bumblebees were able to learn rewarding dry flowers in all environments, but their ability to learn humid rewarding flowers was dependent on the environmental humidity, and they were unable to learn humid rewarding flowers when the environment was highly saturated. This suggests that floral humidity might be masked from bumblebees in humid environments, suggesting that it may be a more useful signal to insect pollinators in arid environments.

Conical petal epidermal cells, regulated by the MYB transcription factor MIXTA, have an ancient origin within the angiosperms

Conical epidermal cells occur on the tepals (perianth organs, typically petals and/or sepals) of the majority of animal-pollinated angiosperms, where they play both visual and tactile roles in pollinator attraction, providing grip to foraging insects, and enhancing colour, temperature, and hydrophobicity. To explore the evolutionary history of conical epidermal cells in angiosperms, we surveyed the tepal epidermis in representative species of the ANA-grade families, the early-diverging successive sister lineages to all other extant angiosperms, and analysed the function of a candidate regulator of cell outgrowth from Cabomba caroliniana (Nymphaeales). We identified conical cells in at least two genera from different families (Austrobaileya and Cabomba). A single SBG9 MYB gene was isolated from C. caroliniana and found to induce strong differentiation of cellular outgrowth, including conical cells, when ectopically expressed in Nicotiana tabacum. Ontogenetic analysis and quantitative reverse transcription-PCR established that CcSBG9A1 is spatially and temporally expressed in a profile which correlates with a role in conical cell development. We conclude that conical or subconical cells on perianth organs are ancient within the angiosperms and most probably develop using a common genetic programme initiated by a SBG9 MYB transcription factor.

Antenna movements as a function of odorants' biological value in honeybees (Apis mellifera L.)

In honeybees, the antennae are highly mobile sensory organs that express scanning movements in various behavioral contexts and toward many stimuli, especially odorants. The rules underlying these movements are still unclear. Using a motion-capture system, we analyzed bees’ antennal responses to a panel of pheromonal and other biologically relevant odorants. We observed clear differences in bees’ antennal responses, with opposite movements to stimuli related to opposite contexts: slow backward movements were expressed in response to alarm pheromones, while fast forward movements were elicited by food related cues as well as brood and queen related pheromones. These responses are reproducible, as a similar pattern of odor-specific responses was observed in bees from different colonies, on different years. We then tested whether odorants’ attractiveness for bees, measured using an original olfactory orientation setup, may predict antenna movements. This simple measure of odorants’ valence did however not correlate with either antennal position or velocity measures, showing that more complex rules than simple hedonics underlie bees’ antennal responses to odorants. Lastly, we show that newly-emerged bees express only limited antennal responses compared to older bees, suggesting that a significant part of the observed responses are acquired during bees’ behavioral development.

Antennal movements can be used as behavioral readout of odor valence in honey bees

The fact that honey bees have a relatively simple nervous system that allows complex behaviors has made them an outstanding model for studying neurobiological processes. Studies on learning and memory routinely use appetitive and aversive learning paradigms that involve recording of the proboscis or the sting extension. However, these protocols are based on all-or-none responses, which has the disadvantage of occluding intermediate and more elaborated behaviors. Nowadays, the great advances in tracking software and data analysis, combined with affordable video recording systems, have made it possible to extract very detailed information about animal behavior. Here we describe antennal movements that are elicited by odor that have no, positive or negative valence. We show that animals orient their antennae towards the source of the odor when it is positive, and orient them in the opposite direction when the odor is negative. Moreover, we found that this behavior was modified between animals that had been trained based on protocols of different strength. Since this procedure allows a more accurate description of the behavioral outcome using a relatively small number of animals, it represents a great tool for studying different cognitive processes and olfactory perception.

Novelty detection in early olfactory processing of the honey bee, Apis mellifera

Animals are constantly bombarded with stimuli, which presents a fundamental problem of sorting among pervasive uninformative stimuli and novel, possibly meaningful stimuli. We evaluated novelty detection behaviorally in honey bees as they position their antennae differentially in an air stream carrying familiar or novel odors. We then characterized neuronal responses to familiar and novel odors in the first synaptic integration center in the brain-the antennal lobes. We found that the neurons that exhibited stronger initial responses to the odor that was to be familiarized are the same units that later distinguish familiar and novel odors, independently of chemical identities. These units, including both tentative projection neurons and local neurons, showed a decreased response to the familiar odor but an increased response to the novel odor. Our results suggest that the antennal lobe may represent familiarity or novelty to an odor stimulus in addition to its chemical identity code. Therefore, the mechanisms for novelty detection may be present in early sensory processing, either as a result of local synaptic interaction or via feedback from higher brain centers.

How and why do bees buzz? Implications for buzz pollination

Buzz pollination encompasses the evolutionary convergence of specialized floral morphologies and pollinator behaviour in which bees use vibrations (floral buzzes) to remove pollen. Floral buzzes are one of several types of vibrations produced by bees using their thoracic muscles. Here I review how bees can produce these different types of vibrations and discuss the implications of this mechanistic understanding for buzz pollination. I propose that bee buzzes can be categorized according to their mode of production and deployment into: (i) thermogenic, which generate heat with little mechanical vibration; (ii) flight buzzes which, combined with wing deployment and thoracic vibration, power flight; and (iii) non-flight buzzes in which the thorax vibrates but the wings remain mostly folded, and include floral, defence, mating, communication, and nest-building buzzes. I hypothesize that the characteristics of non-flight buzzes, including floral buzzes, can be modulated by bees via modification of the biomechanical properties of the thorax through activity of auxiliary muscles, changing the rate of activation of the indirect flight muscles, and modifying flower handling behaviours. Thus, bees should be able to fine-tune mechanical properties of their floral vibrations, including frequency and amplitude, depending on flower characteristics and pollen availability to optimize energy use and pollen collection.

Innate and learnt color preferences in the common green-eyed white butterfly (Leptophobia aripa): experimental evidence

Background

Learning abilities help animals modify their behaviors based on experience and innate sensory biases to confront environmental unpredictability. In a food acquisition context, the ability to detect, learn, and switch is fundamental in a wide range of insect species facing the ever-changing availability of their floral rewards. Here, we used an experimental approach to address the innate color preferences and learning abilities of the common green-eyed white butterfly (Leptophobia aripa).

Methods

In Experiment 1, we conducted innate preference choice-tests to determine whether butterflies had a strong innate color preference and to evaluate whether color preferences differed depending on the array of colors offered. We faced naïve butterflies to artificial flowers of four colors (quadruple choice-test): yellow, pink, white, and red; their choices were assessed. In Experiment 2, we examined the ability of this butterfly species to associate colors with rewards while exploring if the spectral reflectance value of a flower color can slow or accelerate this behavioral response. Butterflies were first trained to be fed from artificial yellow flowers inserted in a feeder. These were later replaced by artificial flowers with a similar (blue) or very different (white) spectral reflectance range. Each preference test comprised a dual-choice test (yellow vs blue, yellow vs white).

Results

Butterflies showed an innate strong preference for red flowers. Both the number of visits and the time spent probing these flowers were much greater than the pink, white, and yellow color flowers. Butterflies learn to associate colors with sugar rewards. They then learned the newly rewarded colors as quickly and proficiently as if the previously rewarded color was similar in spectral reflectance value; the opposite occurs if the newly rewarded color is very different than the previously rewarded color.

Conclusions

Our findings suggest that common green-eyed white butterflies have good learning abilities. These capabilities may allow them to respond rapidly to different color stimulus.

Pollination: Influencing bee behaviour with caffeine

Plant secondary metabolites found in floral nectar can affect the behaviour of pollinating insects, but how these changes benefit plants directly is little understood. An experimental study with bumblebees shows that recalling a caffeine-enhanced odour memory can increase flower visitation.

Bumblebees can detect floral humidity

Floral humidity, a region of elevated humidity in the headspace of the flower, occurs in many plant species and may add to their multimodal floral displays. So far, the ability to detect and respond to floral humidity cues has been only established for hawkmoths when they locate and extract nectar while hovering in front of some moth-pollinated flowers. To test whether floral humidity can be used by other more widespread generalist pollinators, we designed artificial flowers that presented biologically relevant levels of humidity similar to those shown by flowering plants. Bumblebees showed a spontaneous preference for flowers that produced higher floral humidity. Furthermore, learning experiments showed that bumblebees are able to use differences in floral humidity to distinguish between rewarding and non-rewarding flowers. Our results indicate that bumblebees are sensitive to different levels of floral humidity. In this way floral humidity can add to the information provided by flowers and could impact pollinator behaviour more significantly than previously thought.

Spatial patterning of scent in petunia corolla is discriminated by bees and involves the ABCG1 transporter

Floral guides are patterned cues that direct the pollinator to the plant reproductive organs. The spatial distribution of showy visual and olfactory traits allows efficient plant-pollinator interactions. Data on the mechanisms underlying floral volatile patterns or their interactions with pollinators are lacking. Here we characterize the spatial emission patterns of volatiles from the corolla of the model plant Petunia × hybrida and reveal the ability of honeybees to distinguish these patterns. Along the adaxial epidermis, in correlation with cell density, the petal base adjacent to reproductive organs emitted significantly higher levels of volatiles than the distal petal rim. Volatile emission could also be differentiated between the two epidermal surfaces: emission from the adaxial side was significantly higher than that from the abaxial side. Similar emission patterns were also observed in other petunias, Dianthus caryophyllus (carnation) and Argyranthemum frutescens (Marguerite daisy). Analyses of transcripts involved in volatile production/emission revealed lower levels of the plasma-membrane transporter ABCG1 in the abaxial versus adaxial epidermis. Transient overexpression of ABCG1 enhanced emission from the abaxial epidermis to the level of the adaxial epidermis, suggesting its involvement in spatial emission patterns in the epidermal layers. Proboscis extension response experiments showed that differences in emission levels along the adaxial epidermis, that is, petal base versus rim, detected by GC-MS are also discernible by honeybees.

Floral infrared emissivity estimates using simple tools

BACKGROUND

Floral temperature has important consequences for plant biology, and accurate temperature measurements are therefore important to plant research. Thermography, also referred to as thermal imaging, is beginning to be used more frequently to measure and visualize floral temperature. Accurate thermographic measurements require information about the object's emissivity (its capacity to emit thermal radiation with temperature), to obtain accurate temperature readings. However, there are currently no published estimates of floral emissivity available. This is most likely to be due to flowers being unsuitable for the most common protocols for emissivity estimation. Instead, researchers have used emissivity estimates collected on vegetative plant tissue when conducting floral thermography, assuming these tissues to have the same emissivity. As floral tissue differs from vegetative tissue, it is unclear how appropriate and accurate these vegetative tissue emissivity estimates are when they are applied to floral tissue.

RESULTS

We collect floral emissivity estimates using two protocols, using a thermocouple and a water bath, providing a guide for making estimates of floral emissivity that can be carried out without needing specialist equipment (apart from the thermal camera). Both protocols involve measuring the thermal infrared radiation from flowers of a known temperature, providing the required information for emissivity estimation. Floral temperature is known within these protocols using either a thermocouple, or by heating the flowers within a water bath. Emissivity estimates indicate floral emissivity is high, near 1, at least across petals. While the two protocols generally indicated the same trends, the water bath protocol gave more realistic and less variable estimates. While some variation with flower species and location on the flower is observed in emissivity estimates, these are generally small or can be explained as resulting from artefacts of these protocols, relating to thermocouple or water surface contact quality.

CONCLUSIONS

Floral emissivity appears to be high, and seems quite consistent across most flowers and between species, at least across petals. A value near 1, for example 0.98, is recommended for accurate thermographic measurements of floral temperature. This suggests that the similarly high values based on vegetation emissivity estimates used by previous researchers were appropriate.

Tactile active sensing in an insect plant pollinator

The interaction between insects and the flowers they pollinate has driven the evolutionary diversity of both insects and flowering plants, two groups with the most numerous species on Earth. Insects use vision and olfaction to localize host plants, but we know relatively little about how they find the tiny nectary opening in the flower, which can be well beyond their visual resolution. Especially when vision is limited, touch becomes crucial in successful insect-plant pollination interactions. Here, we studied the remarkable feeding behavior of crepuscular hawkmoths Manduca sexta, which use their long, actively controlled, proboscis to expertly explore flower-like surfaces. Using machine vision and 3D-printed artificial flower-like feeders, we revealed a novel behavior that shows moths actively probe surfaces, sweeping their proboscis from the feeder edge to its center repeatedly until they locate the nectary opening. Moreover, naive moths rapidly learn to exploit these flowers, and they adopt a tactile search strategy to more directly locate the nectary opening in as few as three to five consecutive visits. Our results highlight the proboscis as a unique active sensory structure and emphasize the central role of touch in nectar foraging insect-plant pollinator interactions.

Phylogenetic signal in floral temperature patterns

OBJECTIVES

Floral structures may be warmer than their environment, and can show thermal patterning, where individual floral structures show different temperatures across their surface. Pollinators can differentiate between artificial flowers that mimic both naturally warmed and thermally patterned ones, but it has yet to be demonstrated that these patterns are biologically meaningful. To explore the relationship between pollinators and temperature patterning, we need to know whether there is diversity in patterning, and that these patterns are not simply a by-product of floral architecture constrained by ancestry. We analysed a dataset of 97 species to explore whether intrafloral temperature differences were correlated within clades (phylogenetic signal), or whether the variation seen was diverse enough to suggest that floral temperature patterns are influenced by the abiotic or pollinator-related niches to which plant species are adapted.

RESULTS

Some phylogenetic signal was observed, with both the Asteraceae and species of Pelargonium being more similar than expected by chance, but with other species surveyed not showing signal. The Asteraceae tend to have large temperature differences across the floral surface, which may be due to floral architecture constraints within the family. Other families show no correlation, suggesting that patterning is influenced by pollinators and the environment.

Slippery flowers as a mechanism of defence against nectar-thieving ants

BACKGROUND AND AIMS

The great diversity of floral characteristics among animal-pollinated plants is commonly understood to be the result of coevolutionary interactions between plants and pollinators. Floral antagonists, such as nectar thieves, also have the potential to exert an influence upon the selection of floral characteristics, but adaptation against floral antagonists has attracted comparatively little attention. We found that the corollas of hornet-pollinated Codonopsis lanceolata (Campanulaceae) and the tepals of bee-pollinated Fritillaria koidzumiana (Liliaceae) are slippery to nectar-thieving ants living in the plant's habitat; because the flowers of both species have exposed nectaries, slippery perianths may function as a defence against nectar-thieving ants.

METHODS

We conducted a behavioural experiment and observed perianth surface microstructure by scanning electron microscopy to investigate the mechanism of slipperiness. Field experiments were conducted to test whether slippery perianths prevent floral entry by ants, and whether ant presence inside flowers affects pollination.

KEY RESULTS

Scanning electron microscopy observations indicated that the slippery surfaces were coated with epicuticular wax crystals. The perianths lost their slipperiness when wiped with hexane. Artificial bridging of the slippery surfaces using non-slippery materials allowed ants to enter flowers more frequently. Experimental introduction of live ants to the Codonopsis flowers evicted hornet pollinators and shortened the duration of pollinator visits. However, no statistical differences were found in the fruit or seed sets of flowers with and without ants.

CONCLUSIONS

Slippery perianths, most probably based on epicuticular wax crystals, prevent floral entry by ants that negatively affect pollinator behaviour. Experimental evidence of floral defence based on slippery surfaces is rare, but such a mode of defence may be widespread amongst flowering plants.

Conical epidermal cells cause velvety colouration and enhanced patterning in Mandevilla flowers

The majority of angiosperms have flowers with conical epidermal cells, which are assumed to have various functions, such as enhancing the visual signal to pollinators, but detailed optical studies on how conical epidermal cells determine the flower's visual appearance are scarce. Here we report that conical epidermal cells of Mandevilla sanderi flowers effectively reduce surface gloss and create a velvety appearance. Owing to the reduction in surface gloss, the flower further makes more efficient use of floral pigments and light scattering structures inside the flower. The interior backscattering yields a cosine angular dependence of reflected light, meaning that the flowers approximate near-perfect (Lambertian) diffusers, creating a visual signal that is visible across a wide angular space. Together with the large flowers and the tilted corolla tips, this generates a distinct visual pattern, which may enhance the visibility to pollinators.

Iridescence and hydrophobicity have no clear delineation that explains flower petal micro-surface

Plant organs including flowers and leaves typically have a variety of different micro-structures present on the epidermal surface. These structures can produce measurable optical effects with viewing angle including shifts in peak reflectance and intensity; however, these different structures can also modulate hydrophobic properties of the surfaces. For some species optical effects have been proposed to act as signals to enhance pollination interactions, whilst the ability to efficiently shed water provides physiological advantages to plants in terms of gas exchange and reducing infections. Currently, little is known about epidermal surface structure of flowering plants in the Southern Hemisphere, and how micro-surface may be related with either hydrophobicity or visual signalling. We measured four Australian native species and two naturalised species using a combination of techniques including SEM imaging, spectral sampling with a goniometer and contact angle measurements. Spectral data were evaluated in relation to published psychophysics results for important pollinators and reveal that potential visual changes, where present, were unlikely to be perceived by relevant pollinators. Nevertheless, hydrophobicity also did not simply explain petal surfaces as similar structures could in some cases result in very different levels of water repellency.

Australian native flower colours: Does nectar reward drive bee pollinator flower preferences?

Colour is an important signal that flowering plants use to attract insect pollinators like bees. Previous research in Germany has shown that nectar volume is higher for flower colours that are innately preferred by European bees, suggesting an important link between colour signals, bee preferences and floral rewards. In Australia, flower colour signals have evolved in parallel to the Northern hemisphere to enable easy discrimination and detection by the phylogenetically ancient trichromatic visual system of bees, and native Australian bees also possess similar innate colour preferences to European bees. We measured 59 spectral signatures from flowers present at two preserved native habitats in South Eastern Australia and tested whether there were any significant differences in the frequency of flowers presenting higher nectar rewards depending upon the colour category of the flower signals, as perceived by bees. We also tested if there was a significant correlation between chromatic contrast and the frequency of flowers presenting higher nectar rewards. For the entire sample, and for subsets excluding species in the Asteraceae and Orchidaceae, we found no significant difference among colour categories in the frequency of high nectar reward. This suggests that whilst such relationships between flower colour signals and nectar volume rewards have been observed at a field site in Germany, the effect is likely to be specific at a community level rather than a broad general principle that has resulted in the common signalling of bee flower colours around the world.

Multimodal interactions in insect navigation

Animals travelling through the world receive input from multiple sensory modalities that could be important for the guidance of their journeys. Given the availability of a rich array of cues, from idiothetic information to input from sky compasses and visual information through to olfactory and other cues (e.g. gustatory, magnetic, anemotactic or thermal) it is no surprise to see multimodality in most aspects of navigation. In this review, we present the current knowledge of multimodal cue use during orientation and navigation in insects. Multimodal cue use is adapted to a species’ sensory ecology and shapes navigation behaviour both during the learning of environmental cues and when performing complex foraging journeys. The simultaneous use of multiple cues is beneficial because it provides redundant navigational information, and in general, multimodality increases robustness, accuracy and overall foraging success. We use examples from sensorimotor behaviours in mosquitoes and flies as well as from large scale navigation in ants, bees and insects that migrate seasonally over large distances, asking at each stage how multiple cues are combined behaviourally and what insects gain from using different modalities.

Mechanical Ecology-Taking Biomechanics to the Field

Synopsis Interdisciplinary research can have strong and surprising synergistic effects, leading to rapid knowledge gains. Equally important, it can help to reintegrate fragmented fields across increasingly isolated specialist sub-disciplines. However, the lack of a common identifier for research "in between fields" can make it difficult to find relevant research outputs and network effectively. We illustrate and address this issue for the emerging interdisciplinary hotspot of "mechanical ecology," which we define here as the intersection of quantitative biomechanics and field ecology at the organism level. We show that an integrative approach crucially advances our understanding in both disciplines by (1) putting biomechanical mechanisms into a biologically meaningful ecological context and (2) addressing the largely neglected influence of mechanical factors in organismal and behavioral ecology. We call for the foundation of knowledge exchange platforms such as meeting symposia, special issues in journals, and focus groups dedicated to mechanical ecology.

Surprising absence of association between flower surface microstructure and pollination system

The epidermal cells of flowers come in different shapes and have different functions, but how they evolved remains largely unknown. Floral micro-texture can provide tactile cues to insects, and increases in surface roughness by means of conical (papillose) epidermal cells may facilitate flower handling by landing insect pollinators. Whether flower microstructure correlates with pollination system remains unknown. Here, we investigate the floral epidermal microstructure in 29 (congeneric) species pairs with contrasting pollination system. We test whether flowers pollinated by bees and/or flies feature more structured, rougher surfaces than flowers pollinated by non-landing moths or birds and flowers that self-pollinate. In contrast with earlier studies, we find no correlation between epidermal microstructure and pollination system. The shape, cell height and roughness of floral epidermal cells varies among species, but is not correlated with pollinators at large. Intriguingly, however, we find that the upper (adaxial) flower surface that surrounds the reproductive organs and often constitutes the floral display is markedly more structured than the lower (abaxial) surface. We thus conclude that conical epidermal cells probably play a role in plant reproduction other than providing grip or tactile cues, such as increasing hydrophobicity or enhancing the visual signal.

Pollinator parasites and the evolution of floral traits

The main selective force driving floral evolution and diversity is plant-pollinator interactions. Pollinators use floral signals and indirect cues to assess flower reward, and the ensuing flower choice has major implications for plant fitness. While many pollinator behaviors have been described, the impact of parasites on pollinator foraging decisions and plant-pollinator interactions have been largely overlooked. Growing evidence of the transmission of parasites through the shared-use of flowers by pollinators demonstrate the importance of behavioral immunity (altered behaviors that enhance parasite resistance) to pollinator health. During foraging bouts, pollinators can protect themselves against parasites through self-medication, disease avoidance, and grooming. Recent studies have documented immune behaviors in foraging pollinators, as well as the impacts of such behaviors on flower visitation. Because pollinator parasites can affect flower choice and pollen dispersal, they may ultimately impact flower fitness. Here, we discuss how pollinator immune behaviors and floral traits may affect the presence and transmission of pollinator parasites, as well as how pollinator parasites, through these immune behaviors, can impact plant-pollinator interactions. We further discuss how pollinator immune behaviors can impact plant fitness, and how floral traits may adapt to optimize plant fitness in response to pollinator parasites. We propose future research directions to assess the role of pollinator parasites in plant-pollinator interactions and evolution, and we propose better integration of the role of pollinator parasites into research related to pollinator optimal foraging theory, floral diversity and agricultural practices.

Cross-modal transfer in visual and nonvisual cues in bumblebees

Bumblebees Bombus terrestris are good at learning to distinguish between patterned flowers. They can differentiate between flowers that differ only in their patterning of scent, surface texture, temperature, or electrostatic charge, in addition to visual patterns. As recently shown, bumblebees trained to discriminate between nonvisual scent patterns can transfer this learning to visually patterned flowers that show similar spatial patterning to the learnt scent patterns. Bumblebees can, therefore, transfer learnt patterns between different sensory modalities, without needing to relearn them. We used differential conditioning techniques to explore whether cross-modal transfer of learnt patterns also occurred between visual and temperature patterns. Bumblebees that successfully learnt to distinguish rewarding and unrewarding temperature patterns did not show any preferences for the corresponding unlearnt visual pattern. Similarly, bumblebees that learnt visual patterns did not transfer these to temperature patterns, suggesting that they are unable to transfer learning of temperature and visual patterns. We discuss how cross-modality pattern learning may be limited to modalities that have potentially strong neurological links, such as the previously demonstrated transfer between scent and visual patterns.

Ultraviolet patterns of flowers revealed in polymer replica - caused by surface architecture

Angiosperms and their pollinators are adapted in a close co-evolution. For both the plants and pollinators, the functioning of the visual signaling system is highly relevant for survival. As the frequency range of visual perception in many insects extends into the ultraviolet (UV) region, UV-patterns of plants play an important role in the flower-pollinator interaction. It is well known that many flowers contain UV-absorbing pigments in their petal cells, which are localized in vacuoles. However, the contribution of the petal surface microarchitecture to UV-reflection remains uncertain. The correlation between the surface structure and its reflective properties is also relevant for biomimetic applications, for example, in the field of photovoltaics. Based on previous work, we selected three model species with distinct UV-patterns to explore the possible contribution of the surface architecture to the UV-signaling. Using a replication technique, we transferred the petal surface structure onto a transparent polymer. Upon illumination with UV-light, we observed structural-based patterns in the replicas that were surprisingly comparable to those of the original petals. For the first time, this experiment has shown that the parameters of the surface structure lead to an enhancement in the amount of absorbed UV-radiation. Spectrophotometric measurements revealed up to 50% less reflection in the UV-absorbing regions than in the UV-reflecting areas. A comparative characterization of the micromorphology of the UV-reflecting and UV-absorbing areas showed that, in principle, a hierarchical surface structure results in more absorption. Therefore, the results of our experiments demonstrate the structural-based amplification of UV-reflection and provide a starting point for the design of bioinspired antireflective and respectively strongly absorbing surfaces.

Functional significance of the optical properties of flowers for visual signalling

BACKGROUND

Flower coloration is a key enabler for pollinator attraction. Floral visual signals comprise several components that are generated by specific anatomical structures and pigmentation, and often have different functions in pollinator attraction. Anatomical studies have advanced our understanding of the optical properties of flowers, and evidence from behavioural experiments has elucidated the biological relevance of different components of floral visual signals, but these two lines of research are often considered independently.

SCOPE

Here, we review current knowledge about different aspects of the floral visual signals, their anatomical and optical properties, and their functional significance in plant-pollinator visual signalling. We discuss common aspects, such as chromatic and achromatic contrast, hue, saturation and brightness, as well as less common types of visual signals, including gloss, fluorescence, polarization and iridescence in the context of salience of floral colour signals and their evolution, and highlight promising avenues for future research.

UV-light perception is modulated by the odour element of an olfactory-visual compound in restrained honeybees

Honeybees use visual and olfactory cues to detect flowers during foraging trips. Hence, the reward association of a nectar source is a multimodal construct which has at least two major components – olfactory and visual cues. How both sensory modalities are integrated to form a common reward association and whether and how they may interfere, is an open question. The present study used stimulation with UV, blue and green light to evoke distinct photoreceptor activities in the compound eye and two odour components (Geraniol, Citronellol). To test if a compound of both modalities is perceived as the sum of its elements (elemental processing) or as a unique cue (configural processing) we combined monochromatic light with single odour components in positive (PP) and negative patterning (NP) experiments. During PP, the compound of two modalities was rewarded, whereas the single elements were not. For NP, stimuli comprising a single modality were rewarded, whereas the olfactory-visual compound was not. Furthermore, we compared the differentiation abilities between two light stimuli with and without being part of an olfactory-visual compound. Interestingly, the behavioural performances revealed a prominent case of configural processing, but only in those cases when UV light was an element of an olfactory-visual compound. Instead, learning with green- and blue-containing compounds rather supports elemental processing theory.

Reporting of thermography parameters in biology: a systematic review of thermal imaging literature

Infrared (IR) thermography, where temperature measurements are made with IR cameras, has proven to be a very useful and widely used tool in biological science. Several thermography parameters are critical to the proper operation of thermal cameras and the accuracy of measurements, and these must usually be provided to the camera. Failure to account for these parameters may lead to less accurate measurements. Furthermore, the failure to provide information of parameter choices in reports may compromise appraisal of accuracy and replicate studies. In this review, we investigate how well biologists report thermography parameters. This is done through a systematic review of biological thermography literature that included articles published between years 2007 and 2017. We found that in primary biological thermography papers, which make some kind of quantitative temperature measurement, 48% fail to report values used for emissivity (an object's capacity to emit thermal radiation relative to a black body radiator), which is the minimum level of reporting that should take place. This finding highlights the need for life scientists to take into account and report key parameter information when carrying out thermography, in the future.

The yellow specialist: dronefly Eristalis tenax prefers different yellow colours for landing and proboscis extension

Droneflies, imagoes of the hoverfly Eristalis tenax, are known to possess a preference for yellow flowers, i.e. they prefer to visit yellow flowers and prefer to extend the proboscis to yellow colours. In this study we disentangle these colour preferences by investigating the landing reaction and proboscis reflex with particular reference to intensity, spectral purity and dominant wavelength of colour stimuli and their UV reflection properties. In multiple-choice tests, naïve and non-trained flies prefer to land on yellow colours independent of their UV reflection characteristics, but also accept blue, white and pink colours if they absorb UV and are of sufficient brightness. Flies trained to land on colours other than yellow still prefer yellow colours to some extent. Moreover, the flies prefer bright over dark yellow colours even if trained to dark yellow ones. The flies refuse to land on dark colours of all hues. Naïve flies exhibit the proboscis reflex only to pure yellow pollen. These experiments show for the first time that landing in droneflies is triggered by a yellow colour independent of its UV reflection properties, but proboscis extension is triggered by yellow colours strongly absorbing blue and UV. The ability to discriminate colours is better than predicted by the categorical colour vision model. The colour preferences in E. tenax represent a fine-tuned ability to visit yellow flowers displaying a UV bull's-eye pattern.

Nectar trichome structure of aquatic bladderworts from the section Utricularia (Lentibulariaceae) with observation of flower visitors and pollinators

In Utricularia, the flower spur is a nectary and in this organ, nectar is produced and stored. This study aimed to examine the structure of the nectary trichomes in four Utricularia species (Utricularia vulgaris L., U. australis R.Br., U. bremii Heer and U. foliosa L.) from the generic section Utricularia. We have investigated whether species with different spur morphology had similar spur anatomy and nectary trichome structure. In Utricularia flowers, nectar is produced by spur capitate trichomes (sessile or stalked). Our results showed that regardless of the various spur morphology, trichomes have similar architecture and ultrastructure. Head cells of these trichomes are transfer cells with an eccrine nectar secretion. Examined species differed in the micromorphology of papillae in spurs. The fly Eristalis tenax was found to be a pollinator of U. vulgaris. Small Halictidae bees seem to be pollinators of U. foliosa.

Cellulose-Based Biomimetics and Their Applications

Nature has been producing cellulose since long before man walked the surface of the earth. Millions of years of natural design and testing have resulted in cellulose-based structures that are an inspiration for the production of synthetic materials based on cellulose with properties that can mimic natural designs, functions, and properties. Here, five sections describe cellulose-based materials with characteristics that are inspired by gratings that exist on the petals of the plants, structurally colored materials, helical filaments produced by plants, water-responsive materials in plants, and environmental stimuli-responsive tissues found in insects and plants. The synthetic cellulose-based materials described herein are in the form of fibers and films. Fascinating multifunctional materials are prepared from cellulose-based liquid crystals and from composite cellulosic materials that combine functionality with structural performance. Future and recent applications are outlined.

The diversity of floral temperature patterns, and their use by pollinators

Pollinating insects utilise various sensory cues to identify and learn rewarding flower species. One such cue is floral temperature, created by captured sunlight or plant thermogenesis. Bumblebees, honeybees and stingless bees can distinguish flowers based on differences in overall temperature between flowers. We report here that floral temperature often differs between different parts of the flower creating a temperature structure or pattern. Temperature patterns are common, with 55% of 118 plant species thermographed, showing within-flower temperature differences greater than the 2°C difference that bees are known to be able to detect. Using differential conditioning techniques, we show that bumblebees can distinguish artificial flowers differing in temperature patterns comparable to those seen in real flowers. Thus, bumblebees are able to perceive the shape of these within-flower temperature patterns. Floral temperature patterns may therefore represent a new floral cue that could assist pollinators in the recognition and learning of rewarding flowers.

The physics of pollinator attraction

Contents 350 I. 350 II. 350 III. 352 IV. 353 V. 353 353 References 354 SUMMARY: This Tansley Insight focuses on recent advances in our understanding of how flowers manipulate physical forces to attract animal pollinators and ensure reproductive success. Research has traditionally explored the role of chemical pigments and volatile organic compounds as cues for pollinators, but recent reports have demonstrated the importance of physical and structural means of pollinator attraction. Here we explore the role of petal microstructure in influencing floral light capture and optics, analysing colour, gloss and polarization effects. We discuss the interaction between flower, pollinator and gravity, and how petal surface structure can influence that interaction. Finally, we consider the role of electrostatic forces in pollen transfer and pollinator attraction. We conclude that this new interdisciplinary field is evolving rapidly.

The bee, the flower, and the electric field: electric ecology and aerial electroreception

Bees and flowering plants have a long-standing and remarkable co-evolutionary history. Flowers and bees evolved traits that enable pollination, a process that is as important to plants as it is for pollinating insects. From the sensory ecological viewpoint, bee–flower interactions rely on senses such as vision, olfaction, humidity sensing, and touch. Recently, another sensory modality has been unveiled; the detection of the weak electrostatic field that arises between a flower and a bee. Here, we present our latest understanding of how these electric interactions arise and how they contribute to pollination and electroreception. Finite-element modelling and experimental evidence offer new insights into how these interactions are organised and how they can be further studied. Focussing on pollen transfer, we deconstruct some of the salient features of the three ingredients that enable electrostatic interactions, namely the atmospheric electric field, the capacity of bees to accumulate positive charge, and the propensity of plants to be relatively negatively charged. This article also aims at highlighting areas in need of further investigation, where more research is required to better understand the mechanisms of electrostatic interactions and aerial electroreception.

Temporal, but not spatial, changes in expression patterns of petal identity genes are associated with loss of papillate conical cells and the shift to bird pollination in Macaronesian Lotus (Leguminosae)

In the generally bee-pollinated genus Lotus a group of four species have evolved bird-pollinated flowers. The floral changes in these species include altered petal orientation, shape and texture. In Lotus these characters are associated with dorsiventral petal identity, suggesting that shifts in the expression of dorsal identity genes may be involved in the evolution of bird pollination. Of particular interest is Lotus japonicus CYCLOIDEA 2 (LjCYC2), known to determine the presence of papillate conical cells on the dorsal petal in L. japonicus. Bird-pollinated species are unusual in not having papillate conical cells on the dorsal petal. Using RT-PCR at various stages of flower development, we determined the timing of expression in all petal types for the three putative petal identity genes (CYC-like genes) in different species with contrasting floral morphology and pollination syndromes. In bird-pollinated species the dorsal identity gene, LjCYC2, is not expressed at the floral stage when papillate conical cells are normally differentiating in bee-pollinated species. In contrast, in bee-pollinated species, LjCYC2 is expressed during conical cell development. Changes in the timing of expression of the above two genes are associated with modifications in petal growth and lateralisation of the dorsal and ventral petals in the bird-pollinated species. This study indicates that changes in the timing, rather than spatial distribution, of expression likely contribute to the modifications of petal micromorphology and petal size during the transition from bee to bird pollination in Macaronesian Lotus species.

Pollinator shifts drive petal epidermal evolution on the Macaronesian Islands bird-flowered species

Pollinator shifts are considered to drive floral trait evolution, yet little is still known about the modifications of petal epidermal surface at a biogeographic region scale. Here we investigated how independent shifts from insects to passerine birds in the Macaronesian Islands consistently modified this floral trait (i.e. absence of papillate cells). Using current phylogenies and extensive evidence from field observations, we selected a total of 81 plant species and subspecies for petal microscopy and comparative analysis, including 19 of the 23 insular species pollinated by opportunistic passerine birds (Macaronesian bird-flowered element). Species relying on passerine birds as the most effective pollinators (bird-pollinated) independently evolved at least five times and in all instances associated with a loss of papillate cells, whereas species with a mixed pollination system (birds plus insects and/or other vertebrates) evolved at least five times in Macaronesia and papillate cells were lost in only 25% of these transitions. Our findings suggest that petal micromorphology is a labile trait during pollinator shifts and that papillate cells tend to be absent on those species where pollinators have limited mechanical interaction with flowers, including opportunistic passerine birds that forage by hovering or from the ground.

Petal micromorphology and its relationship to pollination

The characteristics of petal epidermal conical cells affect the quality of the signals perceived by various pollinators. This study aimed to identify variations in micromorphological characteristics of flower petals and their relationship to melittophily, ornithophily and chiropterophily pollination systems. The petals of 11 species were analysed using scanning electron microscopy and optical microscopy and the micromorphological traits were described, measured and compared using Tukey's test, PCA and cluster analysis. Unlike chiropterophily, all melittophilous and some ornithophilous species possessed adaxial epidermal conical cells. Cluster grouping separated chiropterophilous flowers from melittophilous and ornithophilous. PCA analysis showed that the two morphometric profile of conical cells was the attribute that most strongly influenced the grouping of species. When considering the data set of the three pollination systems, melittophilous and ornithophilous plants were more similar to each other than they were to chriopterophilous species. The distance between conical cell apices is an important parameter in interactions with pollinators. This study facilitated recognition of smoothing pollinator resource access through petal micromorphological characteristics. Further research regarding the biometry of micromorphological traits related to pollination is required.

Bumblebees require visual pollen stimuli to initiate and multimodal stimuli to complete a full behavioral sequence in close-range flower orientation

Flower visits are complex encounters, in which animals are attracted by floral signals, guided toward the site of the first physical contact with a flower, land, and finally take up floral rewards. At close range, signals of stamens and pollen play an important role to facilitate flower handling in bees, yet the pollen stimuli eliciting behavioral responses are poorly known. In this study, we test the response of flower‐naive bumblebees (Bombus terrestris) toward single and multimodal pollen stimuli as compared to natural dandelion pollen. As artificial pollen stimuli, we used the yellow flavonoid pigment quercetin, the scent compound eugenol, the amino acid proline, the monosaccharide glucose, and the texture of pollen‐grain‐sized glass pellets as a tactile stimulus. Three test stimuli, dandelion pollen, one out of various uni‐ and multimodal stimulus combinations, and a solvent control were presented simultaneously to individual bumblebees, whose response was recorded. The results indicate that bumblebees respond in an irreversible sequence of behavioral reactions. Bumblebees approached the visual stimulus quercetin as often as natural dandelion pollen. An additional olfactory stimulus resulted in slightly more frequent landings. The multimodal stimulus combinations including visual, olfactory, gustatory, and tactile stimuli elicited approaches, antennal contacts, and landings as often as natural pollen. Subsequent reactions like proboscis extension, mandible biting, and buzzing were more often but not regularly observed at dandelion pollen. Our study shows that visual signals of pollen are sufficient to trigger initial responses of bumblebees, whereas multimodal pollen stimuli elicit full behavioral response as compared to natural pollen. Our results suggest a major role of pollen cues for the attraction of bees toward flowers and also explain, why many floral guides mimic the visual signals of pollen and anthers, that is, the yellow and UV‐absorbing color, to direct bumblebees toward the site where they access the floral rewards.

Learning in Insect Pollinators and Herbivores

The relationship between plants and insects is influenced by insects' behavioral decisions during foraging and oviposition. In mutualistic pollinators and antagonistic herbivores, past experience (learning) affects such decisions, which ultimately can impact plant fitness. The higher levels of dietary generalism in pollinators than in herbivores may be an explanation for the differences in learning seen between these two groups. Generalist pollinators experience a high level of environmental variation, which we suggest favors associative learning. Larval herbivores employ habituation and sensitization-strategies useful in their less variable environments. Exceptions to these patterns based on habitats, mobility, and life history provide critical tests of current theory. Relevant plant traits should be under selection to be easily learned and remembered in pollinators and difficult to learn in herbivores. Insect learning thereby has the potential to have an important, yet largely unexplored, role in plant-insect coevolution.

Plant Surfaces: Structures and Functions for Biomimetic Innovations

An overview of plant surface structures and their evolution is presented. It combines surface chemistry and architecture with their functions and refers to possible biomimetic applications. Within some 3.5 billion years biological species evolved highly complex multifunctional surfaces for interacting with their environments: some 10 million living prototypes (i.e., estimated number of existing plants and animals) for engineers. The complexity of the hierarchical structures and their functionality in biological organisms surpasses all abiotic natural surfaces: even superhydrophobicity is restricted in nature to living organisms and was probably a key evolutionary step with the invasion of terrestrial habitats some 350-450 million years ago in plants and insects. Special attention should be paid to the fact that global environmental change implies a dramatic loss of species and with it the biological role models. Plants, the dominating group of organisms on our planet, are sessile organisms with large multifunctional surfaces and thus exhibit particular intriguing features. Superhydrophilicity and superhydrophobicity are focal points in this work. We estimate that superhydrophobic plant leaves (e.g., grasses) comprise in total an area of around 250 million km2, which is about 50% of the total surface of our planet. A survey of structures and functions based on own examinations of almost 20,000 species is provided, for further references we refer to Barthlott et al. (Philos. Trans. R. Soc. A 374: 20160191, 1). A basic difference exists between aquatic non-vascular and land-living vascular plants; the latter exhibit a particular intriguing surface chemistry and architecture. The diversity of features is described in detail according to their hierarchical structural order. The first underlying and essential feature is the polymer cuticle superimposed by epicuticular wax and the curvature of single cells up to complex multicellular structures. A descriptive terminology for this diversity is provided. Simplified, the functions of plant surface characteristics may be grouped into six categories: (1) mechanical properties, (2) influence on reflection and absorption of spectral radiation, (3) reduction of water loss or increase of water uptake, moisture harvesting, (4) adhesion and non-adhesion (lotus effect, insect trapping), (5) drag and turbulence increase, or (6) air retention under water for drag reduction or gas exchange (Salvinia effect). This list is far from complete. A short overview of the history of bionics and the impressive spectrum of existing and anticipated biomimetic applications are provided. The major challenge for engineers and materials scientists, the durability of the fragile nanocoatings, is also discussed.

Sugar concentration in nectar: a quantitative metric of crop attractiveness for refined pollinator risk assessments

Those involved with pollinator risk assessment know that agricultural crops vary in attractiveness to bees. Intuitively, this means that exposure to agricultural pesticides is likely greatest for attractive plants and lowest for unattractive plants. While crop attractiveness in the risk assessment process has been qualitatively remarked on by some authorities, absent is direction on how to refine the process with quantitative metrics of attractiveness. At a high level, attractiveness of crops to bees appears to depend on several key variables, including but not limited to: floral, olfactory, visual and tactile cues; seasonal availability; physical and behavioral characteristics of the bee; plant and nectar rewards. Notwithstanding the complexities and interactions among these variables, sugar content in nectar stands out as a suitable quantitative metric by which to refine pollinator risk assessments for attractiveness. Provided herein is a proposed way to use sugar nectar concentration to adjust the exposure parameter (with what is called a crop attractiveness factor) in the calculation of risk quotients in order to derive crop-specific tier I assessments. This Perspective is meant to invite discussion on incorporating such changes in the risk assessment process. © 2016 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.