Diversity of attachment systems in heelwalkers (Mantophasmatodea) - highly specialized, but uniform

BACKGROUND

Heelwalkers possess a highly modified tarsal attachment system. All extant species lift the distalmost tarsomere permanently off the substrate and primarily use their euplantulae for locomotion. The combination of a smooth adhesive pad (arolium) on the pretarsus and fibrillary attachment pads on the euplantulae offers valuable insights for translational approaches, but its infra-order diversity remains unexplored.

RESULTS

We explored the morphology of the tarsal attachment apparatus of Mantophasmatodea based on a representative taxon sampling spanning a large fraction of species of this group and compared morphological differences in the specialized morphology of this system across species and sexes. Our scanning electron microscope investigation of the tarsi of 11 species (52% of all described extant species) revealed an overall very consistent ground pattern and almost no specific adaptations. There are only minor, but mostly clade-specific differences in the shape of the adhesive setae on the tarsal euplantulae and in the morphology and density of the acanthae on the pretarsal arolium. Both features differ primarily between Austrophasmatidae in comparison to the remaining Mantophasmatodea taxa.

CONCLUSION

We conclude that the strong specialization of the mantophasmatodean tarsal attachment sufficiently copes with the diversity of substrates the insects are exposed to.

Functional morphology of cleaning devices in the damselfly Ischnura elegans (Odonata, Coenagrionidae)

Among the different micro- and nanostructures located on cuticular surfaces, grooming devices represent fundamental tools for insect survival. The present study describes the grooming microstructures of the damselfly Ischnura elegans (Odonata, Coenagrionidae) at the adult stage. These structures, situated on the foreleg tibiae, were observed using scanning electron microscopy, and the presence and distribution of resilin, an elastomeric protein that enhances cuticle flexibility, were analyzed using confocal laser scanning microscopy. Eye and antennal grooming behavior were analyzed to evaluate the particle removal efficiency in intact insects and in insects with ablated grooming devices. The grooming devices are constituted of long setae from which a concave cuticular lamina develops towards the medial side of the leg. Each seta shows a material gradient of resilin from its basal to the distal portion and from the seta to the cuticular lamina. The removal of the grooming devices induces a strong increase in the contaminated areas on the eyes after grooming. Further studies on insect grooming can provide valuable data on the functional morphology of insect micro- and nanostructures and can represent a starting point to develop advanced biomimetic cleaning tools.

Capillary adhesion of stick insects

Scientific progress within the last few decades has revealed the functional morphology of an insect's sticky footpads-a compliant pad that secretes thin liquid films. However, the physico-chemical mechanisms underlying their adhesion remain elusive. Here, we explore these underlying mechanisms by simultaneously measuring adhesive force and contact geometry of the adhesive footpads of live, tethered Indian stick insects, Carausius morosus, spanning more than two orders of magnitude in body mass. We find that the adhesive force we measure is similar to the previous measurements that use a centrifuge. Our measurements afford us the opportunity to directly probe the adhesive stress in vivo and use existing theory on capillary adhesion to predict the surface tension of the secreted liquid and compare it to previous assumptions. From our predictions, we find that the surface tension required to generate the adhesive stresses we observed ranges between 0.68 and 12 mNm - 1 ${\rm m}^{-1}$ . The low surface tension of the liquid would enhance the wetting of the stick insect's footpads and promote their ability to conform to various substrates. Our insights may inform the biomimetic design of capillary-based, reversible adhesives and motivate future studies on the physico-chemical properties of the secreted liquid.

Comparative analysis of the ultrastructure and adhesive secretion pathways of different smooth attachment pads of the stick insect Medauroidea extradentata (Phasmatodea)

The mechanism by which insects achieve attachment and locomotion across diverse substrates has long intrigued scientists, prompting extensive research on the functional morphology of attachment pads. In stick insects, attachment and locomotion are facilitated by two distinct types of smooth cuticular attachment pads: the primary adhesion force-generating arolium and the friction force-generating euplantulae. They are both supported by an adhesive secretion delivered into the interspace between the attachment pads and the substrate. In this study, we analysed and compared internal morphology, material composition and ultrastructure, as well as the transportation pathways in both adhesive organs in the stick insect Medauroidea extradentata using scanning electron microscopy, micro-computed tomography, light microscopy, and confocal laser scanning microscopy. Our observations revealed structural differences between both attachment pads, reflecting their distinct functionality. Furthermore, our results delineate a potential pathway for adhesive secretions, originating from exocrine epidermal cells and traversing various layers before reaching the surface. Within the attachment pad, the fluid may influence the viscoelastic properties of the pad and control the attachment/detachment process. Understanding the material composition of attachment pads and the distribution process of the adhesive secretion can potentially aid in the development of more effective artificial attachment systems.

Characterization of Morphologically Distinct Components in the Tarsal Secretion of Medauroidea extradentata (Phasmatodea) Using Cryo-Scanning Electron Microscopy

Attachment to the substrate is an important phenomenon that determines the survival of many organisms. Most insects utilize wet adhesion to support attachment, which is characterized by fluids that are secreted into the interface between the tarsus and the substrates. Previous research has investigated the composition and function of tarsal secretions of different insect groups, showing that the secretions are likely viscous emulsions that contribute to attachment by generating capillary and viscous adhesion, leveling surface roughness and providing self-cleaning of the adhesive systems. Details of the structural organization of these secretions are, however, largely unknown. Here, we analyzed footprints originating from the arolium and euplantulae of the stick insect Medauroidea extradentata using cryo-scanning electron microscopy (cryo-SEM) and white light interferometry (WLI). The secretion was investigated with cryo-SEM, revealing four morphologically distinguishable components. The 3D WLI measurements of the droplet shapes and volumes over time revealed distinctly different evaporation rates for different types of droplets. Our results indicate that the subfunctionalization of the tarsal secretion is facilitated by morphologically distinct components, which are likely a result of different proportions of components within the emulsion. Understanding these components and their functions may aid in gaining insights for developing adaptive and multifunctional biomimetic adhesive systems.

The tight attachment achieved by the male discoidal setae is possibly a counter-adaptation to the grease layer on female integument surfaces in green dock beetles

Green dock beetles Gastrophysa viridula exhibit sexual dimorphism in tarsal attachment setae: females have only pointed, lanceolate and spatula-like setae, while males additionally possess discoidal ones. The sexual dimorphism is probably attributed to the necessity of male discoidal setae to adhere to the smooth back of the female during copulation. We aimed to understand its possible mechanism of attachment with G. viridula. Pull-off forces of both females and males were measured on (i) alive females, (ii) dead and dried females, and (iii) resin replicas of fresh females. The attachment ability tended to increase on dead and replicated female surfaces in both sexes, which indicates that the epicuticular grease layer on the integument of alive intact beetles decreases the attachment. This tendency was prominent in females. The present study clearly showed that in G. viridula discoidal setae enable the males to adhere stronger to female surfaces. The divergent performance found between the sexes differing in their setal composition is probably caused by the stiffness difference between the setae types and by the specific shape of the setal tips. A peculiar reproductive biology in G. viridula is probably attributed to this remarkable divergence of labour in their attachment pads between the sexes.

Bio-inspired materials to control and minimise insect attachment

More than three quarters of all animal species on Earth are insects, successfully inhabiting most ecosystems on the planet. Due to their opulence, insects provide the backbone of many biological processes, but also inflict adverse impacts on agricultural and stored products, buildings and human health. To countermeasure insect pests, the interactions of these animals with their surroundings have to be fully understood. This review focuses on the various forms of insect attachment, natural surfaces that have evolved to counter insect adhesion, and particularly features recently developed synthetic bio-inspired solutions. These bio-inspired solutions often enhance the variety of applicable mechanisms observed in nature and open paths for improved technological solutions that are needed in a changing global society.

Smart joints: auto-cleaning mechanism in the legs of beetles

The auto-cleaning system in digging forelegs of the Congo rose chafer Pachnoda marginata femoro-tibial joint is described. The cleaning system consists of four subsystems: three external ones represented by microsetal pad, hairy brush and scraper and one internal one. They work proactively not only removing contaminants, but also preventing them from entering the joint. The principle of functioning of the cleaning system is based on the sliding of the contacting surfaces of the joint, equipped with hairs, bristles and scrapers. The mutual movement of such surfaces leads to the shift of contaminating particles and, ultimately, to their removal from surfaces of the joint. The key feature of the joint cleaning system is its complete autonomy, in which cleaning is performed constantly with each movement of the femoro-tibial joint without special actions required from the insect. The difference between the auto-cleaning system and self-cleaning and active grooming is also discussed.

The femoro-tibial joint of the digging legs of the beetle Pachnoda marginata employs an automatic cleaning system to avoid contamination.

Enhanced self-cleaning performance of bio-inspired micropillar-arrayed surface by shear

Inspired by the sliding behavior of gecko feet during climbing, the contribution of the shear effect to the self-cleaning performance of a bio-inspired micropillar-arrayed surface is studied through a load-shear-pull contact process. It is found that self-cleaning efficiency can be enhanced significantly by shear. The efficiency also depends on microparticle size. For the case of relatively large and small microparticles, self-cleaning efficiency increases first and then almost keeps a constant with the increase of shear distance at different preloads. For medium microparticles, shear can effectively improve self-cleaning efficiency only when the preload is small. The mechanical mechanism under such enhancement is mainly due to the varying contact states between microparticles and micropillars with the shear distance. When the shear distance is large enough, the final self-cleaning efficiency is not sensitive to shear distance anymore because the contact state reaches dynamic equilibrium. Based on such a self-cleaning mechanism of large microparticles, a simple and effective manipulator that can efficiently transfer solid particles is further proposed.

Reduction in Insect Attachment Caused by Different Nanomaterials Used as Particle Films (Kaolin, Zeolite, Calcium Carbonate)

In the present investigation, we compared the reduction in attachment ability of the southern green stinkbug Nezara viridula (Hemiptera: Pentatomidae) to glass induced by three different nanoparticle (kaolin, zeolite, and calcium carbonate) films. Using traction force experiments, behavioral experiments, and scanning electron microscopy observations, we analyzed the insect attachment ability and linear speed on untreated and treated glass with the three particle films. The three nanomaterials strongly reduced insect attachment ability mainly owing to contamination of attachment pads. The ability to reduce insect attachment was different for the three tested particle films: kaolin and zeolite induced a significantly higher reduction in N. viridula safety factor than calcium carbonate. The coating of the surface was more uniform and compact in kaolin and zeolite compared to calcium carbonate particle film. Moreover, kaolin and zeolite particles can more readily adhere to N. viridula attachment devices, whereas calcium carbonate particles appeared less adherent to the cuticular surface compared to the two aluminosilicate (kaolin and zeolite) particles. Only the application of kaolin reduced insect linear speed during locomotion. Nanoparticle films have a great potential to reduce insect attachment ability and represent a good alternative to the use of insecticides for the control of pentatomid bugs and other pest insects.

Insect Antiadhesive Surfaces Using Electrosprayed Wrinkled Ethyl Cellulose Particles

A range of plants developed leaves, the surfaces of which prevent or diminish insect adhesion due to their microscopic topography. Well known examples include the leaves of the lychee tree (Litchi chinensis). Here, we report a method to coat substrates with ethyl cellulose microparticles that exhibit wrinkled surfaces, resulting in surface morphologies that closely resemble those of insect repelling plants, i.e., Litchi chinensis. The microparticles were prepared by electrospraying, a method that allowed tuning of the particle size and surface morphology. By measuring the traction forces of Colorado potato beetles walking on these surfaces, the wrinkly microsphere parameters were optimized, resulting in biomimetic surfaces that surpass the antiadhesive properties of the biological role model. This study may pave the way to sustainable, nontoxic insecticide replacements.

Attachment devices and the tarsal gland of the bug Coreus marginatus (Hemiptera: Coreidae)

The present ultrastructural investigation using scanning and transmission electron microscopy as well as light and fluorescence microscopy describes in detail the attachment devices and tarsal gland of the bug Coreus marginatus (L.) (Hemiptera: Coreidae). In particular, the fine structure of pulvilli reveals a ventral surface rich with pore channels, consistent with fluid emission, and a folded dorsal surface, which could be useful to enhance the pulvillus contact area during attachment to the substrate. The detailed description of the tarsal gland cells, whose structure is coherent with an active secretory function, allows us to consider the tarsal gland as the plausible candidate for the adhesive fluid production. Scolopidia strictly adhering to the gland cells are also described. On the basis of the fine structure of the tarsal gland, we hypothesise a fluid emission mechanism based on changes of the hydraulic pressure inside the gland, due to the unguitractor tendon movements. This mechanism could provide the fluid release based on compression of the pad and capillary suction, as demonstrated in other insects. The data here reported can contribute to understanding of insect adhesive fluid production, emission and control of its transport.

Comparison of tarsal attachment in two closely related leaf beetle species

Green dock beetles Gastrophysa viridula and Colorado potato beetles Leptinotarsa decemlineata having distinctly different body mass and gait habits were compared with respect to their tarsal morphology and attachment ability. The focus laid on shapes and dimensions of tenent setae related to the peeling line, i.e., the sum of widths of all thin-film elements participating in contact. High-speed rotation of the two leaf beetle species attached to the horizontal and vertical sides of a Plexiglass drum resulted in higher attachment forces of the heavier beetle species that has a larger number of tarsal setae and a larger peeling line length. However, normalizing the measured forces with the corresponding peeling line lengths led to a reversed relationship. This allowed us to assume that the design of adhesive setae in different leaf beetle species matches the requirements imposed by their habitats. In accordance with the theory of thin film peeling, tangential forces were found to be higher than normal forces. The attachment system of females was found to exhibit stronger functional efficiency, which can be correlated to the morphology of their setae.

Liquid dispensing in the adhesive hairy pads of dock beetles

Many insects can climb on smooth inverted substrates using adhesive hairy pads on their legs. The hair-surface contact is often mediated by minute volumes of liquid, which form capillary bridges in the contact zones and aid in adhesion. The liquid transport to the contact zones is poorly understood. We investigated the dynamics of liquid secretion in the dock beetle Gastrophysa viridula by quantifying the volume of the deposited liquid footprints during simulated walking experiments. The footprint volume increased with pad-surface contact time and was independent of the non-contact time. Furthermore, the footprint volume decreased to zero after reaching a threshold cumulative volume (approx. 30 fl) in successive steps. This suggests a limited reservoir with low liquid influx. We modelled our results as a fluidic resistive system and estimated the hydraulic resistance of a single attachment hair of the order of MPa · s/fl. The liquid secretion in beetle hairy pads is dominated by passive suction of the liquid during the contact phase. The high calculated resistance of the secretion pathway may originate from the nanosized channels in the hair cuticle. Such nanochannels presumably mediate the transport of cuticular lipids, which are chemically similar to the adhesive liquid.

Versatility of Turing patterns potentiates rapid evolution in tarsal attachment microstructures of stick and leaf insects (Phasmatodea)

In its evolution, the diverse group of stick and leaf insects (Phasmatodea) has undergone a rapid radiation. These insects evolved specialized structures to adhere to different surfaces typical for their specific ecological environments. The cuticle of their tarsal attachment pads (euplantulae) is known to possess a high diversity of attachment microstructures (AMS) which are suggested to reflect ecological specializations of different groups within phasmids. However, the origin of these microstructures and their developmental background remain largely unknown. Here, based on the detailed scanning electron microscopy study of pad surfaces, we present a theoretical approach to mathematically model an outstanding diversity of phasmid AMS using the reaction-diffusion model by Alan Turing. In general, this model explains pattern formation in nature. For the first time, we were able to identify eight principal patterns and simulate the transitions among these. In addition, intermediate transitional patterns were predicted by the model. The ease of transformation suggests a high adaptability of the microstructures that might explain the rapid evolution of pad characters. We additionally discuss the functional morphology of the different microstructures and their assumed advantages in the context of the ecological background of species.

Structure and biomechanics of the antennal grooming mechanism in the southern green stink bug Nezara viridula

Insects devote a large amount of time to self-groom to remove foreign material, especially from their sensory appendages. Using various microscopy techniques and behavioural experiments on intact and ablated insects, the present study investigates the antennal grooming of the southern green stinkbug Nezara viridula, which represents a serious pest of different crops in most areas of the world. The antennal grooming behaviour encompasses an action of scraping involving the tibial comb complex (tibial comb + fossula) of both forelegs, generally followed by the tibial comb complex grooming of one leg using the tarsal hairy adhesive pad of the opposite leg (rubbing). From our observations, we can exclude a role in the antennal grooming of other structures such as the foretibial apparatus, while we show an involvement of this last structure in repositioning the stylets inside the labium. The external and internal morphology (cryo-scanning and transmission electron microscopy) and the evidence for the presence of large proportions of the elastic protein resilin (confocal laser scanning microscopy) in some parts of both the tibial comb complex and the foretibial apparatus are shown, and their functional roles are discussed. For the first time we demonstrated here the multipurpose role of the basitarsal hairy adhesive pad that is involved in both antennal grooming and adhesion to the substrate.

Liquid secretion and setal compliance: the beetle's winning combination for a robust and reversible adhesion

This paper is a brief review and discussion of the recent literature on the hairy adhesive pads of beetles, with the focus on two features of these pads, firstly, compliant setal tips and secondly, a liquid secretion, that together guarantee robust cycles of attachment/detachment on smooth and rough substrates. The compliance is required to ensure sufficient contact between the setal tips and the substrate with a minimum of elastically stored energy at the contact interface. The secretion fills potential gaps between both surfaces, generates capillary adhesive forces, and enhances self-cleaning of these microstructures. Furthermore, the secretion might prevent setal dehydration and subsequently maintain setal tip compliancy. The paper also pinpoints a series of open questions on the physical mechanisms at play to passively regulate the contact forces developed by these hairy pads during locomotion.

Adhesion and running speed of a tropical arboreal ant (Cephalotes atratus) on wet substrates

In the tropical forest canopy, wingless worker ants must cling to and run along diverse vegetative surfaces with little protection from sun, wind and rain. Ants rely in part on their tiny adhesive tarsal pads to maintain sufficient contact with substrates to prevent falls under these varied conditions. Here, we examined the effects of substrate wettability and surface water on the tarsal pad adhesive performance of a common tropical arboreal ant. Ant adhesion was consistently higher on an intermediately wetting substrate (static water contact angle ca 90°) when resisting both perpendicular (normal) force and parallel (shear) force. Normal adhesion was maintained on intermediately wetting and hydrophobic substrates following the addition of rain-mimicking water droplets, whereas shear adhesion declined on all substrate types tested after wetting. Ant running speed was slower on wet substrates. On wood substrates, normal and shear adhesion declined with increasing wetness from dry, to misted, to water-soaked. These differences probably contributed to lower ant running speed on wet wood. The results of this study provide the first quantitative assessment of tropical arboreal ant adhesive performance under substrate conditions that are commonly encountered in the rainforest canopy.

It's Not a Bug, It's a Feature: Functional Materials in Insects

Over the course of their wildly successful proliferation across the earth, the insects as a taxon have evolved enviable adaptations to their diverse habitats, which include adhesives, locomotor systems, hydrophobic surfaces, and sensors and actuators that transduce mechanical, acoustic, optical, thermal, and chemical signals. Insect-inspired designs currently appear in a range of contexts, including antireflective coatings, optical displays, and computing algorithms. However, as over one million distinct and highly specialized species of insects have colonized nearly all habitable regions on the planet, they still provide a largely untapped pool of unique problem-solving strategies. With the intent of providing materials scientists and engineers with a muse for the next generation of bioinspired materials, here, a selection of some of the most spectacular adaptations that insects have evolved is assembled and organized by function. The insects presented display dazzling optical properties as a result of natural photonic crystals, precise hierarchical patterns that span length scales from nanometers to millimeters, and formidable defense mechanisms that deploy an arsenal of chemical weaponry. Successful mimicry of these adaptations may facilitate technological solutions to as wide a range of problems as they solve in the insects that originated them.

Soiled adhesive pads shear clean by slipping: a robust self-cleaning mechanism in climbing beetles

Animals using adhesive pads to climb smooth surfaces face the problem of keeping their pads clean and functional. Here, a self-cleaning mechanism is proposed whereby soiled feet would slip on the surface due to a lack of adhesion but shed particles in return. Our study offers an in situ quantification of self-cleaning performance in fibrillar adhesives, using the dock beetle as a model organism. After beetles soiled their pads by stepping into patches of spherical beads, we found that their gait was significantly affected. Specifically, soiled pads slipped 10 times further than clean pads, with more particles deposited for longer slips. Like previous studies, we found that particle size affected cleaning performance. Large (45 μm) beads were removed most effectively, followed by medium (10 μm) and small (1 μm). Consistent with our results from climbing beetles, force measurements on freshly severed legs revealed larger detachment forces of medium particles from adhesive pads compared to a flat surface, possibly due to interlocking between fibres. By contrast, dock leaves showed an overall larger affinity to the beads and thus reduced the need for cleaning. Self-cleaning through slippage provides a mechanism robust to particle size and may inspire solutions for artificial adhesives.

A Review of the State of Dry Adhesives: Biomimetic Structures and the Alternative Designs They Inspire

Robust and inexpensive dry adhesives would have a multitude of potential applications, but replicating the impressive adhesive organs of many small animals has proved challenging. A substantial body of work has been produced in recent years which has illuminated the many mechanical processes influencing a dry adhesive interface. The especially potent footpads of the tokay gecko have inspired researchers to develop and examine an impressive and diverse collection of artificial fibrillar dry adhesives, though study of tree frogs and insects demonstrate that successful adhesive designs come in many forms. This review discusses the current theoretical understanding of dry adhesive mechanics, including the observations from biological systems and the lessons learned by recent attempts to mimic them. Attention is drawn in particular to the growing contingent of work exploring ideas which are complimentary to or an alternative for fibrillar designs. The fundamentals of compliance control form a basis for dry adhesives made of composite and “smart,” stimuli-responsive materials including shape memory polymers. An overview of fabrication and test techniques, with a sampling of performance results, is provided.

Hairiness: the missing link between pollinators and pollination

Background

Functional traits are the primary biotic component driving organism influence on ecosystem functions; in consequence, traits are widely used in ecological research. However, most animal trait-based studies use easy-to-measure characteristics of species that are at best only weakly associated with functions. Animal-mediated pollination is a key ecosystem function and is likely to be influenced by pollinator traits, but to date no one has identified functional traits that are simple to measure and have good predictive power.

Methods

Here, we show that a simple, easy to measure trait (hairiness) can predict pollinator effectiveness with high accuracy. We used a novel image analysis method to calculate entropy values for insect body surfaces as a measure of hairiness. We evaluated the power of our method for predicting pollinator effectiveness by regressing pollinator hairiness (entropy) against single visit pollen deposition (SVD) and pollen loads on insects. We used linear models and AIC_C model selection to determine which body regions were the best predictors of SVD and pollen load.

Results

We found that hairiness can be used as a robust proxy of SVD. The best models for predicting SVD for the flower species Brassica rapa and Actinidia deliciosa were hairiness on the face and thorax as predictors (R² = 0.98 and 0.91 respectively). The best model for predicting pollen load for B. rapa was hairiness on the face (R² = 0.81).

Discussion

We suggest that the match between pollinator body region hairiness and plant reproductive structure morphology is a powerful predictor of pollinator effectiveness. We show that pollinator hairiness is strongly linked to pollination—an important ecosystem function, and provide a rigorous and time-efficient method for measuring hairiness. Identifying and accurately measuring key traits that drive ecosystem processes is critical as global change increasingly alters ecological communities, and subsequently, ecosystem functions worldwide.

Rate-dependence of 'wet' biological adhesives and the function of the pad secretion in insects

Many insects use soft adhesive footpads for climbing. The surface contact of these organs is mediated by small volumes of a liquid secretion, which forms thin films in the contact zone. Here, we investigate the role of viscous dissipation by this secretion and the 'bulk' pad cuticle by quantifying the rate-dependence of the adhesive force of individual pads. Adhesion increased with retraction speed, but this effect was independent of the amount of pad secretion present in the contact zone, suggesting that the secretion's viscosity did not play a significant role. Instead, the rate-dependence can be explained by relating the strain energy release rate to the speed of crack propagation, using an established empirical power law. The 'wet' pads' behaviour was akin to that of 'dry' elastomers, with an equilibrium energy release rate close to that of dry van-der-Waals contacts. We suggest that the secretion mainly serves as a 'release layer', minimising viscous dissipation and thereby reducing the time- and 'loading-history'-dependence of the adhesive pads. In contrast to many commercial adhesives which derive much of their strength from viscous dissipation, we show that the major modulator of adhesive strength in 'wet' biological adhesive pads is friction, exhibiting a much larger effect than retraction speed. A comparison between 'wet' and 'dry' biological adhesives, using both results from this study and the literature, revealed a striking lack of differences in attachment performance under varying experimental conditions. Together, these results suggest that 'wet' and 'dry' biological adhesives may be more similar than previously thought.

Energy saving strategies of honeybees in dipping nectar

The honeybee's drinking process has generally been simplified because of its high speed and small scale. In this study, we clearly observed the drinking cycle of the Italian honeybee using a specially designed high-speed camera system. We analysed the pattern of glossal hair erection and the movement kinematics of the protracting tongue (glossa). Results showed that the honeybee used two special protraction strategies to save energy. First, the glossal hairs remain adpressed until the end of the protraction, which indicates that the hydraulic resistance is reduced to less than 1/3 of that in the case if the hairs remain erect. Second, the glossa protracts with a specific velocity profile and we quantitatively demonstrated that this moving strategy helps reduce the total energy needed for protraction compared with the typical form of protraction with constant acceleration and deceleration. These findings suggest effective methods to optimise the control policies employed by next-generation microfluidic pumps.

Cleanliness is next to godliness: mechanisms for staying clean

Getting dirty is a fundamental problem, and one for which there are few solutions, especially across the enormous range of animal size. How do both a honeybee and a squirrel get clean? In this Review, we discuss two broad types of cleaning, considered from the viewpoint of energetics. Non-renewable cleaning strategies rely upon the organism as an energy source. Examples include grooming motions, wet-dog shaking or the secretion of chemicals. Renewable cleaning strategies depend on environmental sources of energy, such as the use of eyelashes to redirect incoming wind and so reduce deposition onto the eye. Both strategies take advantage of body hair to facilitate cleaning, and honeybees and squirrels, for example, each have around 3 million hairs. This hair mat increases the area on which particles can land by a factor of 100, but also suspends particles above the body, reducing their adhesion and facilitating removal. We hope that the strategies outlined here will inspire energy-efficient cleaning strategies in synthetic systems.

Functional morphology and efficiency of the antenna cleaner in Camponotus rufifemur ants

Contamination of body surfaces can negatively affect many physiological functions. Insects have evolved different adaptations for removing contamination, including surfaces that allow passive self-cleaning and structures for active cleaning. Here, we study the function of the antenna cleaner in Camponotus rufifemur ants, a clamp-like structure consisting of a notch on the basitarsus facing a spur on the tibia, both bearing cuticular 'combs' and 'brushes'. The ants clamp one antenna tightly between notch and spur, pull it through, and subsequently clean the antenna cleaner itself with the mouthparts. We simulated cleaning strokes by moving notch or spur over antennae contaminated with fluorescent particles. The notch removed particles more efficiently than the spur, but both components eliminated more than 60% of the particles with the first stroke. Ablation of bristles, brush and comb strongly reduced the efficiency, indicating that they are essential for cleaning. To study how comb and brush remove particles of different sizes, we contaminated antennae of living ants, and anaesthetized them immediately after they had performed the first cleaning stroke. Different-sized beads were trapped in distinct zones of the notch, consistent with the gap widths between cuticular outgrowths. This suggests that the antenna cleaner operates like a series of sieves that remove the largest objects first, followed by smaller ones, down to the smallest particles that get caught by adhesion.

Underwater attachment using hairs: the functioning of spatula and sucker setae from male diving beetles

Males of Dytiscinae beetles use specialized adhesive setae to adhere to female elytra during underwater courtship. This coevolution of male setae and female elytra has attracted much attention since Darwin. However, there has been little examination of their biomechanical functioning despite increasing knowledge on biofibrillar adhesion. Here, we report and compare, for the first time, the mechanisms of underwater attachment using two hair types, the primitive spatula and derived 'passive' sucker, found in male diving beetles. Results from interspecific scaling of protarsal palettes and adhesion by single seta suggest better performance in the later-evolved circular (sucker) setae. Spatula setae with a modified shallow sucker and channels use the combined mechanisms of suction and viscous resistance for adhesion. Velocity-dependent adhesion provides sufficient control for resisting the female's erratic movements while also detaching easily through slow peeling. Direction-dependent shear resistance helps reorient setae surfaces into a preferred direction for effective adhesion. Seta deformation using different mechanisms for circular and spatula setae reduces the force that is transmitted to the contact interface. A softer spring in spatula setae explains their adhesion at lower preloads and assists in complete substrate contact. Attachment mechanisms revealed in adhesive setae with modified spatula and passive suckers provide insights for bioinspired designs of underwater attachment devices.

Self-drying: a gecko's innate ability to remove water from wet toe pads

When the adhesive toe pads of geckos become wet, they become ineffective in enabling geckos to stick to substrates. This result is puzzling given that many species of gecko are endemic to tropical environments where water covered surfaces are ubiquitous. We hypothesized that geckos can recover adhesive capabilities following exposure of their toe pads to water by walking on a dry surface, similar to the active self-cleaning of dirt particles. We measured the time it took to recover maximum shear adhesion after toe pads had become wet in two groups, those that were allowed to actively walk and those that were not. Keeping in mind the importance of substrate wettability to adhesion on wet surfaces, we also tested geckos on hydrophilic glass and an intermediately wetting substrate (polymethylmethacrylate; PMMA). We found that time to maximum shear adhesion recovery did not differ in the walking groups based on substrate wettability (22.7±5.1 min on glass and 15.4±0.3 min on PMMA) but did have a significant effect in the non-walking groups (54.3±3.9 min on glass and 27.8±2.5 min on PMMA). Overall, we found that by actively walking, geckos were able to self-dry their wet toe pads and regain maximum shear adhesion significantly faster than those that did not walk. Our results highlight a unexpected property of the gecko adhesive system, the ability to actively self-dry and recover adhesive performance after being rendered dysfunctional by water.

Physical principles of fluid-mediated insect attachment - Shouldn't insects slip?

Insects use either hairy or smooth adhesive pads to safely adhere to various kinds of surfaces. Although the two types of adhesive pads are morphologically different, they both form contact with the substrate via a thin layer of adhesive fluid. To model adhesion and friction forces generated by insect footpads often a simple "wet adhesion" model is used, in which two flat undeformable substrates are separated by a continuous layer of fluid. This review summarizes the key physical and tribological principles that determine the adhesion and friction in such a model. Interestingly, such a simple wet-adhesion model falls short in explaining several features of insect adhesion. For example, it cannot predict the observed high static friction forces of the insects, which enable them to cling to vertical smooth substrates without sliding. When taking a closer look at the "classic" attachment model, one can see that it is based on several simplifications, such as rigid surfaces or continuous layers of Newtonian fluids. Recent experiments show that these assumptions are not valid in many cases of insect adhesion. Future tribological models for insect adhesion thus need to incorporate deformable adhesive pads, non-Newtonian properties of the adhesive fluid and/or partially "dry" or solid-like contact between the pad and the substrate.

Functionally different pads on the same foot allow control of attachment: stick insects have load-sensitive "heel" pads for friction and shear-sensitive "toe" pads for adhesion

Stick insects (Carausius morosus) have two distinct types of attachment pad per leg, tarsal “heel” pads (euplantulae) and a pre-tarsal “toe” pad (arolium). Here we show that these two pad types are specialised for fundamentally different functions. When standing upright, stick insects rested on their proximal euplantulae, while arolia were the only pads in surface contact when hanging upside down. Single-pad force measurements showed that the adhesion of euplantulae was extremely small, but friction forces strongly increased with normal load and coefficients of friction were 1. The pre-tarsal arolium, in contrast, generated adhesion that strongly increased with pulling forces, allowing adhesion to be activated and deactivated by shear forces, which can be produced actively, or passively as a result of the insects' sprawled posture. The shear-sensitivity of the arolium was present even when corrected for contact area, and was independent of normal preloads covering nearly an order of magnitude. Attachment of both heel and toe pads is thus activated partly by the forces that arise passively in the situations in which they are used by the insects, ensuring safe attachment. Our results suggest that stick insect euplantulae are specialised “friction pads” that produce traction when pressed against the substrate, while arolia are “true” adhesive pads that stick to the substrate when activated by pulling forces.

Grooming Behavior as a Mechanism of Insect Disease Defense

Grooming is a well-recognized, multipurpose, behavior in arthropods and vertebrates. In this paper, we review the literature to highlight the physical function, neurophysiological mechanisms, and role that grooming plays in insect defense against pathogenic infection. The intricate relationships between the physical, neurological and immunological mechanisms of grooming are discussed to illustrate the importance of this behavior when examining the ecology of insect-pathogen interactions.

Self-cleaning in tree frog toe pads; a mechanism for recovering from contamination without the need for grooming

Tree frogs use adhesive toe pads for climbing on a variety of surfaces. They rely on wet adhesion, which is aided by the secretion of mucus. In nature, the pads will undoubtedly get contaminated regularly through usage, but appear to maintain their stickiness over time. Here, we show in two experiments that the toe pads of White's tree frogs (Litoria caerulea) quickly recover from contamination through a self-cleaning mechanism. We compared adhesive forces prior to and after contamination of (1) the whole animal on a rotatable platform and (2) individual toe pads in restrained frogs mimicking individual steps using a motorised stage. In both cases, the adhesive forces recovered after a few steps but this took significantly longer in single toe pad experiments from restrained frogs, showing that use of the pads increases recovery. We propose that both shear movements and a 'flushing' effect of the secreted mucus play an important role in shedding particles/contaminants.

Mechanisms of self-cleaning in fluid-based smooth adhesive pads of insects

Pressure-sensitive adhesives such as tapes become easily contaminated by dust particles. By contrast, animal adhesive pads are able to self-clean and can be reused millions of times over a lifetime with little reduction in adhesion. However, the detailed mechanisms underlying this ability are still unclear. Here we test in adhesive pads of stick insects (Carausius morosus) (1) whether self-cleaning is enhanced by the liquid pad secretion, and (2) whether alternating push-pull movements aid the removal of particles. We measured attachment forces of insect pads on glass after contamination with 10 µm polystyrene beads. While the amount of fluid present on the pad showed no effect on the pads' susceptibility to contamination, the recovery of adhesive forces after contamination was faster when higher fluid levels were present. However, this effect does not appear to be based on a faster rate of self-cleaning since the number of spheres deposited with each step did not increase with fluid level. Instead, the fluid may aid the recovery of adhesive forces by filling in the gaps between contaminating particles, similar to the fluid's function on rough surfaces. Further, we found no evidence that an alternation of pushing and pulling movements, as found in natural steps, leads to a more efficient recovery of adhesion than repeated pulling slides.

Leaf beetle attachment on wrinkles: isotropic friction on anisotropic surfaces

The influence of surface roughness on the attachment ability of insects has been repeatedly reported. In previous experiments, complex surface topographies were used as test substrates, whereas periodical structures have so far been neglected. In the present study, traction experiments with adult beetles Gastrophysa viridula and Leptinotarsa decemlineata were carried out to study the influence of surfaces, structured with periodical wrinkles, on insect attachment. Force measurements were carried out on male and female insects, both intact and after removal of claws, performing tethered walking on five polydimethylsiloxane substrates: (i) smooth, non-structured (control), (ii–v) structured with wrinkles of different wavelengths (366, 502, 911 and 25,076 nm). In two test series, beetles walked either perpendicular or parallel to the wrinkle alignment. Adults of G. viridula produced generally higher forces than those of L. decemlineata. The results show that the alignment of wrinkles had no significant influence on the force generation by beetles, probably because of the skewed position of their tarsomeres relative to the substrates. In both sexes, the highest force values were obtained on surfaces with wrinkles of 25 μm wavelength. On other wrinkled substrates, forces were significantly reduced in both males and females compared with the smooth, flat control, with the minimum force achieved on wrinkles with a wavelength of 911 nm.

Effect of particulate contamination on adhesive ability and repellence in two species of ant (Hymenoptera; Formicidae)

Tarsal adhesive pads are crucial for the ability of insects to traverse their natural environment. Previous studies have demonstrated that for both hairy and smooth adhesive pads, significant reduction in adhesion can occur because of contamination of these pads by wax crystals present on plant surfaces or synthetic microspheres. In this paper, we focus on the smooth adhesive pads of ants and study systematically how particulate contamination and the subsequent loss of adhesion depends on particle size, particle surface energy, humidity and species size. To this end, workers of ant species Polyrhachis dives and Myrmica scabrinodis (Hymenoptera; Formicidae) were presented with loose synthetic powder barriers with a range of powder diameters (1–500 μm) and surface energies (PTFE or glass), which they would have to cross in order to escape the experimental arena. The barrier experiments were conducted for a range of humidities (10–70%). Experimental results and scanning electron microscopy confirm that particulate powders adversely affect the adhesive ability of both species of ant on smooth substrates via contamination of the arolia. Specifically, the loss of adhesion was found to depend strongly on particle diameter, but only weakly on particle type, with the greatest loss occurring for particle diameters smaller than the claw dimensions of each species, and no effect of humidity was found. We also observed that ants were repelled by the powder barriers which led to a decrease of adhesion prior to their eventual crossing, suggesting that insect antennae may play a role in probing the mechanical fragility of substrates before crossing them.

The influence of surface energy on the self-cleaning of insect adhesive devices

The ability of insects to adhere to surfaces is facilitated by the use of adhesive organs found on the terminal leg segments. These adhesive pads are inherently ‘tacky’ and are expected to be subject to contamination by particulates, leading to loss of function. Here, we investigated the self-cleaning of ants and beetles by comparing the abilities of both hairy and smooth pad forms to self-clean on both high and low energy surfaces after being fouled with microspheres of two sizes and surface energies. We focused on the time taken to regain adhesive potential in unrestrained Hymenopterans (Polyrhachis dives and Myrmica scabrinodis) and Coccinellids (Harmonia axyridis and Adalia bipunctata) fouled with microspheres. We found that the reattainment of adhesion is influenced by particle type and size in Hymenopterans, with an interaction between the surface energy of the contaminating particle and substrate. In Coccinellids, reattainment of adhesion was only influenced by particle size and substrate properties. The adhesive organs of Coccinellids appear to possess superior self-cleaning abilities compared with those of Hymenopterans, although Hymenopterans exhibit better adhesion to both surface types.

Walking on smooth and rough ground: activity and timing of the claw retractor muscle in the beetle Pachnoda marginata peregrina (Coleoptera, Scarabaeidae)

The activity pattern of the claw retractor muscle of Pachnoda marginata peregrina beetles was examined in this study. We found this muscle to be located in the tibia, without a femoral part, as is the case in other insects. Electromyograms (EMGs) of the muscle revealed a rather similar activity pattern during beetle locomotion on rough and smooth substrates. We recorded units with small and large amplitude, with the smaller one being active during almost the entire stance phase, the larger one roughly in the first half of stance. Small but significant differences were found in the precise onset and end of activity. Both small and large units began their activity earlier on the rough surface. While there was no difference at the end of activity in the small unit between both surfaces, the large unit ended its activity significantly earlier on the rough substrate. The spike frequencies on both surfaces were also significantly different for both small and large units. The small unit showed a higher spike frequency on the smooth surface, while the large unit had a higher spike frequency on the rough surface. From our experiments, we conclude that the muscle is controlled by the same basic activity pattern on different surfaces, with some adjustments due to sensory feedback. The adjustments cause differences in onset and end of activity, as well as in spike frequency of the involved muscle units.

Beetle adhesive hairs differ in stiffness and stickiness: in vivo adhesion measurements on individual setae

Leaf beetles are able to climb on smooth and rough surfaces using arrays of micron-sized adhesive hairs (setae) of varying morphology. We report the first in vivo adhesive force measurements of individual setae in the beetle Gastrophysa viridula, using a smooth polystyrene substrate attached to a glass capillary micro-cantilever. The beetles possess three distinct adhesive pads on each leg which differ in function and setal morphology. Visualisation of pull-offs allowed forces to be measured for each tarsal hair type. Male discoidal hairs adhered with the highest forces (919 ± 104 nN, mean ± SE), followed by spatulate (582 ± 59 nN) and pointed (127 ± 19 nN) hairs. Discoidal hairs were stiffer in the normal direction (0.693 ± 0.111 N m(-1)) than spatulate (0.364 ± 0.039 N m(-1)) or pointed (0.192 ± 0.044 N m(-1)) hairs. The greater adhesion on smooth surfaces and the higher stability of discoidal hairs help male beetles to achieve strong adhesion on the elytra of females during copulation. A comparison of pull-off forces measured for single setae and whole pads (arrays) revealed comparable levels of adhesive stress. This suggests that beetles are able to achieve equal load sharing across their adhesive pads so that detachment through peeling is prevented.