Sexual dimorphism in the attachment ability of the Colorado potato beetle Leptinotarsa decemlineata (Coleoptera: Chrysomelidae) to rough substrates

A review of bioinspired dry adhesives: from achieving strong adhesion to realizing switchable adhesion

In the early twenty-first century, extensive research has been conducted on geckos' ability to climb vertical walls with the advancement of microscopy technology. Unprecedented studies and developments have focused on the adhesion mechanism, structural design, preparation methods, and applications of bioinspired dry adhesives. Notably, strong adhesion that adheres to both the principles of contact splitting and stress uniform distribution has been discovered and proposed. The increasing popularity of flexible electronic skins, soft crawling robots, and smart assembly systems has made switchable adhesion properties essential for smart adhesives. These adhesives are designed to be programmable and switchable in response to external stimuli such as magnetic fields, thermal changes, electrical signals, light exposure as well as mechanical processes. This paper provides a comprehensive review of the development history of bioinspired dry adhesives from achieving strong adhesion to realizing switchable adhesion.

The effect of age on the attachment ability of stick insects (Phasmatodea)

Many insect species have found their way into ageing research as small and easy-to-keep model organisms. A major sign of ageing is the loss of locomotory functions due to neuronal disorders or tissue wear. Soft and pliable attachment pads on the tarsi of insects adapt to the substrate texture to maximize their real contact area and, thereby, generate attachment during locomotion. In the majority of stick insects, adhesive microstructures covering those pads support attachment. Stick insects do not molt again after reaching the imaginal stage; hence, the cuticle of their pads is subject to continuous ageing. This study aims to quantify how attachment ability changes with age in the stick insect Sungaya aeta Hennemann, 2023 and elucidate the age effects on the material and microstructure of the attachment apparatus. Attachment performance (adhesion and friction forces) on substrates with different roughnesses was compared between two different age groups, and the change of attachment performance was monitored extending over a larger time frame. Ageing effects on the morphology of the attachment pads and the autofluorescence of the cuticle were documented using light, scanning electron, and confocal laser scanning microscopy. The results show that both adhesion and friction forces decline with age. Deflation of the pads, scarring of the cuticle, and alteration of the autofluorescence, likely indicating stiffening of the cuticle, were observed to accumulate over time. This would reduce the attachment ability of the insect, as pads lose their pliant properties and cannot properly maintain sufficient contact area with the substrate.

Parasitoid attachment ability and the host surface wettability

Climbing animals such as geckos and arthropods developed astonishing adhesive mechanisms which are fundamental for their survival and represent valuable models for biomimetic purposes. A firm adhesion to the host surface, in order to successfully lay eggs is necessary for the reproduction of most parasitoid insects. In the present study, we performed a comparative investigation on the attachment ability of four parasitoid species (the egg parasitoid Anastatus bifasciatus (Eupelmidae), the aphid parasitoid Aphidius ervi (Braconidae), the fly pupal ectoparasitoid Muscidifurax raptorellus (Pteromalidae) and the pupal parasitoid of Drosophila Trichopria drosophilae (Diapriidae)) with hosts characterized by a surface having different wettability properties. The friction force measurements were performed on smooth artificial (glass) surfaces showing different contact angles of water. We found that attachment systems of parasitoid insects are tuned to match the wettability of the host surface. Sexual dimorphism in the attachment ability of some tested species has been also observed. The obtained results are probably related to different microstructure and chemical composition of the host surfaces and to different chemical composition of the parasitoid adhesive fluid. The data here presented can be interpreted as an adaptation, especially in the female, to the physicochemical properties of the host surface and contribute to shed light on the coevolutionary processes of parasitoid insects and their hosts.

Interaction of a non-axisymmetric artificial single spatula with rough surfaces

Hair-like attachment structures are frequently used by animals to create stable contact with rough surfaces. Previous studies focused primarily on axisymmetric biomimetic models of artificial spatulas, such as those with a mushroom-shaped and cylinder-shaped geometry, in order to simulate the so-called gecko effect. Here, two geometric prototypes of artificial adhesive structures with non-axisymmetric properties were designed. The investigation of the prototype's interactions with rough surfaces was carried out using the finite element software ABAQUS. Under increasing vertical displacement, the effect of asperity size on the contact pressure evolution of the spatula was investigated. It has been demonstrated that the contact behaviour is greatly affected by the flexibility of the spatula, which is caused by its variable thickness. The thinner spatula shows a higher nominal contact area and attaches more strongly to various rough surfaces. Although a thicker spatula is more susceptible to the 'leverage' phenomenon, which occurs when excessively applied displacements prematurely reduce the nominal contact area, it obtains the ability to regulate attachment during unidirectional loading. Two non-axisymmetric prototypes provide different design concepts for the artificial adhesives. It is hoped that this study will provide fresh viewpoints and innovations that contribute to the development of biologically inspired adhesives.

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.

Morphological and functional analyses for investigation of sexually selected legs in the frog legged beetle Sagra femorata (Coleoptera: Chrysomelidae)

Mate choice and male-male combat over successful mating often cause disproportionate exaggeration of male trait relative to body size. However, the exaggeration is often not the only trait involved with male-male combat and mate choice: suites of co-expressed traits may function together as a coordinated unit. When this occurs, dimorphism may be expected for these additional, non-exaggerated, structures. S. femorata males have disproportionately large hind-legs used in male-male combat over females. During the fights, fore- and mid-legs are used to keep males in positions where advantageous for leverage. Because use of the exaggerated hind-legs is coordinated with the other legs, they will coevolve as a functional unit. Here, we show that 1) S. femorata has sexual size differences in all three legs; 2) males show positive allometry in the relative sizes of all three legs; and 3) microstructures of tarsi on the fore- and mid-legs are also sexually dimorphic. Despite these differences in the tarsal microstructure, 4) adhesion forces of the tarsi had no sexual difference in flat surface. The microstructure would be specialized on attaching elytra surface. These results suggest that the three pairs of legs function together during fighting behavior, with hind-legs employed primarily for fighting, and the fore- and mid-legs functioning to grip females, keeping males positioned on the back of the female during combat.

Insect attachment on waxy plant surfaces: the effect of pad contamination by different waxes

This study focuses on experimental testing of the contamination hypothesis and examines how the contamination of insect adhesive pads with three-dimensional epicuticular waxes of different plant species contributes to the reduction of insect attachment. We measured traction forces of tethered Chrysolina fastuosa male beetles having hairy adhesive pads on nine wax-bearing plant surfaces differing in both shape and dimensions of the wax structures and examined insect adhesive organs after they have contacted waxy substrates. For comparison, we performed the experiments with the same beetle individuals on a clean glass sample just before (gl1) and immediately after (gl2) the test on a plant surface. The tested insects showed a strong reduction of the maximum traction force on all waxy plant surfaces compared to the reference experiment on glass (gl1). After beetles have walked on waxy plant substrates, their adhesive pads were contaminated with wax material, however, to different extents depending on the plant species. The insects demonstrated significantly lower values of both the maximum traction force and the first peak of the traction force and needed significantly longer time to reach the maximum force value in the gl2 test than in the gl1 test. These effects were especially pronounced in cases of the plant surfaces covered with wax projections having higher aspect ratios. The data obtained clearly indicated the impact of waxy plant surfaces on the insect ability to subsequently attach to the clean smooth surface. This effect is caused by the contamination of adhesive pads and experimentally supports the contamination hypothesis.

Mechanoecology: biomechanical aspects of insect-plant interactions

Plants and herbivorous insects as well as their natural enemies, such as predatory and parasitoid insects, are united by intricate relationships. During the long period of co-evolution with insects, plants developed a wide diversity of features to defence against herbivores and to attract pollinators and herbivores' natural enemies. The chemical basis of insect-plant interactions is established and many examples are studied, where feeding and oviposition site selection of phytophagous insects are dependent on the plant's secondary chemistry. However, often overlooked mechanical interactions between insects and plants can be rather crucial. In the context of mechanoecology, the evolution of plant surfaces and insect adhesive pads is an interesting example of competition between insect attachment systems and plant anti-attachment surfaces. The present review is focused on mechanical insect-plant interactions of some important pest species, such as the polyphagous Southern Green Stinkbug Nezara viridula and two frugivorous pest species, the polyphagous Mediterranean fruit fly Ceratitis capitata and the monophagous olive fruit fly Bactrocera oleae. Their ability to attach to plant surfaces characterised by different features such as waxes and trichomes is discussed. Some attention is paid also to Coccinellidae, whose interaction with plant leaf surfaces is substantial across all developmental stages in both phytophagous and predatory species that feed on herbivorous insects. Finally, the role of different kinds of anti-adhesive nanomaterials is discussed. They can reduce the attachment ability of insect pests to natural and artificial surfaces, potentially representing environmental friendly alternative methods to reduce insect pest impact in agriculture.

Strong attachment as an adaptation of flightless weevils on windy oceanic islands

Enhanced attachment ability is common in plants on islands to avoid potential fatal passive dispersal. However, whether island insects also have increased attachment ability remains unclear. Here we measured the attachment of a flightless weevil, Pachyrhynchus sarcitis kotoensis, from tropical islands, and compared it with documented arthropods from the mainland. We examined the morphology and material gradient of its attachment devices to identify the specific adaptive modifications for attachment. We find that the weevil has much stronger attachment force and higher safety factor than previously studied arthropods, regardless of body size and substrate roughness. This probably results from the specific flexible bases of the adhesive setae on the third footpad of the legs. This softer material on the setal base has not been reported hitherto and we suggest that it acts as a flexible hinge to form intimate contact to substrate more effectively. By contrast, no morphological difference in tarsomeres and setae between the weevil and other beetles is observed. Our results show the remarkably strong attachment of an island insect and highlights the potential adaptive benefits of strong attachment in windy island environment. The unique soft bases of the adhesive hairs may inspire the development of strong biomimetic adhesives.

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.

Effect of the Mechanical Properties of Soft Counter-Faces on the Adhesive Capacity of Mushroom-Shaped Biomimetic Microstructures

The effects of mechanical properties and contact environment conditions on the adhesiveness of the biomimetic adhesive mushroom-shaped micro-structure have been experimentally investigated. The idea is based on the adhesive micro-structures and surfaces inspired by nature after observing the abilities of some animals. Applications are proposed in various fields of engineering and technology. However, to enable unconventional uses of these biomimetic adhesion surfaces, such as in the biomedical field, it is necessary to adjust and optimize their tribological properties (friction, adhesion, and peeling strength) in contact with soft substrates that can simulate the mechanical features of biological tissues. Our work explores the effect of the combinations of the various parameters on the strength of adhesion. Under dry contact conditions, soft counter-faces lead to lower adhesion than hard counter-faces, whereas under wet conditions, soft counter-faces lead to higher adhesion than harder counter-faces.

Leaves that walk and eggs that stick: comparative functional morphology and evolution of the adhesive system of leaf insect eggs (Phasmatodea: Phylliidae)

Phylliidae are herbivorous insects exhibiting impressive cryptic masquerade and are colloquially called "walking leaves". They imitate angiosperm leaves and their eggs often resemble plant seeds structurally and in some cases functionally. Despite overall morphological similarity of adult Phylliidae, their eggs reveal a significant diversity in overall shape and exochorionic surface features. Previous studies have shown that the eggs of most Phylliidae possess a specialised attachment mechanism with hierarchical exochorionic fan-like structures (pinnae), which are mantled by a film of an adhesive secretion (glue). The folded pinnae and glue respond to water contact, with the fibrous pinnae expanding and the glue being capable of reversible liquefaction. In general, the eggs of phylliids appear to exhibit varying structures that were suggested to represent specific adaptations to the different environments the eggs are deposited in. Here, we investigated the diversity of phylliid eggs and the functional morphology of their exochorionic structure. Based on the examination of all phylliid taxa for which the eggs are known, we were able to characterise eleven different morphological types. We explored the adhesiveness of these different egg morphotypes and experimentally compared the attachment performance on a broad range of substrates with different surface roughness, surface chemistry and tested whether the adhesion is replicable after detachment in multiple cycles. Furthermore, we used molecular phylogenetic methods to reconstruct the evolutionary history of different egg types and their adhesive systems within this lineage, based on 53 phylliid taxa. Our results suggest that the egg morphology is congruent with the phylogenetic relationships within Phylliidae. The morphological differences are likely caused by adaptations to the specific environmental requirements for the particular clades, as the egg morphology has an influence on the performance regarding the surface roughness. Furthermore, we show that different pinnae and the adhesive glue evolved convergently in different species. While the evolution of the Phylliidae in general appears to be non-adaptive judging on the strong similarity of the adults and nymphs of most species, the eggs represent a stage with complex and rather diverse functional adaptations including mechanisms for both fixation and dispersal of the eggs.

Morphological and Behavioral Adaptations of Silk-Lovers (Plokiophilidae: Embiophila) for Their Lifestyle in the Silk Domiciles of Webspinners (Embioptera)

The diversity of true bugs gave rise to various lifestyles, including gaining advantage from other organisms. Plokiophilidae are cimicomorphan bugs that live in the silk constructions of other arthropods. One group, Embiophila, exclusively settles in the silk colonies of webspinners (Embioptera). We investigated the lifestyle of Embiophila using microscopy to study the micromorphology and material composition of the leg cuticle, choice assays and retention time measurements based on different characteristics of the embiopteran galleries and tilting experiments with different substrates to quantify the attachment performance of the bugs. Embiophila neither explicitly preferred embiopteran presence, nor required silk for locomotion, but the bugs preferred fibrous substrates during the choice experiments. The hairy attachment pad on the tibia showed the best attachment performance on substrates, with an asperity size of 1 µm. Additionally, very rough substrates enabled strong attachment, likely due to the use of claws. Our findings suggest that Embiophila settle in galleries of webspinners to benefit from the shelter against weather and predators and to feed on mites and other intruders. The combination of behavioral and functional morphological experiments enables insights into the life history of these silk-associated bugs, which would be highly challenging in the field due to the minute size and specialized lifestyle of Embiophila.

An Underactuated Adaptive Microspines Gripper for Rough Wall

Wall attachment has great potential in a broad range of applications such as robotic grasping, transfer printing, and asteroid sampling. Herein, a new type of underactuated bionic microspines gripper is proposed to attach to an irregular, rough wall. Experimental results revealed that the gripper, profiting from its flexible structure and underactuated linkage mechanism, is capable of adapting submillimeter scale roughness to centimeter scale geometry irregularity in both normal and tangential attachment. The rigid-flexible coupling simulation analysis validated that the rough adaptation was achieved by the passive deformation of the zigzag flexible structure, while the centimeter-scale irregularity adaptation come from the underactuated design. The attachment test of a spine confirmed that a 5 mm sliding distance of the spine tip on the fine brick wall promises a saturated tangential attachment force, which can guide the stiffness design of flexible structure and parameter selection of underactuated linkage. Furthermore, the developed microspines gripper was successfully demonstrated to grasp irregular rocks, tree trunks, and granite plates. This work presents a generally applicable and dexterous passive adaption design to achieve rough wall attachment for flat and curved objects, which promotes the understanding and application of wall attachment.

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.

Tarsal attachment structures of the biting midge Forcipomyia paludis (Diptera: Ceratopogonidae), a specialized ectoparasite of Odonata imagines

The female of the biting midge Forcipomyia paludis is a dipteran ectoparasite of West Palaearctic damselflies and dragonflies, sucking haemolymph mainly from wing veins of their hosts. This tiny midge remains firmly attached to the wings even during fast flight and aerial fight maneuvers as shown in the present paper by field studies of the large dragonfly, Cordulegaster boltonii. Since individuals of F. paludis firmly attach themselves to the challenging wing surface of their host and can successfully withstand drag and vibrations during flight, we assume that this midge species has specific microstructural adaptations on its legs for attaching to the wing surface. In our morphological study, we used scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM), to study the structure of F. paludis tarsi, as well as the micro morphology of the wing surfaces of their host. Additionally, for the first time, we were able to show attachment devices of the midges dried out in contact with the host’s surface. The spatulae of the plantar setae and especially the empodial setae, are capable of replicating nanoscale wax crystals of the super hydrophobic wing coverage of the dragonfly wing membrane, in order to increase an effective contact area and therefore adhesion. This ability requires extremely soft materials of the spatula, which seems to be rather unique even in comparison to the leg attachment devices of other dipterans and other insect taxa in general.

Morphological adaptations of the mouthparts to the ectoparasitic lifestyle of the biting midge Forcipomyia paludis (Diptera: Ceratopogonidae), specialized in Odonata

Damselflies and dragonflies are well-known hosts of the West Palaearctic biting midge Forcipomyia paludis. Females of this ectoparasitic dipteran mainly cling to the host’s wings, sucking hemolymph from the wing veins. The midges are firmly attached to the wing surface with specialized tarsi, thus not being flung away during the host’s flight maneuvers. As for another ceratopogonid—F. odonatophila from New Guinea—had been suggested, we assumed that in F. paludis, the attachment would be reinforced by the mouthparts during the suction action. In the present study, we used behavioral field observations, scanning electron microscopy (SEM) and high-resolution micro-computed tomography (µCT), to study the mouthparts of F. paludis. We focused on the mouthpart configuration post sucking and thus on the contact with the host's wing as well as on the piercing process into the wing veins. We foster our understanding of F. paludis being a parasite of Odonata by showing proof of the piercing and therefore the sucking of hemolymph from the wings. Additionally, the mouthparts clearly show contamination with odonate wing wax after the sucking procedure. Furthermore, we discuss probable additional functions of the piercing process for the firm attachment to the flying host of F. paludis.

Gecko Adhesion on Flat and Rough Surfaces: Simulations with a Multi-Scale Molecular Model

A multiscale modeling approach is used to develop a particle-based mesoscale gecko spatula model that is able to link atomistic simulations and mesoscale (0.44 µm) simulations. It is used to study the detachment of spatulae from flat as well as nanostructured surfaces. The spatula model is based on microscopical information about spatulae structure and on atomistic molecular simulation results. Target properties for the coarse-graining result from a united-atom model of gecko keratin in periodic boundary conditions (PBC), previously developed by the authors. Pull-off forces necessary to detach gecko keratin under 2D PBC parallel to the surface are previously overestimated when only a small region of a spatula is examined. It is shown here that this is due to the restricted geometry (i.e., missing peel-off mode) and not model parameters. The spatula model peels off when pulled away from a surface, both in the molecular picture of the pull-off process and in the force-extension curve of non-equilibrium simulations mimicking single-spatula detachment studied with atomic force microscopy equipment. The force field and spatula model can reproduce experimental pull-off forces. Inspired by experimental results, the underlying mechanism that causes pull-off forces to be at a minimum on surfaces of varying roughnesses is also investigated. A clear sigmoidal increase in the pull-off force of spatulae with surface roughness shows that adhesion is determined by the ratio between spatula pad area and the area between surface peaks. Experiments showed a correlation with root-mean-square roughness of the surface, but the results of this work indicate that this is not a causality but depends on the area accessible.

A robotic leg inspired from an insect leg

While most insect-inspired robots come with a simple tarsus, such as a hemispherical foot tip, insect legs have complex tarsal structures and claws, which enable them to walk on complex terrain. Their sharp claws can smoothly attach and detach on plant surfaces by actuating a single muscle. Thus, installing an insect-inspired tarsus on legged robots would improve their locomotion on complex terrain. This paper shows that the tendon-driven ball-socket structure provides the tarsus with both flexibility and rigidity, which is necessary for the beetle to walk on a complex substrate such as a mesh surface. Disabling the tarsus' rigidity by removing the socket and elastic membrane of a tarsal joint, means that the claws could not attach to the mesh securely. Meanwhile, the beetle struggled to draw the claws out of the substrate when we turned the tarsus rigid by tubing. We then developed a cable-driven bio-inspired tarsus structure to validate the function of the tarsus as well as to show its potential application in the legged robot. With the tarsus, the robotic leg was able to attach and retract smoothly from the mesh substrate when performing a walking cycle.

Groovy and Gnarly: Surface Wrinkles as a Multifunctional Motif for Terrestrial and Marine Environments

From large ventral pleats of humpback whales to nanoscale ridges on flower petals, wrinkled structures are omnipresent, multifunctional, and found at hugely diverse scales. Depending on the particulars of the biological system-its environment, morphology, and mechanical properties-wrinkles may control adhesion, friction, wetting, or drag; promote interfacial exchange; act as flow channels; or contribute to stretching, mechanical integrity, or structural color. Undulations on natural surfaces primarily arise from stress-induced instabilities of surface layers (e.g., buckling) during growth or aging. Variation in the material properties of surface layers and in the magnitude and orientation of intrinsic stresses during growth lead to a variety of wrinkling morphologies and patterns which, in turn, reflect the wide range of biophysical challenges wrinkled surfaces can solve. Therefore, investigating how surface wrinkles vary and are implemented across biological systems is key to understanding their structure-function relationships. In this work, we synthesize the literature in a metadata analysis of surface wrinkling in various terrestrial and marine organisms to review important morphological parameters and classify functional aspects of surface wrinkles in relation to the size and ecology of organisms. Building on our previous and current experimental studies, we explore case studies on nano/micro-scale wrinkles in biofilms, plant surfaces, and basking shark filter structures to compare developmental and structure-vs-function aspects of wrinkles with vastly different size scales and environmental demands. In doing this and by contrasting wrinkle development in soft and hard biological systems, we provide a template of structure-function relationships of biological surface wrinkles and an outlook for functionalized wrinkled biomimetic surfaces.

Adhesive performance enhancement of the mushroom-shaped microstructured elastomer by atmospheric plasma treatment

Bioinspired reversible adhesives that have been developed in the course of recent years have found several applications in robotics, transportation, and marine applications. One of their prominent features is strong reversible static adhesion. To fulfill the requirements of various applications, the static adhesive performance of these materials can be enhanced by modifying the material and surface properties. In this work, the mushroom-shaped adhesive microstructured surface was functionalized by atmospheric plasma treatment to enhance its adhesive performances. Through optimizing the duration of the treatment, the pull-off force increase of up to 60% can be reached after the treatment in comparison to the measurements performed on the same mushroom-shaped microstructured sample before the treatment. In comparison to the microstructured samples, the attachment of the unstructured sample made of the same silicone elastomer was enhanced by 16% after plasma treatment. The strong adhesion enhancement on the microstructured sample was attributed to the combination of the changed effective elastic modulus of the material and the specific detachment behavior of microstructures. These results are anticipated to contribute to the further development of bioinspired dry adhesives and may potentially widen their usage in various technological applications.

Physical constraints lead to parallel evolution of micro- and nanostructures of animal adhesive pads: a review

Adhesive pads are functional systems with specific micro- and nanostructures which evolved as a response to specific environmental conditions and therefore exhibit convergent traits. The functional constraints that shape systems for the attachment to a surface are general requirements. Different strategies to solve similar problems often follow similar physical principles, hence, the morphology of attachment devices is affected by physical constraints. This resulted in two main types of attachment devices in animals: hairy and smooth. They differ in morphology and ultrastructure but achieve mechanical adaptation to substrates with different roughness and maximise the actual contact area with them. Species-specific environmental surface conditions resulted in different solutions for the specific ecological surroundings of different animals. As the conditions are similar in discrete environments unrelated to the group of animals, the micro- and nanostructural adaptations of the attachment systems of different animal groups reveal similar mechanisms. Consequently, similar attachment organs evolved in a convergent manner and different attachment solutions can occur within closely related lineages. In this review, we present a summary of the literature on structural and functional principles of attachment pads with a special focus on insects, describe micro- and nanostructures, surface patterns, origin of different pads and their evolution, discuss the material properties (elasticity, viscoelasticity, adhesion, friction) and basic physical forces contributing to adhesion, show the influence of different factors, such as substrate roughness and pad stiffness, on contact forces, and review the chemical composition of pad fluids, which is an important component of an adhesive function. Attachment systems are omnipresent in animals. We show parallel evolution of attachment structures on micro- and nanoscales at different phylogenetic levels, focus on insects as the largest animal group on earth, and subsequently zoom into the attachment pads of the stick and leaf insects (Phasmatodea) to explore convergent evolution of attachment pads at even smaller scales. Since convergent events might be potentially interesting for engineers as a kind of optimal solution by nature, the biomimetic implications of the discussed results are briefly presented.

Biomimetic clotrimazole-loaded PLGA films with enhanced adhesiveness for controlled drug release

Biomimetic adhesive surfaces have a number of potential applications in the pharmaceutical and biomedical fields. Fabrication techniques must be adapted to biocompatible and biodegradable materials required for controlled drug release applications. In this study biomimetic adhesive poly(lactic-co-glycolic acid) (PLGA) films loaded with different concentrations of clotrimazole (CTZ) were prepared without combining other adhesive excipients as a controlled release system for potential local oral drug delivery. The films were fully characterized from morphological point of view, and CTZ-loaded biomimetic films exhibited adequate surface pH values, high drug encapsulation efficiency, and loading content. The adhesion strength of the obtained films was significantly higher compared to a flat film reference under different contact conditions. Thermal analysis indicated a decrease of drug crystallinity upon incorporation into PLGA films. The in vitro release of CTZ from PLGA biomimetic films was tested in simulated saliva, and it exhibited an initial burst release, accompanied by a sustained release phase over 10 days. Finally, the mucoadhesive properties of the obtained films was studied using agar/mucin plate as a representative mucosal substrate, and the results demonstrated superior mucoadhesion potential of CTZ-loaded biomimetic film in comparison to its flat counterpart. Having demonstrated the ability to load CTZ into PLGA biomimetic films with enhanced adhesion capacity, the potential use in local oral drug delivery applications warrants further in vitro and in vivo investigations.

Jurassic scorpionflies (Mecoptera) with swollen first metatarsal segments suggesting sexual dimorphism

BACKGROUND

Sexual dimorphism is widespread in insects. The certain specialized structures may be used as weapons in male-male combats or as ornaments to enhance mating opportunities.

RESULTS

We report striking swollen first tarsal segments in two families, four genera and six species of scorpionflies from the Middle Jurassic Yanliao Biota of Northeastern China. Swollen tarsal segments are restricted to male specimens and to hind leg tarsi. The geometric morphometric analyses reveal that the degree of swelling within the orthophlebiid species possessing swollen first metatarsal segments is species-specific, which can be used as a diagnostic character for taxonomic and phylogenetic studies.

CONCLUSIONS

The new findings indicate that swollen first metatarsal segments are relatively common in the family Orthophlebiidae during the Middle Jurassic. The tarsal swellings are considered to be sexually dimorphic, potentially associated with sexually display by males and/or camouflage of a "nuptial gift" in the mating process.

Revisiting the classification of squamate adhesive setae: historical, morphological and functional perspectives

Research on gecko-based adhesion has become a truly interdisciplinary endeavour, encompassing many disciplines within the natural and physical sciences. Gecko adhesion occurs by the induction of van der Waals intermolecular (and possibly other) forces between substrata and integumentary filaments (setae) terminating in at least one spatulate tip. Gecko setae have increasingly been idealized as structures with uniform dimensions and a particular branching pattern. Approaches to developing synthetic simulacra have largely adopted such an idealized form as a foundational template. Observations of entire setal fields of geckos and anoles have, however, revealed extensive, predictable variation in setal form. Some filaments of these fields do not fulfil the morphological criteria that characterize setae and, problematically, recent authors have applied the term 'seta' to structurally simpler and likely non-adhesively competent fibrils. Herein we briefly review the history of the definition of squamate setae and propose a standardized classificatory scheme for epidermal outgrowths based on a combination of whole animal performance and morphology. Our review is by no means comprehensive of the literature regarding the form, function, and development of the adhesive setae of squamates and we do not address significant advances that have been made in many areas (e.g. cell biology of setae) that are largely tangential to their classification and identification. We contend that those who aspire to simulate the form and function of squamate setae will benefit from a fuller appreciation of the diversity of these structures, thereby assisting in the identification of features most relevant to their objectives.

Crucial role of framework with cytoskeletal actin filaments for shaping microstructure of footpad setae in the ladybird beetle, Harmonia axyridis

Insects that can walk on smooth surfaces have specialized structures, footpads, on their legs. Footpads play an important role in adhesion to the substrate surface. Although the morphology and function of footpads have been studied, the mechanism of their formation is still elusive. In the ladybird beetle (Harmonia axyridis), hairy footpads are present on the first and second tarsal segments of the legs. The footpads are covered with hundreds of hairs, i.e. setae, whose tips consist of four types: pointed, lanceolate, spatular, and discoidal. We examined the formation of the footpad during the pupal stage using immuno-staining and fluorescent-conjugated phalloidin staining. We found that a seta was composed of a shaft and a socket and some setae were accompanied by a neuron. By the mid-pupal stages, the shaft cells elongated to form a setal structure. Cytoskeletal actin bundles ramified to create a framework for the setal tip structure of the cells. We examined the effects of the application of cytochalasin D, which inhibits actin polymerization, on the formation of footpad setal structures. The results showed that the setal tips were deformed by the inhibition of actin polymerization. Our observations reveal that cytoskeletal actin filaments are involved in shaping the setae.

Smooth and slipless walking mechanism inspired by the open-close cycle of a beetle claw

This study investigated the function of the beetle's claw for its smooth and slipless walking and designed an artificial claw open-close cycle mechanism to mimic the beetle's walking. First, the effects of claw opening and closing on beetles' ability to attach to surfaces were examined. A beetle does not have an attachment pad, and only its claws work to grip the ground; its claw opens and closes and attaches with two sharp hooks. With their claws, beetles can smoothly walk, neither slipping on nor having their claws stuck in the surface. How do they perform smooth walking with sharp claws? In this study, we observed that beetles close their claws when they raise and swung their legs forward, while they open their claws when they lowered their legs to the ground. We then conducted non-destructive tests: their claws were forced open or closed. There was a significant difference in the trajectories of forced-closed claws compared to intact claws and forced-open claws. When their claws were forced-closed, this caused slippage in walking. On the other hand, when a claw was forced-open and its rotation was also inhibited, the claw stuck heavily in the surface, and the beetle could not walk. Based on these findings, we designed an artificial claw to open and close in the same cyclic manner as in the case of natural beetles. The performance of the artificial claw was consistent with the conclusions drawn from natural beetles: the locomotive robot with the artificial claw smoothly moved without slippage. Through these observations, non-destructive tests and performance of the bio-inspired artificial claws, this study confirmed the function of the open-close cycle of beetle claws and demonstrated and successfully adopted it for a locomotive robot.

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.

Spatio-temporal development of cuticular ridges on leaf surfaces of Hevea brasiliensis alters insect attachment

Cuticular ridges on plant surfaces can control insect adhesion and wetting behaviour and might also offer stability to underlying cells during growth. The growth of the plant cuticle and its underlying cells possibly results in changes in the morphology of cuticular ridges and may also affect their function. We present spatial and temporal patterns in cuticular ridge development on the leaf surfaces of the model plant, Hevea brasiliensis. We have identified, by confocal laser scanning microscopy of polymer leaf replicas, an acropetally directed progression of ridges during the ontogeny of Hevea brasiliensis leaf surfaces. The use of Colorado potato beetles (Leptinotarsa decemlineata) as a model insect species has shown that the changing dimensions of cuticular ridges on plant leaves during ontogeny have a significant impact on insect traction forces and act as an effective indirect defence mechanism. The traction forces of walking insects are significantly lower on mature leaf surfaces compared with young leaf surfaces. The measured walking traction forces exhibit a strong negative correlation with the dimensions of the cuticular ridges.

The Effects of Roughness and Wetness on Salamander Cling Performance

Animals clinging to natural surfaces have to generate attachment across a range of surface roughnesses in both dry and wet conditions. Plethodontid salamanders can be aquatic, semi-aquatic, terrestrial, arboreal, troglodytic, saxicolous, and fossorial and therefore may need to climb on and over rocks, tree trunks, plant leaves, and stems, as well as move through soil and water. Sixteen species of salamanders were tested to determine the effects of substrate roughness and wetness on maximum cling angle. Substrate roughness had a significant effect on maximum cling angle, an effect that varied among species. Substrates of intermediate roughness (asperity size 100-350 µm) resulted in the poorest attachment performance for all species. Small species performed best on smooth substrates, while large species showed significant improvement on the roughest substrates (asperity size 1000-4000 µm), possibly switching from mucus adhesion on a smooth substrate to an interlocking attachment on rough substrates. Water, in the form of a misted substrate coating and a flowing stream, decreased cling performance in salamanders on smooth substrates. However, small salamanders significantly increased maximum cling angle on wetted substrates of intermediate roughness, compared with the dry condition. Study of cling performance and its relationship to surface properties may cast light onto how this group of salamanders has radiated into the most speciose family of salamanders that occupies diverse habitats across an enormous geographical range.

Traction force measurements on male Strepsiptera (Insecta) revealed higher forces on smooth compared with hairy substrates

The aim of this study was to find out how strongly the parasitic insect Stylopsovinae, which has tarsi equipped with tenent hairs and lacking claws, attaches to different substrates. We investigated adhesion of male S. ovinae to the abdomen of its hymenopteran host (Andrena vaga), the hairier abdomen of a Bombus sp. and two artificial smooth reference surfaces with different degrees of hydrophilicity. In our experiments, the male S. ovinae developed significantly higher forces on smooth surfaces. However, the forces were significantly lower on all the hymenopteran surfaces used in the experiment. The absence of anisotropy in the force grip in cranial/caudal direction relative to the host might indirectly indicate that S. ovinae generate forces by adhesion rather than mechanical interlocking with the host hairs. The tolerance of the attachment system of S. ovinae to the substrate chemistry might be explained by the primary contribution of van der Waals interactions and not capillary forces to adhesion in S. ovinae.

Adhesion Performance in the Eggs of the Philippine Leaf Insect Phyllium Philippinicum (Phasmatodea: Phylliidae)

Leaf insects (Phasmatodea: Phylliidae) exhibit perfect crypsis imitating leaves. Although the special appearance of the eggs of the species Phyllium philippinicum, which imitate plant seeds, has received attention in different taxonomic studies, the attachment capability of the eggs remains rather anecdotical. We herein elucidate the specialized attachment mechanism of the eggs of this species and provide the first experimental approach to systematically characterize the functional properties of their adhesion by using different microscopy techniques and attachment force measurements on substrates with differing degrees of roughness and surface chemistry, as well as repetitive attachment/detachment cycles while under the influence of water contact. We found that a combination of folded exochorionic structures (pinnae) and a film of adhesive secretion contribute to attachment, which both respond to water. Adhesion is initiated by the glue, which becomes fluid through hydration, enabling adaption to the surface profile. Hierarchically structured pinnae support the spreading of the glue and reinforcement of the film. This combination aids the egg’s surface in adapting to the surface roughness, yet the attachment strength is additionally influenced by the egg’s surface chemistry, favoring hydrophilic substrates. Repetitive detachment and water-mediated adhesion can optimize the location of the egg to ensure suitable environmental conditions for embryonic development. Furthermore, this repeatable and water-controlled adhesion mechanism can stimulate further research for biomimeticists, ecologists and conservationalists.

Framework with cytoskeletal actin filaments forming insect footpad hairs inspires biomimetic adhesive device design

Footpads allow insects to walk on smooth surfaces. Specifically, liquid secretions on the footpad mediate adhesiveness through Van der Waals, Coulomb, and attractive capillary forces. Although the morphology and function of the footpad are well defined, the mechanism underlying their formation remains elusive. Here, we demonstrate that footpad hair in Drosophila is formed by the elongation of the hair cells and assembly of actin filaments. Knockdown of Actin5C caused a malformation of the hair structure, resulting in reduced ability to adhere to smooth substrates. We determined that functional footpads are created when hair cells form effective frameworks with actin filament bundles, thereby shaping the hair tip and facilitating cuticular deposition. We adapted this mechanism of microstructure formation to design a new artificial adhesive device⁠—a spatula-like fiber-framed adhesive device supported by nylon fibers with a gel material at the tip. This simple self-assembly mechanism facilitates the energy-efficient production of low-cost adhesion devices.

Kimura et al. demonstrate that footpad hair formation of Drosophila involves elongation of hair cells and assembly of actin filaments, followed by cuticular deposition. They then use this mechanism to design an artificial highly adhesive device with easy attachment and detachment properties.

Role of Fruit Epicuticular Waxes in Preventing Bactrocera oleae (Diptera: Tephritidae) Attachment in Different Cultivars of Olea europaea

The olive fruit fly Bactrocera oleae (Diptera: Tephritidae) is the major pest of cultivated olives (Olea europaea L.), and a serious threat in all of the Mediterranean Region. In the present investigation, we demonstrated with traction force experiments that B. oleae female adhesion is reduced by epicuticular waxes (EWs) fruit surface, and that the olive fruit fly shows a different ability to attach to the ripe olive surface of different cultivars of O. europaea (Arbequina, Carolea, Dolce Agogia, Frantoio, Kalamata, Leccino, Manzanilla, Picholine, Nostrale di Rigali, Pendolino and San Felice) in terms of friction force and adhesion, in relation with different mean values of olive surface wettability. Cryo-scanning morphological investigation revealed that the EW present on the olive surface of the different analyzed cultivars are represented by irregular platelets varying in the orientation, thus contributing to affect the surface microroughness and wettability in the different cultivars, and consequently the olive fruit fly attachment. Further investigations to elucidate the role of EW in olive varietal resistance to the olive fruit fly in relation to the olive developmental stage and environmental conditions could be relevant to develop control methods alternative to the use of harmful pesticides.

Porous substrate affects a subsequent attachment ability of the beetle Harmonia axyridis (Coleoptera, Coccinellidae)

According to literature data, porous substrates can cause a reduction of insect attachment ability. We carried out traction experiments with adult ladybird beetles Harmonia axyridis on the smooth solid glass sample and rough porous Al₂O₃ membrane to prove the primary effect of absorption of the insect pad secretion by porous media, rather than surface roughness, on the attachment force on the porous sample. With each insect individual, a set of five experiments was conducted: (1) on glass; (2) on the porous membrane; (3-5) on glass immediately after the test on the porous surface, then after 30 min and 1 h of recovery time. On the porous substrate, the forces, being similar in females and males, were greatly reduced compared to those measured on glass. A significant difference between the force values obtained in the first (before the test on the porous sample) and second (immediately after the experiment on the porous sample) tests on glass was observed. After 30 min recovery time, beetles completely regained their attachment ability. Females produced significantly lower forces than males in all experiments on glass: the differences are probably caused by the sexual dimorphism in the microstructure of their adhesive pads. The obtained results are of fundamental importance for further application in biomimetics of novel insect-repelling surfaces and in plant protection by using porous materials.

Attachment ability of females and males of the ladybird beetle Cryptolaemus montrouzieri to different artificial surfaces

To test the effects of surface chemistry and surface roughness on the attachment ability of female and male Cryptolaemus montrouzieri beetles that are equipped with hairy adhesive pads, traction force experiments were performed on three artificial substrates: (1) hydrophobic smooth glass, (2) hydrophobic smooth epoxy resin, and (3) hydrophobic microrough epoxy resin. Also the micromorphology of the dorsal body side and adhesive pads in males and females was examined using a scanning electron microscopy. The traction force ranged from 0.13 to 3.60 mN in females and from 0.28 to 3.20 mN in males. The force values obtained on different test substrates showed highly significant differences and decreased in the following order of substrates: glass - smooth epoxy resin - microrough epoxy resin. In both females and males, the effect of surfaces was similar. The obtained results clearly showed that both surface parameters, chemistry and roughness, affected the attachment ability of beetles. Similar microstructure of adhesive pads in both sexes resulted in similar attachment performance of males and females on all test substrates.

Mechanical ecology of fruit-insect interaction in the adult Mediterranean fruit fly Ceratitis capitata (Diptera: Tephritidae)

Fruit features represent a trade-off between dispersal and protection against frugivore insects. To prevent insect attack, plants evolved chemical and physical barriers, mainly studied in leaves, while limited knowledge is available for fruits, especially concerning mechanical barriers. We used the Mediterranean fruit fly to shed light on the mechanical ecology of insect-fruit attachment in a pest species. We tested the following hypotheses: is there any sexual dimorphism in attachment devices and attachment ability? Can the attachment ability of females of Ceratitis capitata to fruits of various host plants vary according to fruit surfaces with different morphology (smooth, hairy, waxy) or physico-chemical properties? The tarsal attachment devices were studied using Cryo-SEM and TEM. The maximum friction forces of C. capitata females on fruit surfaces of typical host plants were evaluated using a load cell force transducer. The attachment ability of both sexes on artificial surfaces was evaluated using a centrifugal force tester. Our data revealed sexual dimorphism in the size of pulvilli, which are wider in females. A higher friction force is exerted by females in comparison with males, in agreement with the need to firmly adhere to the host plant fruit during oviposition. Among the tested fruits, the stronger friction force was recorded on hairy or rough surfaces while a force reduction was recorded on waxy fruits. To unravel the mechanical ecology of insect-plant interaction between plants and species of Tephritidae can be useful to develop non-chemical methods to control these important crop pests.

Removal of heavy oil using rhinoceros beetle, Oryctes rhinoceros

The remnant of heavy oil, often expressed as total petroleum hydrocarbons (TPHs), in soil and/or sludge is commonly encountered by and often ruins the habitat of some indigenous insects. Recent studies indicate that some of the insects could stand the impact and some not just survived but might serve as an oil degrader. The potential of using indigenous insects in treating oil pollution is of interests in this study. In Asia, the potential oil-exposed rhinoceros beetle, Oryctes rhinoceros, was studied herein due to two major interests: one is its tolerance to heavy oil, and the other its ability to remove the oil. Two sets of the experiments spiked with different amounts of heavy oil were conducted in series, and the results of the larval mortality rate, growth rate, feces production, substrate conversion ratio and percent of oil removal were monitored as performance indicators during these treatments. The obtained results showed that the ingestion of heavy oil at a 5000 mg/kg level significantly deteriorated the survival rate (37% left) of the tested larvae, yet oil-experienced larvae could survive in up to 10,000 mg/kg of heavy oil (100% survived). As for the heavy oil removal via larval ingestion, at a 10,000 mg/kg level of oil in the substrate, the percent of weekly removal of heavy oil by larva ingestion was consistent throughout the second run at 56.4 ± 5.6%. These results demonstrated the potential use of beetle larvae as a bioreactor in pollutant removal.

Evidence for a sexually selected function of the attachment system in bedbugs Cimex lectularius (Heteroptera, Cimicidae)

Attachment to surfaces is a major aspect of an animal's interaction with the environment. Consequently, shaping of the attachment system in relation to weight load and substrate is considered to have occurred mainly by natural selection. However, sexual selection may also be important because many animals attach to their partner during mating. The two hypotheses generate opposing predictions in species where males are smaller than females. Natural selection predicts that attachment ability will scale positively with load, and hence body size, and so will be larger in females than males. Sexual selection predicts attachment forces in males will be larger than those in females, despite the males' smaller size because males benefit from uninterrupted copulation by stronger attachment to the female. We tested these predictions in the common bedbug Cimex lectularius, a species in which both sexes, as well as nymphs, regularly carry large loads: blood meals of up to 3 times their body weight. By measuring attachment forces to smooth surfaces and analysing in situ fixed copulating pairs and the morphology of attachment devices, we show that: (i) males generate twice the attachment force of females, despite weighing 15% less; (ii) males adhere to females during copulation using hairy tibial adhesive pads; (iii) there are more setae, and more setae per unit area, in the pads of males than in those of females but there is no difference in the shape of the tarsal setae; and (iv) there is an absence of hairy tibial attachment pads and a low attachment force in nymphs. These results are consistent with a sexually selected function of attachment in bedbugs. Controlling sperm transfer and mate guarding by attaching to females during copulation may also shape the evolution of male attachment structures in other species. More generally, we hypothesise the existence of an arms race in terms of male attachment structures and female counterparts to impede attachment, which may result in a similar evolutionary diversification to male genitalia.

Estimating the maximum attachment performance of tree frogs on rough substrates

Tree frogs can attach to smooth and rough substrates using their adhesive toe pads. We present the results of an experimental investigation of tree frog attachment to rough substrates, and of the role of mechanical interlocking between superficial toe pad structures and substrate asperities in the tree frog species Litoria caerulea and Hyla cinerea. Using a rotation platform setup, we quantified the adhesive and frictional attachment performance of whole frogs clinging to smooth, micro-, and macrorough substrates. The transparent substrates enabled quantification of the instantaneous contact area during detachment by using frustrated total internal reflection. A linear mixed-effects model shows that the adhesive performance of the pads does not differ significantly with roughness (for nominal roughness levels of 0-15 µm) in both species. This indicates that mechanical interlocking does not contribute to the attachment of whole animals. Our results show that the adhesion performance of tree frogs is higher than reported previously, emphasising the biomimetic potential of tree frog attachment. Overall, our findings contribute to a better understanding of the complex interplay of attachment mechanisms in the toe pads of tree frogs, which may promote future designs of tree-frog-inspired adhesives.

A comparison of tarsal morphology and traction force in the two burying beetles Nicrophorus nepalensis and Nicrophorus vespilloides (Coleoptera, Silphidae)

Our aim was to compare friction and traction forces between two burying beetle species of the genus Nicrophorus exhibiting different attachment abilities during climbing. Specifically, the interaction of adhesive hairs and claws during attachment with respect to various surface properties was investigated by using a 2 × 3 experimental design. Traction force was measured for two different surface energies (hydrophilic vs hydrophobic) varying in roughness from smooth to micro-rough to rough. Nanotribometric tests on single legs were also performed. The external morphology of the attachment devices investigated by scanning electron microscopy suggested higher intra-specific (intersexual) than inter-specific differences. Whereas differences between the two species in traction force were high on smooth surfaces, no differences could be detected between males and females within each species. With claws intact, both species showed the highest forces on rough surfaces, although N. nepalensis with clipped claws performed best on a smooth surface. However, N. nepalensis beetles outperformed N. vespilloides, which showed no differences between smooth and rough surfaces with clipped claws. Both species demonstrated poor traction forces on micro-rough surfaces. Results concerning the impact of surface polarity were inconclusive, whereas roughness more strongly affected the attachment performance in both species. Nanotribometric analyses of the fore tarsi performed on micro-rough and rough surfaces revealed higher friction in the proximal (pull) direction compared with the distal (push) direction. In these experiments, we detected neither differences in friction performance between the two species, nor clear trends concerning the influence of surface polarity. We conclude that the investigated morphological traits are not critical for the observed interspecific difference in attachment ability on smooth surfaces. Furthermore, interspecific differences in performance are only clear on smooth surfaces and vanish on micro-rough and rough surfaces. Our results suggest that even subtle differences in the adhesion-mediating secretion in closely related species might result in qualitative performance shifts.

Critical roughness in animal hairy adhesive pads: a numerical modeling approach

General understanding of adhesion failure of setal attachment pads at some particular substrate roughness is important for construction of biologically active adhesive and anti-adhesive surfaces. Here we present a numerical model which is capable of explaining experimentally found effects of the adhesion drop on the level of an individual adhesive spatula. The model incorporates the interaction of an elastic spatula with a rough substrate in 2D. The contact area and the work of adhesion have been determined for two different types of the model substrates. The surfaces were either composed of particles of different sizes or obtained from real rough substrate by smothering with Gaussians having varying widths. Surface profiles of real substrates with different roughness measured using white light interferometry were utilized as a reference. The interaction of a spatula with a substrate was found to be independent of the substrate model type. Similar to that observed in experiments on real animals, numerically found adhesion drops at some particular substrate roughness. It was shown that the adhesion drop is related to the incomplete contact between the spatula and the substrate at the scale when the characteristic roughness wavelength is comparable to the size of the spatula.

Material gradients in fibrillar insect attachment systems: the role of joint-like elements

Insects have developed elaborate fibrillar (or hairy) attachment systems that allow them to attach reliably on a variety of different and unpredictable surfaces. These hairy adhesive pads consist of fine and long surface outgrowths (setae), terminated by thin plate-like tips of different shapes. Besides structural adaptations, recent work revealed material gradients along the length of the setae with spatula-shaped and pointed tip structures. It was shown that these setae have a rigid base and soft setal tips, which is assumed to enhance the adaptability to rough surfaces and prevent clustering of the setae. Here, we show a not yet described type of material gradient found in discoidal (or mushroom-shaped) setae of male leaf beetles. In contrast to the previously shown gradient, the setal tips and the basal and central seta sections are composed of relatively stiff chitinous materials, whereas the transition zones between the central seta sections and the setal tips contain large proportions of the rather soft and elastic protein resilin, forming a joint-like element. Having performed adhesion measurements with artificial macroscopic mushroom-shaped adhesive structures with different implemented joint-like elements, we show that the main functional role of these joint-like elements is to facilitate the adaptability to uneven and non-parallel substrates, rather than to provide detachment tolerance towards pull-off forces applied under various tilt angles.

Investigating the efficiency of a bio-inspired insect repellent surface structure

Most insects with smooth or hairy adhesive pads have very little problems in attaching to smooth substrates. A careful selection of surface roughness, however, can effectively limit the contact area of the adhesive organs with the surface. In comparison to conventional toxin-based insect repelling methods, biologically inspired micro- and nanostructured insect repellent surface structures, thus, offer a great potential to effective and environmentally-friendly control insect pests. We here present a simple experimental approach to qualitatively and quantitatively analyse the efficiency of a micro- and nanorough surface structure. Nauphoeta cinerea and Gastrophysa viridula as model organisms for insects with smooth and hairy adhesive pads, respectively, were placed in an enclosed environment. The escape movements of freely running insects over either the repellent or a control surfaces were counted and analyzed in detail. The tested surface structures were able to significantly reduce the escape of cockroaches with smooth adhesive pads by 44.1%. Interestingly, the data indicates that N. cinerea might use mechanical cues from the antenna to discriminate between surfaces before making contact with the adhesive pads. G. viridula with hairy adhesive pads were not significantly affected by the surface structure. By carefully adjusting 'critical' surface topography parameters relevant for insect adhesion, more efficient and selective repellent surfaces might be achieved. Such nanostructure-based insect repellent surfaces could also help to utilize recruitment behavior of certain insect species and might present a novel approach to effectively control insect pests.

Attachment ability of the polyphagous bug Nezara viridula (Heteroptera: Pentatomidae) to different host plant surfaces

The present investigation tests through friction experiments the attachment ability of adults of the southern green stink bug Nezara viridula L. (Heteroptera: Pentatomidae), a polyphagous insect representing a cosmopolitan pest, on different host plant species characterized by smooth, hairy and waxy surfaces. Surfaces of different tested plants have been studied in Cryo-Scanning Electron Microscope (Cryo-SEM). The load cell force transducer was used to evaluate the potential damage to the insect attachment devices induced by walking on the different leaf surfaces. In case of the plant Phaseolus vulgaris, where insects showed a strong reduction in their adhesion ability during and after walking on the leaf, the damage to the insect by two cultivars with different morphological features and the insect ability to recover after 24 h has been evaluated. The ability to recover notwithstanding the damage to attachment devices, shown by Cryo-SEM investigations, together with the strong attachment forces produced on various plant leaves, characterized by different morphological features, is in agreement with the great adaptability and ecological plasticity of this widely-spread bug species. The present study, increasing our knowledge on the mechanical interaction of this species with different host plant species, can help to develop new strategies to control this insect pest.

Functional morphology of tarsal adhesive pads and attachment ability in ticks Ixodes ricinus (Arachnida, Acari, Ixodidae)

The presence of well-developed, elastic claws on ticks and widely pilose hosts led us to hypothesise that ticks are mostly adapted to attachment and locomotion on rough, strongly corrugated and hairy, felt-like substrates. However, by using a combination of morphological and experimental approaches, we visualised the ultrastructure of attachment devices of Ixodes ricinus and showed that this species adheres more strongly to smooth surfaces than to rough ones. Between paired, elongated, curved, elastic claws, I. ricinus bears a large, flexible, foldable adhesive pad, which represents an adaptation to adhesion on smooth surfaces. Accordingly, ticks attached strongest to glass and to surface profiles similar to those of the human skin, generating safety factors (attachment force relative to body weight) up to 534 (females). Considerably lower attachment force was found on silicone substrates and as a result of thanatosis after jolting.