Pressure-induced silk spinning mechanism in webspinners (Insecta: Embioptera).

The articulated appendages of arthropods are highly adaptable and potentially multifunctional, used for walking, swimming, feeding, prey capture, or other functions. Webspinners (Order Embioptera) are a paragon in this context. In contrast to other arthropods producing silk, they utilize their front feet for silk production. However, employing the same leg for alternative functions rather than for pure locomotion potentially imposes constraints and compromises. We here present morphological and experimental evidence for a "passive" pressure-induced silk spinning mechanism induced by external mechanical stimuli. Furthermore, we demonstrate that, as a consequence of the conflicting functions for their front feet, webspinners have evolved a unique style of walking that reduces the potentially problematic contact between silk ejectors and the substrate. Here we answer for the first time a long-term question within this enigmatic group of insects-how webspinners can use their front feet to spin their nanoscale silk. This knowledge may open the door for experimental studies on an artificial spinning process and for future utilization in applied fields of robotics or chemistry.

The effect of INDEHISCENT point mutations on silique shatter resistance in oilseed rape (Brassica napus).

KEY MESSAGE: This study elucidates the influence of indehiscent mutations on rapeseed silique shatter resistance. A phenotype with enlarged replum-valve joint area and altered cell dimensions in the dehiscence zone is described. Silique shattering is a major factor reducing the yield stability of oilseed rape (Brassica napus). Attempts to improve shatter resistance often include the use of mutations in target genes identified from Arabidopsis (Arabidopsis thaliana). A variety of phenotyping methods assessing the level of shatter resistance were previously described. However, a comparative and comprehensive evaluation of the methods has not yet been undertaken. We verified the increase of shatter resistance in indehiscent double knock-down mutants obtained by TILLING with a systematic approach comparing three independent phenotyping methods. A positive correlation of silique length and shatter resistance was observed and accounted for in the analyses. Microscopic studies ruled out the influence of different lignification patterns. Instead, we propose a model to explain increased shattering resistance of indehiscent rapeseed mutants by altered cell shapes and sizes within the contact surfaces of replum and valves.

Holding tight to feathers - structural specializations and attachment properties of the avian ectoparasite Crataerina pallida (Diptera, Hippoboscidae).

The louse fly Crataerina pallida is an obligate blood-sucking ectoparasite of the common swift Apus apus As a result of reduction of the wings, C. pallida is unable to fly; thus, an effective and reliable attachment to their host's plumage is of utmost importance. The attachment system of C. pallida shows several modifications in comparison to that of other calyptrate flies, notably the large tridentate claws and the dichotomously shaped setae located on the pulvilli. Based on data from morphological analysis, confocal laser scanning microscopy, cryo-scanning electron microscopy and attachment force experiments performed on native (feathers) as well as artificial substrates (glass, epoxy resin and silicone rubber), we showed that the entire attachment system is highly adapted to the fly's lifestyle as an ectoparasite. The claws in particular are the main contributor to strong attachment to the host. Resulting attachment forces on feathers make it impossible to detach C. pallida without damage to the feathers or to the legs of the louse fly itself. Well-developed pulvilli are responsible for the attachment to smooth surfaces. Both dichotomously shaped setae and high setal density explain high attachment forces observed on smooth substrates. For the first time, we demonstrate a material gradient within the setae, with soft, resilin-dominated apical tips and stiff, more sclerotized bases in Diptera. The empodium seems not to be directly involved in the attachment process, but it might operate as a cleaning device and may be essential to maintain the functionality of the entire attachment system.

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.

Ultrastructure of dragonfly wing veins: composite structure of fibrous material supplemented by resilin.

Dragonflies count among the most skilful of the flying insects. Their exceptional aerodynamic performance has been the subject of various studies. Morphological and kinematic investigations have showed that dragonfly wings, though being rather stiff, are able to undergo passive deformation during flight, thereby improving the aerodynamic performance. Resilin, a rubber-like protein, has been suggested to be a key component in insect wing flexibility and deformation in response to aerodynamic loads, and has been reported in various arthropod locomotor systems. It has already been found in wing vein joints, connecting longitudinal veins to cross veins, and was shown to endow the dragonfly wing with chordwise flexibility, thereby most likely influencing the dragonfly's flight performance. The present study revealed that resilin is not only present in wing vein joints, but also in the internal cuticle layers of veins in wings of Sympetrum vulgatum (SV) and Matrona basilaris basilaris (MBB). Combined with other structural features of wing veins, such as number and thickness of cuticle layers, material composition, and cross-sectional shape, resilin most probably has an effect on the vein's material properties and the degree of elastic deformations. In order to elucidate the wing vein ultrastructure and the exact localisation of resilin in the internal layers of the vein cuticle, the approaches of bright-field light microscopy, wide-field fluorescence microscopy, confocal laser-scanning microscopy, scanning electron microscopy and transmission electron microscopy were combined. Wing veins were shown to consist of up to six different cuticle layers and a single row of underlying epidermal cells. In wing veins of MBB, the latter are densely packed with light-scattering spheres, previously shown to produce structural colours in the form of quasiordered arrays. Longitudinal and cross veins differ significantly in relative thickness of exo- and endocuticle, with cross veins showing a much thicker exocuticle. The presence of resilin in the unsclerotised endocuticle suggests its contribution to an increased energy storage and material flexibility, thus to the prevention of vein damage. This is especially important in the highly stressed longitudinal veins, which have much lower possibility to yield to applied loads with the aid of vein joints, as the cross veins do. These results may be relevant not only for biologists, but may also contribute to optimise the design of micro-air vehicles.

Challenges and solutions for joining polymer materials.

The different mechanisms contributing to adhesion between two polymer surfaces are summarized and described in individual examples, which represent either seminal works in the field of adhesion science or novel approaches to achieve polymer-polymer adhesion. A further objective of this article is the development of new methodologies to achieve strong adhesion between low surface energy polymers.

Adhesion failure at 180,000 frames per second: direct observation of the detachment process of a mushroom-shaped adhesive.

Nature has successfully evolved the mushroom-shaped contact geometry in many organisms in order to solve the attachment problem. We studied the detachment process of individual bioinspired artificial mushroom-shaped adhesive microstructures (MSAMSs) resolving the failure dynamics at high spatiotemporal resolution. The experimental data provide strong evidence for a homogeneous stress distribution in MSAMS, which was recently proposed. Our results allow us to explain the advantage of such contact geometry and provide a suggestion for the widely observed mushroom-shaped contact geometry.

Bioinspired Granular Media Friction Pad: A Universal System for Friction Enhancement on Variety of Substrates.

The granular media friction pad (GMFP) inspired by the biological smooth attachment pads of cockroaches and grasshoppers employs passive jamming, to create high friction forces on a large variety of substrates. The granular medium inside the pad is encased by a flexible membrane which at contact formation greatly adapts to the substrate profile. Upon applying load, the granular medium undergoes the jamming transition and changes from fluid-like to solid-like properties. The jammed granular medium, in combination with the deformation of the encasing elastic membrane, results in high friction forces on a multitude of substrate topographies. Here we explore the effect of elasticity variation on the generation of friction by varying granular media filling quantity as well as membrane modulus and thickness. We systematically investigate contact area and robustness against substrate contamination, and we also determine friction coefficients for various loading forces and substrates. Depending on the substrate topography and loading forces, a low filling quantity and a thin, elastic membrane can be favorable, in order to generate the highest friction forces.

Dry under water: air retaining properties of large-scale elastomer foils covered with mushroom-shaped surface microstructures.

Superhydrophobic surfaces are well known for most different functions in plants, animals, and thus for biomimetic technical applications. Beside the Lotus Effect, one of their features with great technical, economic and ecologic potential is the Salvinia Effect, the capability to keep a stable air layer when submerged under water. Such air layers are of great importance, e.g., for drag reduction (passive air lubrication), antifouling, sensor applications or oil-water separation. Some biological models, e.g., the floating fern Salvinia or the backswimmer Notonecta, show long term stable air retention even under hydrodynamic conditions. Therefore, they are ideal models for the development of technical biomimetic air retaining surfaces. Up to now, several prototypes of such surfaces have been developed, but none provides both, stable air retention and cost effective large scale production. Meanwhile, a novel biomimetic surface is commercially available and produced on a large scale: an adhesive elastomeric film with mushroom-shaped surface microstructures that mimic the adhesion system of animals. In this study, we show that these films, which have been initially developed for a different purpose, due to their specific geometry at the microscale, are capable of stable air retention under water. We present first results concerning the capabilities of mushroom-shaped surface microstructures and show that this elastomer foil is able to stabilize a permanent air layer under water for more than two weeks. Further, the stability of the air layer under pressure was investigated and these results are compared with the predicted theoretical values for air retention of microstructured surfaces. Here, we could show that they fit to the theoretical predictions and that the biomimetic elastomer foil is a promising base for the development of an economically and efficient biomimetic air retaining surface for a broad range of technical applications.

Sand-throwing behaviour in pit-building antlion larvae: insights from finite-element modelling.

Sandy pitfall traps of antlions are elaborate constructions to capture prey. Antlions exploit the interactions between the particles in their habitat and build a stable trap. This trap is close to the unstable state; prey items will slide towards the centre-where the antlion ambushes-when entering the trap. This is efficient but requires permanent maintenance. According to the present knowledge, antlions throw sand, mainly to cause sandslides towards the centre of the pit. We hypothesized that: (i) sand-throwing causes sandslides towards the centre of the pit and (ii) sand-throwing constantly maintains the pitfall trap and thus keeps its efficiency high. Using laboratory experiments, as well as finite-element analysis, we tested these hypotheses. We show, experimentally and numerically, that sand that accumulates at the centre of the pit will be removed continuously by sand-throwing, this maintenance is leading to slope condition close to an unstable state. This keeps the slope angle steep and the efficiency of the trap constant. Furthermore, the resulting sandslides can relocate the trapped prey towards the centre of the pit. This study adds further insights from specific mechanical properties of a granular medium into the behavioural context of hunting antlion larvae.

Porous silicone substrates inhibit permanent barnacle attachment under natural conditions.

Barnacles are able to effectively adhere to most surfaces underwater. Dewetting of the corresponding surface prior to the release of their permanent adhesive plays an important role in the attachment process. Possibly, a surface that is able to interfere with this process may have exceptional fouling repellence and fouling release abilities. Therefore, open-pored foams made from polydimethylsiloxane (PDMS) were tested together with flat PDMS samples as controls in a 13-week-long field experiment in the Baltic Sea. On a weekly basis, both settlement and fouling density development of the bay barnacle Balanus (=Amphibalanus) improvisus were monitored. The overall settlement was close to zero on PDMS foams and the few attached barnacles were not able to stay on the PDMS foams longer than 1 week after initial settlement. Changes in the stiffness of the PDMS foams did not affect these results. Open-pored PDMS foam systems may be a promising tool in the development of new, innovative antifouling strategies.

Adaptations of dragonfly larvae and their exuviae (Insecta: Odonata), attachment devices and their crucial role during emergence.

Moulting, especially in 'hemimetabolous' insects that emerge upside down, is a crucial moment in their live. Losing their attachment during this situation can be fatal for survival. We here studied the emergence of dragonfly adults, describe structures involved in larval attachment to the substrate, and biomechanically test the pull-off forces of exuviae to natural substrates. Confocal laser scanning microscopy and scanning electron microscopy were used to describe both morphology and material composition of the leg cuticle of Anax imperator larvae. The results show that the combination of morphological and behavioral adaptations provides reliable anchorage of exuviae to the substrates. We determined a safety factor of 14, and demonstrated that this staggered safety system experiencing several unlocking and relocking events withstand multiple disturbances before the entire exuvia is completely detaches. This furthers our understanding of interlocking and anchorage of insects in general and may allow for future applications.

Local deformation and stiffness distribution in fly wings.

Mechanical properties of insect wings are essential for insect flight aerodynamics. During wing flapping, wings may undergo tremendous deformations, depending on the wings' spatial stiffness distribution. We here show an experimental evaluation of wing stiffness in three species of flies using a micro-force probe and an imaging method for wing surface reconstruction. Vertical deflection in response to point loads at 11 characteristic points on the wing surface reveals that average spring stiffness of bending lines between wing hinge and point loads varies ∼77-fold in small fruit flies and up to ∼28-fold in large blowflies. The latter result suggests that local wing deformation depends to a considerable degree on how inertial and aerodynamic forces are distributed on the wing surface during wing flapping. Stiffness increases with an increasing body mass, amounting to ∼0.6 Nm-1 in fruit flies, ∼0.7 Nm-1 in house flies and ∼2.6 Nm-1 in blowflies for bending lines, running from the wing base to areas near the center of aerodynamic pressure. Wings of house flies have a ∼1.4-fold anisotropy in mean stiffness for ventral versus dorsal loading, while anisotropy is absent in fruit flies and blowflies. We present two numerical methods for calculation of local surface deformation based on surface symmetry and wing curvature. These data demonstrate spatial deformation patterns under load and highlight how veins subdivide wings into functional areas. Our results on wings of living animals differ from previous experiments on detached, desiccated wings and help to construct more realistic mechanical models for testing the aerodynamic consequences of specific wing deformations.

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.

Tarsal attachment devices of the southern green stink bug Nezara viridula (Heteroptera: Pentatomidae).

Based on analyses with cryo-scanning and transmission electron microscopy, the present study reports on the morphology and ultrastructure of the attachment structures of the green stinkbug Nezara viridula L. (Heteroptera: Pentatomidae), a cosmopolitan pest of different crops in most areas of the world. In addition, the presence and distribution of large proportions of the elastic protein resilin in these structures was revealed by confocal laser scanning microscopy. The attachment structures of each leg comprise two sclerotised claws, a pair of smooth flexible pulvilli and a hairy adhesive pad located at the ventral side of the basitarsus. No sexual dimorphism is evident. Contact areas of resting individuals on a smooth surface show that N. viridula creates contact to the substrate with the ventral surface of (a) the distal portions of the pulvilli, (b) the setae of the hairy adhesive pad, (c) the two paraempodia representing mechanosensory setae, and (d) the tips of the claws. Each pulvillus is a sac-like structure formed by complex cuticular layers that vary in their structure and resilin content. The dorsal side consists of sclerotised chitinous material, while the ventral cuticle consists mainly of resilin and shows a very thin epicuticle and a thick exocuticle. The setae of the hairy adhesive pad are pointed and socketed. They exhibit a pronounced longitudinal gradient in the material composition, with large proportions of resilin being present in the setal tips. In most of these setae, especially in those of the distal-most part of the pad, also a transverse gradient in the material composition is visible.

Competing with barnacle cement: wetting resistance of a re-entrant surface reduces underwater adhesion of barnacles.

Surfaces with re-entrant topographies can repel liquids even of extremely low surface tension, almost independently of the material's inherent wettability. We show that this topography-based wetting resistance can also be applied to underwater applications, reducing the permanent adhesion of marine hardfouling organisms. Having combined a biofouling assay in the marine environment with microscopic analyses, we demonstrate how a synergistic effect of a soft silicone-based material with a re-entrant mushroom-shaped surface topography strongly increases the fouling release ability of such coatings compared with a smooth control made from the same material. Our coating inhibited the complete wetting of the solidifying glues produced by marine organisms, which resulted in a decreased contact area and, consequently, low adhesion. Our work suggests that topography-induced wetting resistance of surfaces may become a viable approach in preventing permanent adhesion of marine hardfouling organisms.

Inversion of friction anisotropy in a bio-inspired asymmetrically structured surface.

Friction anisotropy is an important property of many surfaces that usually facilitate the generation of motion in a preferred direction. Such surfaces are very common in biological systems and have been the templates for various bio-inspired materials with similar tribological properties. So far friction anisotropy is considered to be the result of an asymmetric arrangement of surface nano- and microstructures. However, here we show by using bio-inspired sawtooth-structured surfaces that the anisotropic friction properties are not only controlled by an asymmetric surface topography, but also by the ratio of the sample-substrate stiffness, the aspect ratio of surface structures, and by the substrate roughness. Systematically modifying these parameters, we were able to demonstrate a broad range of friction anisotropies, and for specific sample-substrate combinations even an inversion of the anisotropy. This result highlights the complex interrelation between the different material and topographical parameters on friction properties and sheds new light on the conventional design paradigm of tribological systems. Finally, this result is also of great importance for understanding functional principles of biological materials and surfaces, as such inversion of friction anisotropy may correlate with gait pattern and walking behaviour in climbing animals, which in turn may be used in robotic applications.

Influence of ambient humidity on the attachment ability of ladybird beetles (Coccinella septempunctata).

Many insects possess adhesive foot pads, which enable them to scale smooth vertical surfaces. The function of these organs may be highly affected by environmental conditions. Ladybird beetles (Coccinellidae) possess dense tarsal soles of tenent setae, supplemented with an adhesive fluid. We studied the attachment ability of the seven-spotted ladybird beetle (Coccinella septempunctata) at different humidities by horizontal traction experiments. We found that both low (15%) and high (99%) relative humidities lead to a decrease of attachment ability. The significantly highest attachment forces were revealed at 60% humidity. This relationship was found both in female and male beetles, despite of a deviating structure of adhesive setae and a significant difference in forces between sexes. These findings demonstrate that not only dry adhesive setae are affected by ambient humidity, but also setae that stick due to the capillarity of an oily secretion.

Enhanced Locomotion Efficiency of a Bio-inspired Walking Robot using Contact Surfaces with Frictional Anisotropy.

Based on the principles of morphological computation, we propose a novel approach that exploits the interaction between a passive anisotropic scale-like material (e.g., shark skin) and a non-smooth substrate to enhance locomotion efficiency of a robot walking on inclines. Real robot experiments show that passive tribologically-enhanced surfaces of the robot belly or foot allow the robot to grip on specific surfaces and move effectively with reduced energy consumption. Supplementing the robot experiments, we investigated tribological properties of the shark skin as well as its mechanical stability. It shows high frictional anisotropy due to an array of sloped denticles. The orientation of the denticles to the underlying collagenous material also strongly influences their mechanical interlocking with the substrate. This study not only opens up a new way of achieving energy-efficient legged robot locomotion but also provides a better understanding of the functionalities and mechanical properties of anisotropic surfaces. That understanding will assist developing new types of material for other real-world applications.

Friction behavior of a microstructured polymer surface inspired by snake skin.

The aim of this study was to understand the influence of microstructures found on ventral scales of the biological model, Lampropeltis getula californiae, the California King Snake, on the friction behavior. For this purpose, we compared snake-inspired anisotropic microstructured surfaces to other microstructured surfaces with isotropic and anisotropic geometry. To exclude that the friction measurements were influenced by physico-chemical variations, all friction measurements were performed on the same epoxy polymer. For frictional measurements a microtribometer was used. Original data were processed by fast Fourier transformation (FFT) with a zero frequency related to the average friction and other peaks resulting from periodic stick-slip behavior. The data showed that the specific ventral surface ornamentation of snakes does not only reduce the frictional coefficient and generate anisotropic frictional properties, but also reduces stick-slip vibrations during sliding, which might be an adaptation to reduce wear. Based on this extensive comparative study of different microstructured polymer samples, it was experimentally demonstrated that the friction-induced stick-slip behavior does not solely depend on the frictional coefficient of the contact pair.