Fracture toughness of locust cuticle

Performance of biological food processing interfaces: Perspectives on the science of mollusc radula

The Mollusca comprises a diverse range of organisms, with the class Gastropoda alone boasting approximately 80 000 extant species. Their adaptability across various habitats is facilitated by the evolution of the radula, a key structure for food acquisition. The radula's composition and mechanical properties, including its chitinous membrane, teeth, and supporting structures, enable efficient food gathering and processing. Through adaptive tooth morphology and composition, an interplay between radular components is facilitated, which results in collective effects to withstand forces encountered during feeding and reduce structural failure, with the broad range of variations reflecting ecological niches. Furthermore, teeth consist of composite materials with sometimes high contents of iron, calcium, or silicon to reduce wear. During interaction with the food, the radula performs complex three-dimensional motions, challenging to document. Here, we provide a review on the morphology, the mechanical properties, the composition, and various other parameters that contribute to radular performance. Due to, e.g., the smallness of these structures, there are, however, limitations to radular research. However, numerical simulations and physical models tested on substrates offer avenues for further understanding radular function and performance during feeding. These studies not only advance our knowledge of molluscan biology and ecology but also provide inspirations for biomimetic design and further advances in materials engineering.

Insect exoskeletons react to hypergravity

A typical feature of biological materials is their ability to adapt to mechanical load. However, it is not known whether the cuticle exoskeleton, one of the most common biological structures, also shares this trait. Here, we show direct experimental evidence that prolonged exposure to hypergravity conditions affects the morphology and biomechanics of an insect exoskeleton. Locusts were raised for several weeks in a custom-designed centrifuge at various levels of hypergravity. Biomechanical measurements and X-ray microtomography show that up to 3 g load Young's modulus of the tibiae increased by about 67%. Higher gravitational loads however decreased the survival rate, body mass and endocuticle thickness. These results directly show that cuticle exoskeletons can react to hypergravity. This ability has so far only been known for bone endoskeletons and plants. Our findings thus add important context to the discussion on general ultimate factors in the evolution of adaptive biological materials and skeletal systems.

Biochemical compositions and histopathology of the young and aged nymphs of the desert locust Schistocerca gregaria Forsskål (Orthoptera: Acrididae) affected by Photorhabdus luminescens (Enterobacterales: Morganellaceae)

Locusts occasionally represent a danger in Africa despite intensive management measures, leading to severe yield loss and a commensurate loss of food and money. The laboratory assessment of the toxicity of the Photorhabdus luminescens bacteria and its cell-free filtrate used Schistocerca gregaria nymphs in the second and fifth nymphs as test insects. Greater mortality was seen in locust nymphs of the second and fifth instars due to the high levels of toxicity produced by the bacterial suspension and its cell-free filtrate. The amounts of protein, fat, and carbohydrates in the treated locusts were drastically reduced. For the treated second and fifth instar nymphs of the desert locust, adverse effects on the muscular layers of the midgut and the muscles in the jumping legs were investigated.

Complex hemolymph circulation patterns in grasshopper wings

An insect's living systems-circulation, respiration, and a branching nervous system-extend from the body into the wing. Wing hemolymph circulation is critical for hydrating tissues and supplying nutrients to living systems such as sensory organs across the wing. Despite the critical role of hemolymph circulation in maintaining healthy wing function, wings are often considered "lifeless" cuticle, and flows remain largely unquantified. High-speed fluorescent microscopy and particle tracking of hemolymph in the wings and body of the grasshopper Schistocerca americana revealed dynamic flow in every vein of the fore- and hindwings. The global system forms a circuit, but local flow behavior is complex, exhibiting three distinct types: pulsatile, aperiodic, and "leaky" flow. Thoracic wing hearts pull hemolymph from the wing at slower frequencies than the dorsal vessel; however, the velocity of returning hemolymph (in the hindwing) is faster than in that of the dorsal vessel. To characterize the wing's internal flow mechanics, we mapped dimensionless flow parameters across the wings, revealing viscous flow regimes. Wings sustain ecologically important insect behaviors such as pollination and migration. Analysis of the wing circulatory system provides a template for future studies investigating the critical hemodynamics necessary to sustaining wing health and insect flight.

Methods for Silk Property Analyses across Structural Hierarchies and Scales

Silk from silkworms and spiders is an exceptionally important natural material, inspiring a range of new products and applications due to its high strength, elasticity, and toughness at low density, as well as its unique conductive and optical properties. Transgenic and recombinant technologies offer great promise for the scaled-up production of new silkworm- and spider-silk-inspired fibres. However, despite considerable effort, producing an artificial silk that recaptures the physico-chemical properties of naturally spun silk has thus far proven elusive. The mechanical, biochemical, and other properties of pre-and post-development fibres accordingly should be determined across scales and structural hierarchies whenever feasible. We have herein reviewed and made recommendations on some of those practices for measuring the bulk fibre properties; skin-core structures; and the primary, secondary, and tertiary structures of silk proteins and the properties of dopes and their proteins. We thereupon examine emerging methodologies and make assessments on how they might be utilized to realize the goal of developing high quality bio-inspired fibres.

Temporal changes in the physical and mechanical properties of beetle elytra during maturation

Changes in physical properties of Tenebrio molitor and Tribolium castaneum elytra (hardened forewings) were studied to understand how the development of microstructure and chemical interactions determine cuticle mechanical properties. Analysis of these properties supports a model in which cuticular material is continuously secreted from epidermal cells to produce an extracellular matrix so that the outermost layers mature first. It is hypothesized that enzymatic crosslinking and pigmentation reactions along with dehydration help to stabilize the protein-chitin network within the initial layers of cuticle shortly after eclosion. Mature layers are proposed to bear most of the mechanical loads. The frequency dependence of the storage modulus and the tan δ values decreased during the beginning of maturation, reaching constant values after 48 h post-eclosion. A decrease of tan δ indicates an increase in crosslinking of the material. The water content declined from 75% to 31%, with a significant portion lost from within the open spaces between the dorsal and ventral cuticular layers. Dehydration had a less significant influence than protein crosslinking on the mechanical properties of the elytron during maturation. When Tribolium cuticular protein TcCP30 expression was decreased by RNAi, the tan δ and frequency dependence of E' of the elytron did not change during maturation. This indicates that TcCP30 plays a role in the crosslinking process of the beetle's exoskeleton. This study was inspired by previous work on biomimetic multicomponent materials and helps inform future work on creating robust lightweight materials derived from natural sources. STATEMENT OF SIGNIFICANCE: Examination of changes in the physical properties of the elytra (hardened forewings) of two beetle species advanced understanding of how the molecular interactions influence the mechanical properties of the elytra. Physical characterization, including dynamic mechanical analysis, determined that the outer portion of the elytra matured first, while epidermal cells continued to secrete reactive components until the entire structure reached maturation. RNA interference was used to identify the role of a key protein in the elytra. Suppression of its expression reduced the formation of crosslinked polymeric components in the elytra. Identifying the molecular interactions in the matrix of proteins and polysaccharides in the elytra together with their hierarchical architecture provides important design concepts in the development of biomimetic materials.

The forewing of a black cicada Cryptotympana atrata (Hemiptera, Homoptera: Cicadidae): Microscopic structures and mechanical properties

Insects in nature flap their wings to generate lift force and driving torque to adjust their attitude and control stability. An insect wing is a biomaterial composed of flexible membranes and tough veins. In this paper, we study the microscopic structures and mechanical properties of the forewing of the black cicada, Cryptotympana atrata. The thickness of the wing membranes and the diameter of veins varied from the wing root to the tip. The thickness of the wing membranes ranged from 6.0 to 29.9 μm, and the diameter of the wing veins decreased in a gradient from the wing root to the tip, demonstrating that the forewing of the black cicada is a nonuniform biomaterial. The elastic modulus of the membrane near the wing root ranged from 4.45 to 5.03 GPa, which is comparable to that of some industrial membranes. The microstructure of the wing vein exhibited a hollow tubular structure with flocculent structure inside. The "fresh" sample stored more water than the "dry" sample, resulting in a significant difference in the elastic modulus between the fresh and dried veins. The different membrane thicknesses and elastic moduli of the wing veins near the root and tip resulted in varied degrees of deformation on both sides of the flexion line of the forewing during twisting. The measurements of the forewing of the cicada may serve as a guide for selecting airfoil materials for the bionic flapping-wing aircraft and promote the design and manufacture of more durable bionic wings in the future.

Insect antennae: Coupling blood pressure with cuticle deformation to control movement

Insect antennae are hollow, blood-filled fibers with complex shape. Muscles in the two basal segments control antennal movement, but the rest (flagellum) is muscle-free. The insect can controllably flex, twist, and maneuver its antennae laterally. To explain this behavior, we performed a comparative study of structural and tensile properties of the antennae of Periplaneta americana (American cockroach), Manduca sexta (Carolina hawkmoth), and Vanessa cardui (painted lady butterfly). These antennae demonstrate a range of distinguishable tensile properties, responding either as brittle or strain-adaptive fibers that stiffen when stretched. Scanning electron microscopy and high-speed imaging of antennal breakup during stretching revealed complex coupling of blood pressure and cuticle deformation in antennae. A generalized Lamé theory of solid mechanics was developed to include the force-driven deformation of blood-filled antennal tubes. We validated the theory against experiments with artificial antennae with no adjustable parameters. Blood pressure increased when the insect inflated its antennae or decreased below ambient pressure when an external tensile load was applied to the antenna. The pressure-cuticle coupling can be controlled through changes of the blood volume in the antennal lumen. In insects that do not fill the antennal lumen with blood, this blood pressure control is lacking, and the antennae react only by muscular activation. We suggest that the principles we have discovered for insect antennae apply to other appendages that share a leg-derived ancestry. Our work offers promising new applications for multifunctional fiber-based microfluidics that could transport fluids and be manipulated by the same fluid on demand. STATEMENT OF SIGNIFICANCE: Insect antennae are blood-filled, segmented fibers with muscles in the two basal segments. The long terminal segment is muscle-free but can be flexed. To explain this behavior, we examined structure-function relationships of antennae of cockroaches, hawkmoths, and butterflies. Hawkmoth antennae behaved as brittle fibers, but butterfly and cockroach antennae showed strain-adaptive behavior like fibers that stiffen when stretched. Videomicroscopy of antennal breakup during stretching revealed complex coupling of blood pressure and cuticle deformation. Our solid mechanics model explains this behavior. Because antennae are leg-derived appendages, we suggest that the principles we found apply to other appendages of leg-derived ancestry. Our work offers new applications for multifunctional fiber-based microfluidics that could transport fluids and be manipulated by the fluid on demand.

Effect of sample treatment on the elastic modulus of locust cuticle obtained by nanoindentation

Cuticle is one of the most abundant, but least studied, biological composites. As a result, it has contributed very little to the field of biomimetics. An important step to overcome this problem is to study cuticle biomechanics by means of accurate mechanical measurements. However, due to many reasons, mechanical testing on fresh cuticle specimens is not always possible. Hence, researchers often use stored specimens to measure properties of arthropod cuticle. Our knowledge about the influence of different treatment methods on cuticle properties is currently very limited. In this study, we investigated the effect of freezing, desiccation, and rehydration on the elastic modulus of the hind tibial cuticle of locusts obtained by nanoindentation. We found that all the mentioned treatments significantly influence cuticle properties. This is in contrast to previous reports suggesting that freezing did not significantly influence the elastic modulus of native cuticle specimens tested in bending. In the light of our data, we suggest that changes of the elastic modulus of cuticle are not solely due to changes of the water content. Our results provide a platform for more accurate measurements of cuticle properties.

The biomechanics of the locust ovipositor valves: a unique digging apparatus

The female locust has a unique mechanism for digging in order to deposit its eggs deep in the ground. It uses two pairs of sclerotized valves to displace the granular matter, while extending its abdomen as it propagates underground. This ensures optimal conditions for the eggs to incubate and provides them with protection from predators. Here, the direction-dependent biomechanics of the locust's major, dorsal digging valves are quantified and analysed under forces in the physiological range and beyond, considering the hydration level as well as the females' sexual maturation state. Our findings reveal that the responses of the valves to compression forces in the digging and propagation directions change upon sexual maturation to follow their function and depend on environmental conditions. In addition, mature females, which lay eggs, have stiffer valves, up to approximately 19 times the stiffness of the pre-mature locusts. The valves are stiffer in the major working direction, corresponding to soil shuffling and compression, compared with the direction of propagation. Hydration of the valves reduces their stiffness but increases their resilience against failure. These findings provide mechanical and materials guidelines for the design of novel non-drilling burrowing tools, including three-dimensionally printed anisotropic materials based on composites

Multiscale engineered artificial tooth enamel

Tooth enamel, renowned for its high stiffness, hardness, and viscoelasticity, is an ideal model for designing biomimetic materials, but accurate replication of complex hierarchical organization of high-performance biomaterials in scalable abiological composites is challenging. We engineered an enamel analog with the essential hierarchical structure at multiple scales through assembly of amorphous intergranular phase (AIP)-coated hydroxyapatite nanowires intertwined with polyvinyl alcohol. The nanocomposite simultaneously exhibited high stiffness, hardness, strength, viscoelasticity, and toughness, exceeding the properties of enamel and previously manufactured bulk enamel-inspired materials. The presence of AIP, polymer confinement, and strong interfacial adhesion are all needed for high mechanical performance. This multiscale design is suitable for scalable production of high-performance materials.

Improving Polysaccharide-Based Chitin/Chitosan-Aerogel Materials by Learning from Genetics and Molecular Biology

Improved wound healing of burnt skin and skin lesions, as well as medical implants and replacement products, requires the support of synthetical matrices. Yet, producing synthetic biocompatible matrices that exhibit specialized flexibility, stability, and biodegradability is challenging. Synthetic chitin/chitosan matrices may provide the desired advantages for producing specialized grafts but must be modified to improve their properties. Synthetic chitin/chitosan hydrogel and aerogel techniques provide the advantages for improvement with a bioinspired view adapted from the natural molecular toolbox. To this end, animal genetics provide deep knowledge into which molecular key factors decisively influence the properties of natural chitin matrices. The genetically identified proteins and enzymes control chitin matrix assembly, architecture, and degradation. Combining synthetic chitin matrices with critical biological factors may point to the future direction with engineering materials of specific properties for biomedical applications such as burned skin or skin blistering and extensive lesions due to genetic diseases.

Semi-automated differentiation of insect exo- and endocuticle in X-ray microtomography

One of the most versatile and complex biological composite materials, the insect exoskeleton shows a huge range in biomechanical properties. The cuticle exoskeleton can be differentiated into two main histologically different layers with distinct properties: the outer, more sclerotized exocuticle and inner, softer endocuticle. For most biomechanical research questions, it is of great importance to be able to selectively characterize geometrical features of these layers. However, most conventional preparation methods (cross-sections, histological staining, SEM) require complex and destructive sample preparation, which provides only two-dimensional information. Here, we present a novel, simple staining method using X-ray microtomography to distinguish between exo- and endocuticle in a 3D environment without sample destruction. We illustrate the power of our method using locust (Locusta migratoria) hindleg tibia, a well characterized biomechanical sample. Our method allows an easy and direct measurement of exo- and endocuticle and their respective geometric features. Applying our method will help to understand the biomechanical role of exo- and endocuticle within an insect exoskeleton and will allow us to understand its composition and morphological features in more detail.

Collective effect of damage prevention in taenioglossan radular teeth is related to the ecological niche in Paludomidae (Gastropoda: Cerithioidea)

The molluscan radula, a thin membrane with embedded rows of teeth, is the structure for food processing and gathering. For proper functioning, radular failures must be either avoided or reduced when interacting with the preferred food, as this might be of high significance for the individual fitness. Thus, the analysis of structural failure in radular teeth could be included in studies on trophic specializations. Here, we tested the failure of non-mineralized, chitinous radular teeth from taxa, belonging to an African paludomid species flock from Lake Tanganyika and surrounding river systems. These species are of high interest for evolutionary biologists since they represent a potential result of an adaptive radiation including trophic specialisations to distinct substrates, the food is attached to. In a biomechanical experiment a shear load was applied to tooth cusps with a force transducer connected to a motorized stage until structural failure occurred. Subsequently broken areas were measured and breaking stress was calculated. As the experiments were carried out under dry and wet conditions, the high influence of the water content on the forces, teeth were capable to resist, could be documented. Wet teeth were able to resist higher forces, because of their increased flexibility and the flexibility of the embedding membrane, which enabled them either to slip away or to gain support from adjacent teeth. This mechanism can be understood as collective effect reducing structural failure without the mineralisation with wear-minimizing elements, as described for Polyplacophora and Patellogastropoda. Since the documented mechanical behaviour of radular teeth and the maximal forces, teeth resist, can directly be related to the gastropod ecological niche, both are here identified as an adaptation to preferred feeding substrates. STATEMENT OF SIGNIFICANCE: The radula, a chitinous membrane with teeth, is the molluscan feeding structure. Here we add onto existing knowledge about the relationship between tooth's mechanical properties and species' ecology by determining the tooth failure resistance. Six paludomid species (Gastropoda) of a prominent species flock from Lake Tanganyika, foraging on distinct feeding substrates, were tested. With a force transducer wet and dry teeth were broken, revealing the high influence of water content on mechanical behaviour and force resistance of teeth. Higher forces were needed to break wet radulae due to an increased flexibility of teeth and membrane, which resulted in an interlocking or twisting of teeth. Mechanical behaviour and force resistance were both identified as trophic adaptations to feeding substrate.

The damping and structural properties of dragonfly and damselfly wings during dynamic movement

For flying insects, stability is essential to maintain the orientation and direction of motion in flight. Flight instability is caused by a variety of factors, such as intended abrupt flight manoeuvres and unwanted environmental disturbances. Although wings play a key role in insect flight stability, little is known about their oscillatory behaviour. Here we present the first systematic study of insect wing damping. We show that different wing regions have almost identical damping properties. The mean damping ratio of fresh wings is noticeably higher than that previously thought. Flight muscles and hemolymph have almost no 'direct' influence on the wing damping. In contrast, the involvement of the wing hinge can significantly increase damping. We also show that although desiccation reduces the wing damping ratio, rehydration leads to full recovery of damping properties after desiccation. Hence, we expect hemolymph to influence the wing damping indirectly, by continuously hydrating the wing system.

Reducing the risk of rostral bending failure in Curculio Linnaeus, 1758

With over 300 species worldwide, the genus Curculio Linnaeus, 1758 is a widespread, morphologically diverse lineage of weevils that mainly parasitize nuts. Females use the rostrum, an elongate cuticular extension of the head, to excavate oviposition sites. This process causes extreme bending and deformation of the rostrum, without apparent harm to the structure. The cuticle of the rostral apex exhibits substantial modifications to its composite structure that enhance the elasticity and resiliency of this structure. Here we develop finite element models of the head and rostrum for three Curculio species representing disparate North American clades and rostral morphotypes. The models were subjected to varying apical loads and to prescribed dislocation of the head capsule, with and without representing the cuticular modifications of the rostral apex. We found that the altered layer thicknesses and macrofiber orientation angles of the rostral apex fully explain the observed elasticity of the rostrum. These modifications have a synergistic effect that greatly enhances the flexibility of the rostral apex. Consequently, the cuticle composite profile of the rostral apex substantially mitigates the risk of fracture in dorso-apical flexion. Removing the cuticular modifications, in turn, causes a negative margin of safety for rostral bending, implying strong risk of catastrophic structural failure. The occipital sulci were identified as an important source of biomechanical constraint upon the elasticity of the rostrum, and exhibit the greatest risk of failure within this structure. The apical cuticle profile greatly reduced the maximum stresses and strain energy accumulated in the rostrum, thereby resulting in a positive margin of safety and reducing the risk of fracture. Our findings imply that the primary selective pressure influencing the evolution of the rostral cuticle was most likely negative selection of structural failure caused by bending. STATEMENT OF SIGNIFICANCE: Weevils are among the most diverse and evolutionarily successful animal lineages on Earth. Their success is driven in part by a structure called the rostrum, which gives weevil heads a characteristic "snout-like" appearance. Nut weevils in the genus Curculio use the rostrum to drill holes into developing fruits and nuts, into which they deposit their eggs. During oviposition this exceedingly slender structure is bent into a straightened configuration - in some species up to 90^∘ - but does not suffer any damage during this process. Using finite element models of the rostra of three morphologically distinct species, we show that the Curculio rostrum is only able to withstand repeated, extreme bending because of modifications to the composite structure of the cuticle in the rostral apex. These modifications were shown previously to enhance the intrinsic toughness of the cuticle; in this study, we demonstrate that modification of the rostral cuticle also results in more evenly distributed bending stresses, further reducing the risk of fracture. This is the first time that the laminate profile, orthotropic behavior, and functional gradation of the cuticle have been incorporated into a three-dimensional finite element model of an insect cuticular structure. Our models highlight the significance of biomechanical constraint - i.e., avoidance of catastrophic structural failure - on the evolution of insect morphology.

Influence of water content on mechanical behaviour of gastropod taenioglossan radulae

One molluscan autapomorphy is the radula, the organ used for feeding. Here, for the first time, the performance and failure of taenioglossan radular teeth were tested in a biomechanical experiment which in turn allowed building hypotheses about tooth functionalities. Shear load was applied to tooth cusps with a force transducer until structural failure occurred, the broken area was measured, and finally breaking stress was calculated. These experiments were carried out under dry and wet conditions. Our results show that certain tooth types can resist higher stresses and are rather specialised to loosen food items from a surface, whereas other teeth can only gather food particles. The experiments additionally illustrate the high influence of the water content on the resulting breaking stress. When wet teeth were tested, their ductility and ability to avoid being fractured by an obstacle increased. Their flexibility also allowed them support from teeth of adjacent tooth rows, which made the whole system less prone to failure. Our results were compared with the previous data on the mechanical properties and feeding simulations. This study provides a keystone for further comparative studies aiming at connecting diversity of radulae with their possible adaptations to the ingesta.

Structural analysis across length scales of the scorpion pincer cuticle

Biological structures such as bone, nacre and exoskeletons are organized hierarchically, with the degree of isotropy correlating with the length-scale. In these structures, the basic components are nanofibers or nanoplatelets, which are strong and stiff but anisotropic, whereas at the macrolevel, isotropy is preferred because the direction and magnitude of loads is unpredictable. The structural features and mechanisms, which drive the transition from anisotropy to isotropy across length scales, raise fundamental questions and are therefore the subject of the current study. Focusing on the tibia (fixed finger) of the scorpion pincer, bending tests of cuticle samples confirm the macroscale isotropy of the strength, stiffness, and toughness. Imaging analysis of the cuticle reveals an intricate multilayer laminated structure, with varying chitin-protein fiber orientations, arranged in eight hierarchical levels. We show that the cuticle flexural stiffness is increased by the existence of a thick intermediate layer, not seen before in the claws of crustaceans. Using laminate analysis to model the cuticle structure, we were able to correlate the nanostructure to the macro-mechanical properties, uncovering shear enhancing mechanisms at different length scales. These mechanisms, together with the hierarchical structure, are essential for achieving macro-scale isotropy. Interlaminar failure (ILF) analysis of the cuticle leads to an estimation of the protein matrix shear strength, previously not measured. A similar structural approach can be adopted to the design of future synthetic composites with balanced strength, stiffness, toughness, and isotropy.

Circulation in Insect Wings

Insect wings are living, flexible structures composed of tubular veins and thin wing membrane. Wing veins can contain hemolymph (insect blood), tracheae, and nerves. Continuous flow of hemolymph within insect wings ensures that sensory hairs, structural elements such as resilin, and other living tissue within the wings remain functional. While it is well known that hemolymph circulates through insect wings, the extent of wing circulation (e.g., whether flow is present in every vein, and whether it is confined to the veins alone) is not well understood, especially for wings with complex wing venation. Over the last 100 years, scientists have developed experimental methods including microscopy, fluorescence, and thermography to observe flow in the wings. Recognizing and evaluating the importance of hemolymph movement in insect wings is critical in evaluating how the wings function both as flight appendages, as active sensors, and as thermoregulatory organs. In this review, we discuss the history of circulation in wings, past and present experimental techniques for measuring hemolymph, and broad implications for the field of hemodynamics in insect wings.

Time-scale mechanical behaviors of locust semi-lunar process cuticles under power amplification for rapid movements

The semi-lunar process (SLP) is a key component in the power amplification of locusts to achieve rapid movements. Its mechanical properties determine the amount of the power amplification and the subsequent locomotion performance. As previously reported, the SLP cuticle endures physiological dynamic loadings. However, the time-scale mechanical properties of the SLP are still unknown, especially under stress relaxation and cyclic loadings. In this paper, the SLP cuticles of adult desert locusts (Schistocerca gregaria) were studied using stress relaxation and cyclic tests, with loadings corresponding to the physiological loading conditions of the power amplification. The SLP cuticle was found to show pronounced stress relaxation behavior with the resultant force and an evident time shift between the maximal displacement and the maximal resultant force. The number of loading cycles before mechanical failure (life cycle number) increases when the SLP cuticle is cyclically loaded by a lower stress level. Moreover, the failure strength of the SLP at low cycles equals the physiological stress level in the power amplification, implying that the healing of the cuticle might contribute to the successful performance of numerous jumps in the course of the adult locust life. This study not only deepens our understanding of the power amplification mechanism of locust locomotion but also provides valuable knowledge for the design optimization of bioinspired jumping robots and elastic energy storage devices.

SAUV-A Bio-Inspired Soft-Robotic Autonomous Underwater Vehicle

Autonomous and remotely operated underwater vehicles allow us to reach places which have previously been inaccessible and perform complex repair, exploration and analysis tasks. As their navigation is not infallible, they may cause severe damage to themselves and their often fragile surroundings, such as flooded caves, coral reefs, or even accompanying divers in case of a collision. In this study, we used a shallow neural network, consisting of interlinking PID controllers, and trained by a genetic algorithm, to control a biologically inspired AUV with a soft and compliant exoskeleton. Such a compliant structure is a versatile and passive solution which reduces the accelerations induced by collisions to 56% of the original mean value acting upon the system, thus, notably reducing the stress on its components and resulting reaction forces on its surroundings. The segmented structure of this spherical exoskeleton protects the encased system without limiting the use of cameras, sensors or manipulators.

Cuticle sclerotization determines the difference between the elastic moduli of locust tibiae

A striking characteristic of insect cuticle is the wide range of its material property values, with respect to stiffness, strength and toughness. The elastic modulus of cuticle, for instance, ranges over seven orders of magnitude in different structures and different species. Previous studies suggested that this characteristic is influenced by the microstructure and sclerotization of cuticle. However, the relative role of the two factors in determining the material properties of cuticle is unknown. Here we used a combination of scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM) and nanoindentation, to investigate the effect of microstructure and sclerotization on the elastic modulus of tibiae of desert locusts. Our results showed that tibial cuticle is an anisotropic material with the highest elastic modulus along the tibial axis. This is likely because majority of the fibers in the cuticle are oriented along this axis. We also found that the hind tibia has a significantly higher elastic modulus, compared with the fore and mid tibiae. This is likely due to the higher sclerotization level of the hind tibia cuticle, and seems to be an adaptation to the locust locomotion by jumping, in which axial loads in the hind tibiae may reach several times the insect body weight. Our results suggest that while sclerotization determines the difference between the elastic moduli of the tibiae, anisotropic properties of each tibia is controlled by the specific fiber orientation. Our study provides one of only a few comprehensive investigations on insect cuticle, and helps to better understand the structure-material-function relationship in this complex biological composite. STATEMENT OF SIGNIFICANCE: Insect cuticle is a biological composite with strong anisotropy and wide ranges of material properties. Using an example of the tibial cuticle of desert locusts, we examined the role of two influential factors on the elastic modulus of cuticle: microstructure and sclerotization. Our results suggested the strong influence of sclerotization on the variation of the elastic modulus among fore, mid and hind tibiae, and that of the microstructure on the anisotropy of each tibia. Our results deepens the current understanding of the structure-material-function relationship in complex insect cuticle.

Repair of microdamage caused by cyclic loading in insect cuticle

It is well known that repeated loading cycles can reduce the strength of a material and cause eventual failure by the gradual build-up of damage. Previous work has shown that mammalian bone is able to extend its life almost indefinitely by continuously repairing microdamage, preventing the development of macroscopic cracks. However, no study has been conducted until now to investigate repair of microdamage in any other biological material. We applied cyclic bending loads to the hind tibiae of desert locusts (Schistocerca gregaria). We observed a significant decrease in the elastic stiffness (Young's modulus) of the cuticle during the five applied loading cycles, indicating that microdamage had been induced. The tibiae were then left to rest for various time periods: 1 hr, 24 hr, 1 week, and 4 weeks. When tested again after up to 24 hr, there was still a significant decrease in stiffness, showing that some microdamage remained. However, in the samples left for 1 week or 4 weeks before retesting, this decrease in stiffness had disappeared, indicating that the microdamage had been repaired. This is the first ever indication that insects are capable of repairing microdamage. It is a highly significant finding-insects such as locusts rely on the stiffness and strength of their hind legs for jumping. This study suggests that, within a time period of order of a few days, the insect can fully restore the mechanical function of an overloaded leg and thus return to normal activities.

A simple, high-resolution, non-destructive method for determining the spatial gradient of the elastic modulus of insect cuticle

Nature has evolved structures with high load-carrying capacity and long-term durability. The principles underlying the functionality of such structures, if studied systematically, can inspire the design of more efficient engineering systems. An important step in this process is to characterize the material properties of the structure under investigation. However, direct mechanical measurements on small complex-shaped biological samples involve numerous technical challenges. To overcome these challenges, we developed a method for estimation of the elastic modulus of insect cuticle, the second most abundant biological composite in nature, through simple light microscopy. In brief, we established a quantitative link between the autofluorescence of different constituent materials of insect cuticle, and the resulting mechanical properties. This approach was verified using data on cuticular structures of three different insect species. The method presented in this study allows three-dimensional visualisation of the elastic modulus, which is impossible with any other available technique. This is especially important for precise finite-element modelling of cuticle, which is known to have spatially graded properties. Considering the simplicity, ease of implementation and high-resolution of the results, our method is a crucial step towards a better understanding of material-function relationships in insect cuticle, and can potentially be adapted for other graded biological materials.

Avoidance of Catastrophic Structural Failure as an Evolutionary Constraint: Biomechanics of the Acorn Weevil Rostrum

The acorn weevil (Curculio Linnaeus, 1758) rostrum (snout) exhibits remarkable flexibility and toughness derived from the microarchitecture of its exoskeleton. Modifications to the composite profile of the rostral cuticle that simultaneously enhance the flexibility and toughness of the distal portion of the snout are characterized. Using classical laminate plate theory, the effect of these modifications on the elastic behavior of the exoskeleton is estimated. It is shown that the tensile behavior of the rostrum across six Curculio species with high morphological variation correlates with changes in the relative layer thicknesses and orientation angles of layers in the exoskeleton. Accordingly, increased endocuticle thickness is strongly correlated with increased tensile strength. Rostrum stiffness is shown to be inversely correlated with work of fracture; thus allowing a highly curved rostrum to completely straighten without structural damage. Finally, exocuticle rich invaginations of the occipital sutures are identified both as a likely site of crack initiation in tensile failure and as a source of morphological constraint on the evolution of the rostrum in Curculio weevils. It is concluded that avoidance of catastrophic structural failure, as initiated in these sutures under tension, is the driving selective pressure in the evolution of the female Curculio rostrum.

What goes up must come down: biomechanical impact analysis of falling locusts

Many insects are able to precisely control their jumping movements. Once in the air, the properties of the actual landing site, however, are almost impossible to predict. Falling insects thus have to cope with the situation at impact. In particular, for insects jumping to escape predators, a controlled landing movement appears to be a major evolutionary advantage. A quick recovery into an upright and stable body posture minimizes the time to prepare for the next escape jump. In this study, we used high-speed recordings to investigate the falling and in particular the impact behavior of Schistocerca gregaria locusts, a common model organism for studies on the biomechanics of jumping. Detailed impact analyses of free-falling locusts show that most insects typically crashed onto the substrate. Although free-falling locusts tended to spread their legs, they mostly fell onto the head and thorax first. The presence of wings did not significantly reduce impact speed; however, it did affect the orientation of the body at impact and significantly reduced the time to recover. Our results also show that alive warm locusts fell significantly faster than inactive or dead locusts. This indicates a possible tradeoff between active control versus reduced speed. Interestingly, alive insects also tended to perform a characteristic bending movement of the body at impact. This biomechanical adaptation might reduce the rebound and shorten the time to recover. The adhesive pads also play an important role in reducing the time to recover by allowing the insect to anchor itself to the substrate.

Time-lapse three-dimensional imaging of crack propagation in beetle cuticle

Arthropod cuticle has extraordinary properties. It is very stiff and tough whilst being lightweight, yet it is made of rather ordinary constituents. This desirable combination of properties results from a hierarchical structure, but we currently have a poor understanding of how this impedes damage propagation. Here we use non-destructive, time-lapse in situ tensile testing within an X-ray nanotomography (nCT) system to visualise crack progression through dry beetle elytron (wing case) cuticle in 3D. We find that its hierarchical pseudo-orthogonal laminated microstructure exploits many extrinsic toughening mechanisms, including crack deflection, fibre and laminate pull-out and crack bridging. We highlight lessons to be learned in the design of engineering structures from the toughening methods employed. STATEMENT OF SIGNIFICANCE: We present the first comprehensive study of the damage and toughening mechanisms within arthropod cuticle in a 3D time-lapse manner, using X-ray nanotomography during crack growth. This technique allows lamina to be isolated despite being convex, which limits 2D analysis of microstructure. We report toughening mechanisms previously unobserved in unmineralised cuticle such as crack deflection, fibre and laminate pull-out and crack bridging; and provide insights into the effects of hierarchical microstructure on crack propagation. Ultimately the benefits of the hierarchical microstructure found here can not only be used to improve biomimetic design, but also helps us to understand the remarkable success of arthropods on Earth.

Age-related responses to injury and repair in insect cuticle

We evaluated the ability of female adult desert locusts (Schistocerca gregaria) to repair injuries to their exoskeletons and restore mechanical strength over the course of their natural life. We discovered that younger insects are more capable of repairing injuries, displaying no significant decreases in failure strength, stiffness or bending moment to failure after 3 weeks of repair. Older insects, in contrast, were only capable of repairing to ∼70% of their original strength. Both older and younger insects carry out targeted deposition to repair injuries. We also examined different mechanisms of failure, and we discovered that the cuticle of older insects is more susceptible to crack growth due to a large decrease in fracture toughness with age, making them more sensitive to scalpel cuts and punctures. The biological mechanisms that drive these changes are still under investigation.

Biomechanical Strategies Underlying the Robust Body Armour of an Aposematic Weevil

Robust body armor is one of many anti-predator strategies used among animal taxa. The exoskeleton of insects can serve as the secondary defense mechanism in combination with the primary defense such as warning color. Aposematic Pachyrhynchus weevils advertise their unprofitability and use their robust exoskeleton for effective defense against lizard predators. While the mature weevils survive after the predatory attack, the soft teneral ones can easily be consumed. To reveal how the mature weevils achieve such effective protection, we investigated the ontogenetic changes in the microstructure and material properties of the exoskeleton of the adult weevils. We also tested the functional role of a weevil-specific structure, the fibrous ridge, in the robustness of the elytral cuticle of the mature weevils. The results showed that the mature weevils have thicker, stiffer and more sclerotized cuticle than the teneral ones. The fibrous ridges in the endocuticle considerably increase the overall stiffness of their cuticle. Together these biomechanical strategies enable Pachyrhynchus weevils to achieve robust body armor that efficiently protects them from lizard predation.

Biomechanical Evaluation of Wasp and Honeybee Stingers

In order to design a painless and mechanically durable micro syringe-needle system for biomedical applications, the study of insect stingers is of interest because of their elegant structures and functionalities. In the present work, the structure, mechanical properties and the mechanical behavior during insertion of wasp and honeybee stingers have been investigated. The non-invasive imaging tool, micro-computed tomography has been employed to reveal the 3D-structures of wasp and honeybee stingers. A quasi-static nanoindentation instrument was used to measure the nanomechanical properties. Both wasp and honeybee stingers have graded mechanical properties, decreasing along their longitudinal direction starting from the base. The computed tomography images and the measured material properties from nanoindentation were fed into a computational framework to determine the mechanical behavior of the stingers during penetration. The computation results predicted the penetration angle of +10° for the wasp stinger and −6° for the honeybee stinger, which mimics the practical insertion mechanism of both stingers. Based on this understanding, a wasp and honeybee stringer inspired micro syringe-needle design has also been proposed.

Both stiff and compliant: morphological and biomechanical adaptations of stick insect antennae for tactile exploration

Active tactile exploration behaviour is constrained to a large extent by the morphological and biomechanical properties of the animal's somatosensory system. In the model organism Carausius morosus, the main tactile sensory organs are long, thin, seemingly delicate, but very robust antennae. Previous studies have shown that these antennae are compliant under contact, yet stiff enough to maintain a straight shape during active exploration. Overcritical damping of the flagellum, on the other hand, allows for a rapid return to the straight shape after release of contact. Which roles do the morphological and biomechanical adaptations of the flagellum play in determining these special mechanical properties? To investigate this question, we used a combination of biomechanical experiments and numerical modelling. A set of four finite-element (FE) model variants was derived to investigate the effect of the distinct geometrical and material properties of the flagellum on its static (bending) and dynamic (damping) characteristics. The results of our numerical simulations show that the tapered shape of the flagellum had the strongest influence on its static biomechanical behaviour. The annulated structure and thickness gradient affected the deformability of the flagellum to a lesser degree. The inner endocuticle layer of the flagellum was confirmed to be essential for explaining the strongly damped return behaviour of the antenna. By highlighting the significance of two out of the four main structural features of the insect flagellum, our study provides a basis for mechanical design of biomimetic touch sensors tuned to become maximally flexible while quickly resuming a straight shape after contact.

Review on nanoparticles and nanostructured materials: history, sources, toxicity and regulations

Nanomaterials (NMs) have gained prominence in technological advancements due to their tunable physical, chemical and biological properties with enhanced performance over their bulk counterparts. NMs are categorized depending on their size, composition, shape, and origin. The ability to predict the unique properties of NMs increases the value of each classification. Due to increased growth of production of NMs and their industrial applications, issues relating to toxicity are inevitable. The aim of this review is to compare synthetic (engineered) and naturally occurring nanoparticles (NPs) and nanostructured materials (NSMs) to identify their nanoscale properties and to define the specific knowledge gaps related to the risk assessment of NPs and NSMs in the environment. The review presents an overview of the history and classifications of NMs and gives an overview of the various sources of NPs and NSMs, from natural to synthetic, and their toxic effects towards mammalian cells and tissue. Additionally, the types of toxic reactions associated with NPs and NSMs and the regulations implemented by different countries to reduce the associated risks are also discussed.

Stiffness distribution in insect cuticle: a continuous or a discontinuous profile?

Insect cuticle is a biological composite with a high degree of complexity in terms of both architecture and material composition. Given the complex morphology of many insect body parts, finite-element (FE) models play an important role in the analysis and interpretation of biomechanical measurements, taken by either macroscopic or nanoscopic techniques. Many previous studies show that the interpretation of nanoindentation measurements of this layered composite material is very challenging. To develop accurate FE models, it is of particular interest to understand more about the variations in the stiffness through the thickness of the cuticle. Considering the difficulties of making direct measurements, in this study, we use the FE method to analyse previously published data and address this issue numerically. For this purpose, sets of continuous or discontinuous stiffness profiles through the thickness of the cuticle were mathematically described. The obtained profiles were assigned to models developed based on the cuticle of three insect species with different geometries and layer configurations. The models were then used to simulate the mechanical behaviour of insect cuticles subjected to nanoindentation experiments. Our results show that FE models with discontinuous exponential stiffness gradients along their thickness were able to predict the stress and deformation states in insect cuticle very well. Our results further suggest that, for more accurate measurements and interpretation of nanoindentation test data, the ratio of the indentation depth to cuticle thickness should be limited to 7% rather than the traditional '10% rule'. The results of this study thus might be useful to provide a deeper insight into the biomechanical consequences of the distinct material distribution in insect cuticle and also to form a basis for more realistic modelling of this complex natural composite.

How does a slender tibia resist buckling? Effect of material, structural and geometric characteristics on buckling behaviour of the hindleg tibia in stick insect postembryonic development

During the lifespan of the stick insect Carausius morosus, their long and narrow tibiae experience substantial compressive loads. The mechanical load on the tibiae increases as the weight of the insect rises. The increase in body weight is accompanied by a notable increase in the insect's body size and, accordingly, by an increase in the length of the tibiae. Both of these changes can raise the risk of buckling of the tibiae. In this study, we tracked changes in the material and geometric properties of the hindleg tibia of C. morosus during growth. The results show that although buckling (either by Euler buckling or local buckling) is the dominant failure mode under compression, the tibia is very capable of maintaining its buckling resistance in each postembryonic developmental stage. This is essentially the result of a compromise between the increasing slenderness of the tibia and its increasing material stiffness. The use of an optimal radius to thickness ratio, a soft resilin-dominated core, and chitin fibres oriented in both longitudinal and circumferential directions are presumably additional strategies preventing buckling of the tibia. This study, providing the first quantitative data on changes in the biomechanical properties of cuticle during the entire life of an insect, is expected to shed more light on the structure-property-function relationship in this complex biological composite.

Dragonfly wing nodus: A one-way hinge contributing to the asymmetric wing deformation

Dragonfly wings are highly specialized locomotor systems, which are formed by a combination of several structural components. The wing components, also known as structural elements, are responsible for the various aspects of the wing functionality. Considering the complex interactions between the wing components, modelling of the wings as a whole is only possible with inevitable huge oversimplifications. In order to overcome this difficulty, we have recently proposed a new approach to model individual components of complex wings comparatively. Here, we use this approach to study nodus, a structural element of dragonfly wings which has been less studied to date. Using a combination of several imaging techniques including scanning electron microscopy (SEM), wide-field fluorescence microscopy (WFM), confocal laser scanning microscopy (CLSM) and micro-computed tomography (micro-CT) scanning, we aim to characterize the spatial morphology and material composition of fore- and hindwing nodi of the dragonfly Brachythemis contaminata. The microscopy results show the presence of resilin in the nodi, which is expected to help the deformability of the wings. The computational results based on three-dimensional (3D) structural data suggest that the specific geometry of the nodus restrains its displacements when subjected to pressure on the ventral side. This effect, resulting from an interlocking mechanism, is expected to contribute to the dorso-ventral asymmetry of wing deformation and to provide a higher resistance to aerodynamic forces during the downstroke. Our results provide an important step towards better understanding of the structure-property-function relationship in dragonfly wings.

STATEMENT OF SIGNIFICANCE

In this study, we investigate the wing nodus, a specialized wing component in dragonflies. Using a combination of modern imaging techniques, we demonstrate the presence of resilin in the nodus, which is expected to facilitate the wing deformability in flight. The specific geometry of the nodus, however, seems to restrain its displacements when subjected to pressure on the ventral side. This effect, resulting from an interlocking mechanism, is suggested to contribute to dorso-ventral asymmetry of wing deformations and to provide a higher resistance to aerodynamic forces during the downstroke. Our results provide an important step towards better understanding of the structure-property-function relationship in dragonfly wings and might help to design more efficient wings for biomimetic micro-air vehicles.

On the relationship between indentation hardness and modulus, and the damage resistance of biological materials

The remarkable mechanical performance of biological materials is based on intricate structure-function relationships. Nanoindentation has become the primary tool for characterising biological materials, as it allows to relate structural changes to variations in mechanical properties on small scales. However, the respective theoretical background and associated interpretation of the parameters measured via indentation derives largely from research on 'traditional' engineering materials such as metals or ceramics. Here, we discuss the functional relevance of indentation hardness in biological materials by presenting a meta-analysis of its relationship with indentation modulus. Across seven orders of magnitude, indentation hardness was directly proportional to indentation modulus. Using a lumped parameter model to deconvolute indentation hardness into components arising from reversible and irreversible deformation, we establish criteria which allow to interpret differences in indentation hardness across or within biological materials. The ratio between hardness and modulus arises as a key parameter, which is related to the ratio between irreversible and reversible deformation during indentation, the material's yield strength, and the resistance to irreversible deformation, a material property which represents the energy required to create a unit volume of purely irreversible deformation. Indentation hardness generally increases upon material dehydration, however to a larger extent than expected from accompanying changes in indentation modulus, indicating that water acts as a 'plasticiser'. A detailed discussion of the role of indentation hardness, modulus and toughness in damage control during sharp or blunt indentation yields comprehensive guidelines for a performance-based ranking of biological materials, and suggests that quasi-plastic deformation is a frequent yet poorly understood damage mode, highlighting an important area of future research.

STATEMENT OF SIGNIFICANCE

Instrumented indentation is a widespread tool for characterising the mechanical properties of biological materials. Here, we show that the ratio between indentation hardness and modulus is approximately constant in biological materials. A simple elastic-plastic series deformation model is employed to rationalise part of this correlation, and criteria for a meaningful comparison of indentation hardness across biological materials are proposed. The ratio between indentation hardness and modulus emerges as the key parameter characterising the relative amount of irreversible deformation during indentation. Despite their comparatively high hardness to modulus ratio, biological materials are susceptible to quasiplastic deformation, due to their high toughness: quasi-plastic deformation is hence hypothesised to be a frequent yet poorly understood phenomenon, highlighting an important area of future research.

Site-specific characterization of beetle horn shell with micromechanical bending test in focused ion beam system

Biological materials are the result of years of evolution and possess a number of efficient features and structures. Researchers have investigated the possibility of designing biomedical structures that take advantage of these structural features. Insect shells, such as beetle shells, are among the most promising types of biological material for biomimetic development. However, due to their intricate geometries and small sizes, it is challenging to measure the mechanical properties of these microscale structures. In this study, we developed an in-situ testing platform for site-specific experiments in a focused ion beam (FIB) system. Multi-axis nano-manipulators and a micro-force sensor were utilized in the testing platform to allow better results in the sample preparation and data acquisition. The entire test protocol, consisting of locating sample, ion beam milling and micro-mechanical bending tests, can be carried out without sample transfer or reattachment. We used our newly devised test platform to evaluate the micromechanical properties and structural features of each separated layer of the beetle horn shell. The Young's modulus of both the exocuticle and endocuticle layers was measured. We carried out a bending test to characterize the layers mechanically. The exocuticle layer bent in a brick-like manner, while the endocuticle layer exhibited a crack blunting effect.

STATEMENT OF SIGNIFICANCE

This paper proposed an in-situ manipulation/test method in focused ion beam for characterizing micromechanical properties of beetle horn shell. The challenge in precise and accurate fabrication for the samples with complex geometry was overcome by using nano-manipulators having multi-degree of freedom and a micro-gripper. With the aid of this specially designed test platform, bending tests were carried out on cantilever-shaped samples prepared by focused ion beam milling. Structural differences between exocuticle and endocuticle layers of beetle horn shell were explored and the results provided insight into the structural advantages of each biocomposite structure.

The effect of aging on the mechanical behaviour of cuticle in the locust Schistocerca gregaria

Despite some previous work on the morphology and mechanical properties of parts of the insect exoskeleton, there is very little known about how these properties change over time during the life of the insect. We examined the hind tibia of the adult desert locust (Schistocerca gregaria) as a function of time up to 63 days following the final moult, a much longer period that previously studied. We identified an initial growth phase, lasting on average 21 days, in which leg thickness increased rapidly (averaging 1.8μm/day) by endocuticle deposition, and a subsequent mature phase in which the deposition rate slowed to 0.3μm/day. Cantilever bending tests revealed that Young's modulus and failure stress also increased rapidly during the growth phase, but remained almost constant during the mature phase, with average values of 8.3GPa (± 2.3GPa) and 175MPa (±31.5MPa) respectively, which are considerably higher than previously measured for fresh insect cuticle. Biomechanical analysis showed that the failure mode also changed, from local buckling of the tubular leg during the growth phase to failure at the material's ultimate strength in the mature phase. Over time, the ratio of radius/thickness of the leg decreased, passing through the estimated optimal value which would confer the best strength/weight ratio. This is the first ever biomechanical study to track changes in arthropod cuticle over a large part of adult life of the animal, and has revealed some unexpected and complex changes which may shed light on how arthropods regulate their load-bearing skeletal parts during aging.

Effect of sample treatment on biomechanical properties of insect cuticle

Experimental limitations often prevent to perform biomechanical measurements on fresh arthropod cuticle samples. Hence, in many cases short- or long-term storage of samples is required. So far, it is not known whether any of the standard lab-techniques commonly used to fix or store insect cuticle samples in any way affects the biomechanical properties of the respective samples. In this paper we systematically address this question for the first time, with a focus on practical, easily accessible and common lab-methods including storage in water, ethanol, glutaraldehyde, freezing and desiccation. We performed a comprehensive and sensitive non-destructive Dynamic Mechanical Analysis (DMA) on locust hind leg tibiae using a three-point-bending setup. Our results show that from all tested treatments, freezing samples at -20 °C was the best option to maintain the original values for Young's modulus and damping properties of insect cuticle. In addition, our results indicate that the damping properties of locust hind legs might be mechanically optimized in respect to the jumping and kicking direction.

Damage, repair and regeneration in insect cuticle: The story so far, and possibilities for the future

The exoskeleton of an insect can contain countless specializations across an individual, across developmental stages, and across the class Insecta. Hence, the exoskeleton's building material cuticle must perform a vast variety of functions. Cuticle displays a wide range of material properties which are determined by several known factors: the amount and orientation of the chitin fibres, the constituents and degree of cross-linking and hydration of the protein matrix, the relative amounts of exo- and endocuticle, and the shape of the structures themselves. In comparison to other natural materials such as wood and mammal bone, relatively few investigations into the mechanical properties of insect cuticle have been carried out. Of these, very few have focussed on the need for repair and its effectiveness at restoring mechanical stability to the cuticle. Insect body parts are often subject to prolonged repeated cyclic loads when running and flying, as well as more extreme "emergency" behaviours necessary for survival such as jumping, wedging (squeezing through small holes) and righting (when overturned). What effects have these actions on the cuticle itself? How close to the limits of failure does an insect push its body parts? Can an insect recover from minor or major damage to its exoskeleton "bones"? No current research has answered these questions conclusively.

Resilin microjoints: a smart design strategy to avoid failure in dragonfly wings

Dragonflies are fast and manoeuvrable fliers and this ability is reflected in their unique wing morphology. Due to the specific lightweight structure, with the crossing veins joined by rubber-like resilin patches, wings possess strong deformability but can resist high forces and large deformations during aerial collisions. The computational results demonstrate the strong influence of resilin-containing vein joints on the stress distribution within the wing. The presence of flexible resilin in the contact region of the veins prevents excessive bending of the cross veins and significantly reduces the stress concentration in the joint.

Structures, properties, and energy-storage mechanisms of the semi-lunar process cuticles in locusts

Locusts have excellent jumping and kicking abilities to survive in nature, which are achieved through the energy storage and release processes occurring in cuticles, especially in the semi-lunar processes (SLP) at the femorotibial joints. As yet, however, the strain energy-storage mechanisms of the SLP cuticles remain unclear. To decode this mystery, we investigated the microstructure, material composition, and mechanical properties of the SLP cuticle and its remarkable strain energy-storage mechanisms for jumping and kicking. It is found that the SLP cuticle of adult Locusta migratoria manilensis consists of five main parts that exhibit different microstructural features, material compositions, mechanical properties, and biological functions in storing strain energy. The mechanical properties of these five components are all transversely isotropic and strongly depend on their water contents. Finite element simulations indicate that the two parts of the core region of the SLP cuticle likely make significant contributions to its outstanding strain energy-storage ability. This work deepens our understanding of the locomotion behaviors and superior energy-storage mechanisms of insects such as locusts and is helpful for the design and fabrication of strain energy-storage devices.

Basal Complex and Basal Venation of Odonata Wings: Structural Diversity and Potential Role in the Wing Deformation

Dragonflies and damselflies, belonging to the order Odonata, are known to be excellent fliers with versatile flight capabilities. The ability to fly over a wide range of speeds, high manoeuvrability and great agility are a few characteristics of their flight. The architecture of the wings and their structural elements have been found to play a major role in this regard. However, the precise influence of individual wing components on the flight performance of these insects remains unknown. The design of the wing basis (so called basal complex) and the venation of this part are responsible for particular deformability and specific shape of the wing blade. However, the wing bases are rather different in representatives of different odonate groups. This presumably reflects the dimensions of the wings on one hand, and different flight characteristics on the other hand. In this article, we develop the first three-dimensional (3D) finite element (FE) models of the proximal part of the wings of typical representatives of five dragonflies and damselflies families. Using a combination of the basic material properties of insect cuticle, a linear elastic material model and a nonlinear geometric analysis, we simulate the mechanical behaviour of the wing bases. The results reveal that although both the basal venation and the basal complex influence the structural stiffness of the wings, it is only the latter which significantly affects their deformation patterns. The use of numerical simulations enabled us to address the role of various wing components such as the arculus, discoidal cell and triangle on the camber formation in flight. Our study further provides a detailed representation of the stress concentration in the models. The numerical analysis presented in this study is not only of importance for understanding structure-function relationship of insect wings, but also might help to improve the design of the wings for biomimetic micro-air vehicles (MAVs).

Biomechanical Factors in the Adaptations of Insect Tibia Cuticle

Insects are among the most diverse groups of animals on Earth. Their cuticle exoskeletons vary greatly in terms of size and shape, and are subjected to different applied forces during daily activities. We investigated the biomechanics of the tibiae of three different insect species: the desert locust (Schistocerca gregaria), American cockroach (Periplaneta americana) and Death's Head cockroach (Blaberus discoidalis). In a previous work, we showed that these tibiae vary not only in geometry (length, radius and thickness) but also in material quality (Young's modulus) and in the applied stress required to cause failure when loaded in bending. In the present work we used kinematic data from the literature to estimate the forces and stresses arising in vivo for various different activities, and thus calculated factors of safety defined as the ratio between the failure stress and the in vivo stress, adjusting the failure stress to a lower value to allow for fatigue failure in the case of frequently repeated activities. Factors of safety were found to vary considerably, being as little as 1.7 for the most strenuous activities, such as jumping or escaping from tight spaces. Our results show that these limbs have evolved to the point where they are close to optimal, and that instantaneous failure during high-stress activities is more critical than long-term fatigue failure. This work contributes to the discussion on how form and material properties have evolved in response to the mechanical functions of the same body part in different insects.

Bridging the gap: wound healing in insects restores mechanical strength by targeted cuticle deposition

If an insect is injured, can it repair its skeleton in a manner which is mechanically strong and viable? Previous work has described the biological processes that occur during repair of insect cuticle, but until now, there has been no biomechanical assessment of the repaired area. We analysed the biomechanics of the injury repair process in the desert locust (Schistocerca gregaria). We show that after an incision, a healing process occurred which almost doubled the mechanical strength of locust tibial cuticle, restoring it to 66% of the original, intact strength. This repair process occurred by targeted cuticle deposition, stimulated by the presence of the injury. The cut surfaces remained unrepaired, but a patch of endocuticle was deposited, reinforcing the area and thus increasing the effective fracture toughness. The deposition rate of endocuticle inside the tibia increased fourfold compared with uninjured controls, but only on the dorsal side, where the incision was placed. The limb is highly loaded during jumping, so this partial restoration of strength will have a profound effect on the fitness of the insect. A finite-element model provided insights into the mechanics of the repair, predicting that the patch material reaches its ultimate strength before the fracture toughness of the existing cuticle is exceeded.

Effects of multiple vein microjoints on the mechanical behaviour of dragonfly wings: numerical modelling

Dragonfly wings are known as biological composites with high morphological complexity. They mainly consist of a network of rigid veins and flexible membranes, and enable insects to perform various flight manoeuvres. Although several studies have been done on the aerodynamic performance of Odonata wings and the mechanisms involved in their deformations, little is known about the influence of vein joints on the passive deformability of the wings in flight. In this article, we present the first three-dimensional finite-element models of five different vein joint combinations observed in Odonata wings. The results from the analysis of the models subjected to uniform pressures on their dorsal and ventral surfaces indicate the influence of spike-associated vein joints on the dorsoventral asymmetry of wing deformation. Our study also supports the idea that a single vein joint may result in different angular deformations when it is surrounded by different joint types. The developed numerical models also enabled us to simulate the camber formation and stress distribution in the models. The computational data further provide deeper insights into the functional role of resilin patches and spikes in vein joint structures. This study might help to more realistically model the complex structure of insect wings in order to design more efficient bioinspired micro-air vehicles in future.