Natural arrangement of micro-strips reduces shear strain in the locust cuticle during power amplification

Functional morphology and biomechanics of arthropods

Representatives of arthropods, the largest animal phylum, occupy terrestrial, aquatic, arboreal, and subterranean niches. Their evolutionary success depends on specific morphological and biomechanical adaptations related to their materials and structures. Biologists and engineers have become increasingly interested in exploring these natural solutions to understand relationships between structures, materials, and their functions in living organisms. The aim of this special issue is to present the state-of-the-art research in this interdisciplinary field using modern methodology, such as imaging techniques, mechanical testing, movement capture, and numerical modeling. It contains nine original research reports covering diverse topics, including flight, locomotion, and attachment of the arthropods. The research achievements are essential not only to understand ecological adaptations, and evolutionary and behavioral traits, but also to drive prominent advances for engineering from exploitation of numerous biomimetic ideas.

A Tunable, Simplified Model for Biological Latch Mediated Spring Actuated Systems

We develop a model of latch-mediated spring actuated (LaMSA) systems relevant to comparative biomechanics and bioinspired design. The model contains five components: two motors (muscles), a spring, a latch, and a load mass. One motor loads the spring to store elastic energy and the second motor subsequently removes the latch, which releases the spring and causes movement of the load mass. We develop freely available software to accompany the model, which provides an extensible framework for simulating LaMSA systems. Output from the simulation includes information from the loading and release phases of motion, which can be used to calculate kinematic performance metrics that are important for biomechanical function. In parallel, we simulate a comparable, directly actuated system that uses the same motor and mass combinations as the LaMSA simulations. By rapidly iterating through biologically relevant input parameters to the model, simulated kinematic performance differences between LaMSA and directly actuated systems can be used to explore the evolutionary dynamics of biological LaMSA systems and uncover design principles for bioinspired LaMSA systems. As proof of principle of this concept, we compare a LaMSA simulation to a directly actuated simulation that includes either a Hill-type force-velocity trade-off or muscle activation dynamics, or both. For the biologically-relevant range of parameters explored, we find that the muscle force-velocity trade-off and muscle activation have similar effects on directly actuated performance. Including both of these dynamic muscle properties increases the accelerated mass range where a LaMSA system outperforms a directly actuated one.

Hyphal systems and their effect on the mechanical properties of fungal sporocarps

Little is known about the mechanical and material properties of hyphae, the single constituent material of Agaricomycetes fungi, despite a growing interest in fungus-based materials. In the Agaricomycetes (the mushrooms and allies), there are three types of hyphae that make up sporocarps: generative, skeletal, and ligative. All filamentous Agaricomycetes can be categorized into one of three categories of hyphal systems that compose them: monomitic, dimitic, and trimitic. Monomitic systems have only generative hyphae. Dimitic systems have generative and either skeletal (most common) or ligative. Trimitic systems are composed of all three kinds of hyphae. SEM imaging, compression testing, and theoretical modeling were used to characterize the material and mechanical properties of representative monomitic, dimitic, and trimitic sporocarps. Compression testing revealed an increase in the compression modulus and compressive strength with the addition of more hyphal types (monomitic to dimitic and dimitic to trimitic). The mesostructure of the trimitic sporocarp was tested and modeled, suggesting that the difference in properties between the solid material and the microtubule mesostructure is a result of differences in structure and not material. Theoretical modeling was completed to estimate the mechanical properties of the individual types of hyphae and showed that skeletal hyphae make the largest contribution to mechanical properties of fungal sporocarps. Understanding the contributions of the different types of hyphae may help in the design and application of fungi-based or bioinspired materials. STATEMENT OF SIGNIFICANCE: This research studies the material and mechanical properties of fungal sporocarps and their hyphae, the single constituent material of Agaricomycetes fungi. Though some work has been done on fungal hyphae, this research studies hyphae in context of the three hyphal systems found in Agaricomycetes fungi and estimates the properties of the hyphal filaments, which has not been done previously. This characterization was performed by analyzing the structures and mechanical properties of fungal sporocarps and calculating the theoretical mechanical properties of their hyphae. This data and the resulting conclusions may lead to a better design and implementation process of fungi-based materials in various applications using the properties now known or calculated.

The melanized layer of Armillaria ostoyae rhizomorphs: Its protective role and functions

Armillaria ostoyae (Romagn.) Herink is a highly pathogenic fungus that uses exploratory, cordlike structures called rhizomorphs to seek out new sources of nutrition, posing a parasitic threat to natural stands of trees, orchards, and vineyards. Rhizomorphs are notoriously difficult to destroy, and this resilience is due in large part to a melanized layer that protects the rhizomorph. While this structure has been previously observed, its structural and chemical defenses are yet to be discerned. Research was conducted on both lab-cultured and wild-harvested rhizomorph samples. While both environments produce rhizomorphs, only the wild-harvested rhizomorphs produced the melanized layer, allowing for direct investigation of its structure and properties. Imaging, chemical analysis, mechanical testing, and finite element modeling were used to understand the defense mechanisms provided by the melanized layer. Imaging showed a porous outer layer in both types of rhizomorphs, though the pores were smaller in the harvested melanized layer. This melanized layer contained calcium, which provides chemical defense against both human and natural control methods, but was absent from cultured samples. Nanoindentation resulted in a larger variance of hardness values for cultured rhizomorphs than for wild-harvested. Finite element analysis proved that the smaller pore structure of the melanized porous layer had the best balance between maximum deformation and resulting permanent deformation. These results allow for a better understanding of the defenses of this pathogenic fungus, which may lead to better control methods.

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.