The homogenous alternative to biomineralization: Zn- and Mn-rich materials enable sharp organismal "tools" that reduce force requirements

Multicopper oxidase-2 mediated cuticle formation: Its contribution to evolution and success of insects as terrestrial organisms

The insect cuticle is a non-cellular matrix composed of polysaccharide chitins and proteins. The cuticle covers most of the body surface, including the trachea, foregut, and hindgut, and it is the body structure that separates the intraluminal environment from the external environment. The cuticle is essential to sustain their lives, both as a physical barrier to maintain homeostasis and as an exoskeleton that mechanically supports body shape and movement. Previously, we proposed a theory about the possibility that the cuticle-forming system contributes to the "evolution and success of insects." The main points of our theory are that 1) insects evolved an insect-specific system of cuticle formation and 2) the presence of this system may have provided insects with a competitive advantage in the early land ecosystems. The key to this theory is that insects utilize molecular oxygen abundant in the atmosphere, which differs from closely related crustaceans that form their cuticles with calcium ions. With newly obtained knowledge, this review revisits the significance of the insect-specific system for insects to adapt to terrestrial environments and also discusses the long-standing question in entomology as to why, despite their great success in terrestrial environments, they poorly adapt to marine environments.

The puncture mechanics: an example from the bed bug Cimex lectularius showing traumatic insemination using the paramere

Cimicidae are well-known for traumatic insemination, and males pierce females with their parameres and transfer sperm through them. The shape of parameres is relatively stable in the family, but in some genera, the paramere is elongated, appearing less resistant against lateral deflection. To understand the mechanical limitations of the paramere, we studied its penetration mechanics of the common bed bug, Cimex lectularius. We examined the post-abdominal morphology, paramere geometry and material properties and conducted breaking stress experiments on the paramere under wet and dry conditions. Mechanical property gradients are present with the paramere tip as the stiffest region and the base as the most flexible one. These mechanical properties relate to the presence of Ca, Zn and Si. The basal wing-shaped structure is flexible, enabling it to interlock with the anal region during mating. The paramere is slightly twisted; the tip region is circular in cross-section, and the geometry of the rest is rather complex. In the mechanical tests, wet parameres mainly buckled, while dried parameres broke off. The level of structural failures depended on directions from which the compression forces were applied. Structural, material and mechanical strengthening mechanisms preventing the paramere from mechanical failure are discussed.

Mechanical and elemental characterization of ant mandibles: consequences for bite mechanics

Mandible morphology has an essential role in biting performance, but the mandible cuticle can have regional differences in its mechanical properties. The effects of such a heterogeneous distribution of cuticle material properties in the mandible responses to biting loading are still poorly explored in chewing insects. Here, we tested the mechanical properties of mandibles of the ant species Formica cunicularia by nanoindentation and investigated the effects of the cuticular variation in Young's modulus (E) under bite loading with finite-element analysis (FEA). The masticatory margin of the mandible, which interacts with the food, was the hardest and stiffest region. To unravel the origins of the mechanical property gradients, we characterized the elemental composition by energy-dispersive X-ray spectroscopy. The masticatory margin possessed high proportions of Cu and Zn. When incorporated into the FEA, variation in E effectively changed mandible stress patterns, leading to a relatively higher concentration of stresses in the stiffer mandibular regions and leaving the softer mandible blade with relatively lower stress. Our results demonstrated the relevance of cuticle E heterogeneity in mandibles under bite loading, suggesting that the accumulation of transition metals such as Cu and Zn has a relevant correlation with the mechanical characteristics in F. cunicularia mandibles.

Mandible elemental composition and mechanical properties from distinct castes of the leafcutter ant Atta laevigata (Attini; Formicidae)

Leafcutter ant colonies are divided into castes with the individuals performing different tasks, based mostly on size. With the mandibles, the small minims care for the brood or the fungus, whereas the larger minors and mediae cut and transport plant material, with the ant size positively related to the material size. The mechanical properties and composition of the mandible cuticle have been previously tested in the soldiers as the largest caste, revealing that the cutting edges contained high contents of the cross-linking transition metal zinc (Zn). With regard to the smaller castes, no data are present. To study how the mandible size and function relates to its mechanical properties, we here tested the mandibles of minims, minors and mediae by nanoindentation. We found that the hardness (H) and Young's modulus (E) values increased with increasing ant size and that the mandible cutting edges in each caste have the highest H- and E-values. To gain insight into the origins of these properties, we characterized the elemental composition by energy-dispersive X-ray analysis, revealing that minors and mediae possessed higher content of Zn in the cutting edges in contrast to the minims containing significantly less Zn. This shows, that Zn content relates to higher mechanical property values. Additionally, it shows that all of these parameters can differ within a single species.

Wear patterns of radular teeth in Loligo vulgaris (Cephalopoda; Mollusca) are related to their structure and mechanical properties

Radular teeth have to cope with wear, when interacting with ingesta. In some molluscan taxa, wear-coping mechanisms, related to the incorporation of high contents of iron or silica, have been previously determined. For most species, particularly for those which possess radulae without such incorporations, wear-coping mechanisms are understudied. In the present study, we documented and characterized the wear on radular teeth in the model species Loligo vulgaris (Cephalopoda). By applying a range of methods, the elementary composition and mechanical properties of the teeth were described, to gain insight into mechanisms for coping with abrasion. It was found that the tooth regions that are prone to wear are harder and stiffer. Additionally, the surfaces interacting with the ingesta possessed a thin coating with high contents of silicon, probably reducing abrasion. The here presented data may serve as an example of systematic study of radular wear, in order to understand the relationship between the structure of radular teeth and their properties.

Desert locusts (Schistocerca gregaria) feed with self-sharpening, scissor-like mandibles

The mandibles of the desert locust Schistocerca gregaria (Forsskål, 1775) are digger-shovel-shaped mouthparts that are part of the locust's exoskeleton formed by the insect cuticle. The cuticle is a polymer-fibre composite, which supports, encases and protects the entire body. Mandibles experience heavy loading and wear due to direct contact with hard and abrasive food, just like teeth, their mineralized analogues in vertebrates. With dual-energy X-ray tomography, we image well-defined regions of zinc (Zn)-enriched cuticle at the mandible cutting edges and quantify the Zn concentrations in these regions. Zn is known to increase stiffness, hardness and wear resistance of the otherwise purely polymeric insect cuticle. In S. gregaria, the position of the Zn-enriched cutting-edge regions relative to one another suggests that the mandibles form a scissor-like cutting tool, which sharpens itself as the mouthparts shear past one another during feeding. Comparing the architecture of these purely polymeric mandibles with the mineralized incisors of rodents, we find fundamental design differences in cutting-tool structure and performance. Locusts' scissors and rodents' carving knives perform different functions, because they act on food that differs significantly in properties and shape: softer, sheet-like material in the case of locusts and harder bulk material in the case of rodents.

Mandible mechanical properties and composition of the larval Glossosoma boltoni (Trichoptera, Insecta)

Insect feeding structures, such as mandibles, interact with the ingesta (food or/and substrate) and can be adapted in morphology, composition of material and mechanical properties. The foraging on abrasive ingesta, as on algae covering rocks, is particularly challenging because the mandibles will be prone to wear and structural failure, thus suggesting the presence of mandibular adaptations to accompany this feeding behavior. Adaptations to this are well studied in the mouthparts of molluscs and sea urchins, but for insects there are large gaps in our knowledge. In this study, we investigated the mandibles of a grazing insect, the larvae of the trichopteran Glossosoma boltoni. Using scanning electron microscopy, wear was documented on the mandibles. The highest degree was identified on the medial surface of the sharp mandible tip. Using nanoindentation, the mechanical properties, such as hardness and Young's modulus, of the medial and lateral mandible cuticles were tested. We found, that the medial cuticle of the tip was significantly softer and more flexible than the lateral one. These findings indicate that a self-sharpening mechanism is present in the mandibles of this species, since the softer medial cuticle is probably abraded faster than the harder lateral one, leading to sharp mandible tips. To investigate the origins of these properties, we visualized the degree of tanning by confocal laser scanning microscopy. The autofluorescence signal related to the mechanical property gradients. The presence of transition and alkaline earth metals by energy dispersive X-ray spectroscopy was also tested. We found Ca, Cl, Cu, Fe, K, Mg, Mn, P, S, Si, and Zn in the cuticle, but the content was very low and did not correlate with the mechanical property values.

Biomechanics of cutting: sharpness, wear sensitivity and the scaling of cutting forces in leaf-cutter ant mandibles

Herbivores large and small need to mechanically process plant tissue. Their ability to do so is determined by two forces: the maximum force they can generate, and the minimum force required to fracture the plant tissue. The ratio of these forces determines the relative mechanical effort; how this ratio varies with animal size is challenging to predict. We measured the forces required to cut thin polymer sheets with mandibles from leaf-cutter ant workers which vary by more than one order of magnitude in body mass. Cutting forces were independent of mandible size, but differed by a factor of two between pristine and worn mandibles. Mandibular wear is thus likely a more important determinant of cutting force than mandible size. We rationalize this finding with a biomechanical analysis, which suggests that pristine mandibles are ideally 'sharp'-cutting forces are close to a theoretical minimum, which is independent of tool size and shape, and instead solely depends on the geometric and mechanical properties of the cut tissue. The increase of cutting force due to mandibular wear may be particularly problematic for small ants, which generate lower absolute bite forces, and thus require a larger fraction of their maximum bite force to cut the same plant. This article is part of the theme issue 'Food processing and nutritional assimilation in animals'.

Simulated biomechanical performance of morphologically disparate ant mandibles under bite loading

Insects evolved various modifications to their mouthparts, allowing for a broad exploration of feeding modes. In ants, workers perform non-reproductive tasks like excavation, food processing, and juvenile care, relying heavily on their mandibles. Given the importance of biting for ant workers and the significant mandible morphological diversity across species, it is essential to understand how mandible shape influences its mechanical responses to bite loading. We employed Finite Element Analysis to simulate biting scenarios on mandible volumetric models from 25 ant species classified in different feeding habits. We hypothesize that mandibles of predatory ants, especially trap-jaw ants, would perform better than mandibles of omnivorous species due to their necessity to subdue living prey. We defined simulations to allow only variation in mandible morphology between specimens. Our results demonstrated interspecific differences in mandible mechanical responses to biting loading. However, we found no evident differences in biting performance between the predatory and the remaining ants, and trap-jaw mandibles did not show lower stress levels than other mandibles under bite loading. These results suggest that ant feeding habit is not a robust predictor of mandible biting performance, a possible consequence of mandibles being employed as versatile tools to perform several tasks.

Chemo-mechanical-microstructural coupling in the tarsus exoskeleton of the scorpion Scorpio palmatus

The multiscale structure of biomaterials enables their exceptional mechanical robustness, yet the impact of each constituent at their relevant length scale remains elusive. We used SAXD analysis to expose the intact chitin-fiber architecture within the exoskeleton on a scorpion's claw, revealing varying orientations, including Bouligand and unidirectional regions different from other arthropod species. We uncovered the contribution of individual components' constituent behavior to its mechanical properties from the micro- to the nanoscale. At the microscale, in-situ micromechanical experiments were used to determine site-specific stiffness, strength, and failure of the biocomposite due to fiber orientation, while metal-crosslinking of proteins is characterized via fluorescence maps. At the constituent level, combined with FEA simulations, we uncovered the behavior of fiber-matrix deformation with fiber diameter <53.7 nm and protein modulus in the range 1.4-11 MPa. The unveiled microstructure-mechanics relationship sheds light on the evolved structural functionalities and constituents' interactions within the scorpion cuticle. STATEMENT OF SIGNIFICANCE: The pincer exoskeleton is a fundamental part of the scorpion's body due to its multifunctionality. Precise structural and compositional analysis within the hierarchy is paramount to understand the fundamentals of the mechanical properties of the composite exoskeleton. Here, we expose the intact chitin-fiber architecture of the pincer exoskeleton using nondestructive analysis. In-situ mechanical characterization was performed at nanometer levels within the exoskeleton hierarchy, which complemented with simulations, uncovered the elastic modulus of the protein matrix. Our findings confirm the presence and distribution of metal ions and their role as reinforcements in the protein matrix via ligand coordinate bonds. In future work, these findings can be of great potential to inspire the design of composite materials.

Comparative Material and Mechanical Properties among Cicada Mouthparts: Cuticle Enhanced with Inorganic Elements Facilitates Piercing through Woody Stems for Feeding

Adult cicadas pierce woody stems with their mouthparts to feed on xylem, suggesting the presence of cuticular adaptations that could increase hardness and elastic modulus. We tested the following hypotheses: (a) the mouthpart cuticle includes inorganic elements, which augment the mechanical properties; (b) these elements are abundant in specific mouthpart structures and regions responsible for piercing wood; (c) there are correlations among elements, which could provide insights into patterns of element colocalization. We used scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) to investigate mouthpart morphology and quantify the elemental composition of the cuticle among four cicada species, including periodical cicadas (Magicicada sp.). Nanoindentation was used to quantify hardness and elastic modulus of the mandibles. We found 12 inorganic elements, including colocalized manganese and zinc in the distal regions of the mandible, the structure most responsible for piercing through wood; nanoindentation determined that these regions were also significantly harder and had higher elastic modulus than other regions. Manganese and zinc abundance relates to increased hardness and stiffness as in the cuticle of other invertebrates; however, this is one of the first reports of cuticular metals among insects with piercing-sucking mouthparts (>100,000 described species). The present investigation provides insight into the feeding mechanism of cicadas, an important but understudied component of their life traits.

Unusual bromine enrichment in the gastric mill and setae of the hadal amphipod Hirondellea gigas

The hadal amphipod Hirondellea gigas is an emblematic animal of the Pacific trenches, and has a number of special adaptations to thrive in this ‘extreme’ environment, which includes the deepest part of the Earth’s ocean. One such adaptation that has been suggested is the presence of an ‘aluminum gel shield’ on the surface of its body in order to prevent the dissolution of calcitic exoskeleton below the carbonate compensation depth. However, this has not been investigated under experimental conditions that sufficiently prevent aluminum artefacts, and the possibility of other elements with similar characteristic X-ray energy as aluminum (such as bromine) has not been considered. Here, we show with new electron microscopy data gathered under optimized conditions to minimize aluminum artefacts that H. gigas actually does not have an aluminum shield–instead many parts of its body are enriched in bromine, particularly gastric ossicles and setae. Results from elemental analyses pointed to the use of calcite partially substituted with magnesium by H. gigas in its exoskeleton, in order to suppress dissolution. Our results exemplify the necessity of careful sample preparation and analysis of the signals in energy-dispersive X-ray spectroscopic analysis, and the importance of analyses at different electron energies.

The mechanical characterization of the legs, fangs, and prosoma in the spider Harpactira curvipes (Pocock 1897)

The exoskeleton of spiders is the primary structure that interacts with the external mechanical stimuli, thus playing a crucial role in spider life. In particular, fangs, legs, and prosoma are the main rigid structures of the exoskeleton and their properties must be measured to better understand their mechanical behaviours. Here we investigate, by means of nanoindentation, the mechanical properties of the external sclerotized cuticles of such parts in the spider Harpactira curvipes. Interestingly, the results show that the leg’s cuticle is stiffer than the prosoma and has a stiffness similar to the one of the tip fangs. This could be explained by the legs’ function in perceiving vibrations that could be facilitated by higher stiffness. From a broader perspective, this characterization could help to understand how the same basic material (the cuticle, i.e. mainly composed of chitin) can be tuned to achieve different mechanical functions, which improves the animal’s adaptation to specific evolutive requirements. We, thus, hope that this work stimulates further comparative analysis. Moreover, these results may also be potentially important to inspire the design of graded materials with superior mechanical properties.

Artificial and natural silk materials have high mechanical property variability regardless of sample size

Silk fibres attract great interest in materials science for their biological and mechanical properties. Hitherto, the mechanical properties of the silk fibres have been explored mainly by tensile tests, which provide information on their strength, Young’s modulus, strain at break and toughness modulus. Several hypotheses have been based on these data, but the intrinsic and often overlooked variability of natural and artificial silk fibres makes it challenging to identify trends and correlations. In this work, we determined the mechanical properties of Bombyx mori cocoon and degummed silk, native spider silk, and artificial spider silk, and compared them with classical commercial carbon fibres using large sample sizes (from 10 to 100 fibres, in total 200 specimens per fibre type). The results confirm a substantial variability of the mechanical properties of silk fibres compared to commercial carbon fibres, as the relative standard deviation for strength and strain at break is 10–50%. Moreover, the variability does not decrease significantly when the number of tested fibres is increased, which was surprising considering the low variability frequently reported for silk fibres in the literature. Based on this, we prove that tensile testing of 10 fibres per type is representative of a silk fibre population. Finally, we show that the ideal shape of the stress–strain curve for spider silk, characterized by a pronounced exponential stiffening regime, occurs in only 25% of all tested spider silk fibres.

Material Properties and Morphology of Prestomal Teeth in Relation to the Feeding Habits of Diptera (Brachycera)

Simple Summary

The prestomal teeth are structures on the mouthparts of some fly species that rasp surfaces to expose liquids for ingestion. Here, we investigated the material properties of prestomal teeth, including their hardness, elastic modulus, the extent of sclerotization, and elemental composition, and combined these results with morphology to determine their relationship to fly feeding habits. The results indicated that the prestomal teeth are heavily sclerotized and have hardness and elastic modulus values similar to the polymer polycarbonate. Although the presence of inorganic elements contributes to a harder cuticle in other insect species, the fly species studied here had only low concentrations of inorganic elements. We found evidence that the material properties and morphology of prestomal teeth relate to feeding habits, not phylogeny. In particular, flies that pierce tissues for liquids have larger prestomal teeth relative to their mouthpart sizes when compared to species that generally feed on exposed liquids. Flies are one of the most successful groups of organisms and their success likely relates to their ability to feed on a large array of nutritional liquids. Given their importance in ecology systems and relevance to medical and veterinary entomology, the functional morphology of fly mouthparts warrants additional studies.

Abstract

Prestomal teeth are cuticular projections on the mouthparts of some fly species that rasp surfaces when feeding. Although prestomal teeth morphology has been reported for several fly species, their material properties have not been investigated. Here we report the morphology, elemental composition, extent of sclerotization, hardness, and elastic modulus of prestomal teeth and relate these findings to feeding habits. Scanning electron microscopy revealed that species categorized as flower visitors have a large labellum with numerous pseudotracheae and lack prestomal teeth, generalist species have these same features but with prestomal teeth, and specialist species that feed on blood or other insects have a smaller labellum with few or no pseudotracheae and relatively large prestomal teeth. Confocal microscopy revealed that prestomal teeth are heavily sclerotized and the labellum contains resilin, an elastomeric protein. Hardness and elastic modulus were explored with nanoindentation and showed that the insectivorous Scathophaga stercoraria had the hardest prestomal teeth and the highest modulus. Energy dispersive x-ray spectroscopy revealed that prestomal teeth had low concentrations of inorganic elements, suggesting that hardness might be partially supplemented by inorganic elements. Our findings indicate that prestomal teeth morphology and material properties relate more to feeding habits than to phylogeny.