Structural characteristics of claw tooth exoskeleton

The claw tooth of mole crickets has a sharp tip and a thick base, which forms a wedge shape. The cross-sectional shape of claw teeth is approximately triangular. There is a triangular cavity inside the claw tooth that forms a shell-like structure (Figs 5 and 6). The hollow structure has obvious advantages over solid structure in bearing loads when the total amount of the material is constant; moreover, the triangular cross-section makes the structure more bending resistant than it would have been with other cross-section (e.g. circular) [17,18].The area ratio of the cavity on the cross section increases with the distance from the tip to end[5]. Additionally, the inner aspect exoskeleton of teeth is thicker than the outer aspect, especially at the ribs and corner positions. The exoskeleton of insects is composed of a composite material with a gradient multilayer structure[19,20]. The outermost layer of the exoskeleton is termed epicuticle, which is a thin wax layer that provides the main waterproof barrier to the environment. The procuticle is located under the epidermis and is the main structural part of the exoskeleton that is used to withstand the mechanical load from the environment. The procuticle is further divided into the exocuticle and endocuticle, which are similar in structure and material composition. However, the texture of the exocuticle is stacked more densely, as shown in Fig 5c.

Fig 5 shows the fracture cross-section of the base region of claw teeth. The procuticle ridge on the inner aspect is clearly visible. However, the boundary between the epicuticle and the procuticle is not obvious, as shown in Fig 5c. The structure of the exocuticle is uniform and dense, and the layers are densely stacked (Fig 5d). The structure of the endocuticle is less dense than the exocuticle. The upper part of the endocuticle shows the obvious corrugated board-like layered structure, and the layers are connected by laminates (Fig 5e). In the lower part of endocuticle, the structure gradually becomes looser with depth, and the porous fibres are assembled in a spiral stacked pattern (Bouligand structure). In the example shown in Fig 5c, the thickness of the exocuticle is approximately 15.4 μm, and the thickness of the endocuticle is approximately 45 μm; the ratio of the two layers is thus approximately 1:2.9.

The layered structure, transition of fibre orientation and irregular fracture surface of the cuticle of the claw tooth present as a laminate structure. The insect exoskeleton is a complex structure consisting of chitin fibres and protein matrix. The mechanical properties of the cuticle are determined by the properties of the protein matrix and the orientation of the chitosan fibres. The protein matrix can exhibit a great hardness range with a certain degree of hydration, and chitin fibres have different stacking patterns on the sub-micron scale, including parallel and helical patterns[21,22]. The Bouligand (helical stacking) arrangement provides structural strength and is in-plane isotropic[19].

Fig 6 shows the fracture cross section of the tip of claw teeth. The internal cavity is smaller than the base. The cuticle structure of the tip is not obvious, and there are some oriented structural differences between inside and outside of the cross section (Fig 6a). The stacking direction/growth direction of the material seems to be vertical to the outer surface in the outside of the cross section (Fig 6b), and parallel to the longitudinal direction of claw in the inside (Fig 6c).

The cuticle structure of claw teeth in mole crickets is a special form of exoskeleton and is similar to the cuticle of other body parts because the chitosan fibres are stacked in parallel or spiral models. The main function of the claw is digging. It is assumed that there are different structural forms in different locations, and the combination of structures is conducive to the realization of biological functions. The mole cricket uses a "digging-expanding" type of excavation[23], where the tip of the claw teeth is mainly subjected to compression and friction when wedging into soil and the base of the claw teeth is subjected to shear force in expanding motion. Upon comparing Figs 5 and 6, the chitosan fibres in the tip of claw teeth are arranged in a dense and parallel manner, which may helpful in with standing compressive stress. The cross-section in the base shows a distinct laminated structure with deflected and complex fibres, which may helpful in resisting fracture[24].