Microstructure and nanoindentation of the rostrum of Curculio longinasus Chittenden, 1927 (Coleoptera: Curculionidae)

Abstract

The rostrum is an extension of the cuticle of the head of weevils (Coleoptera: Curculionidae) and is often used to bore holes for oviposition (the process of laying eggs) into host plant tissue where larval development occurs. In members of the genus Curculio Linnaeus, 1758, the rostrum is long, slender, and strongly curved, but is nevertheless used to excavate straight bore-holes in the fruit of various host plants, through significant deformation of this structure. In this study, scanning electron microscopy (SEM) was used to examine the rostrum of Curculio longinasus Chittenden, 1927, leading to a microstructural model that describes its deformation behavior.Specifically, we used the continuous stiffness measurement (CSM) technique in nanoindentation to measure the Young's modulus and hardness of rostrum. The values of Young's modulus and hardness for the endocuticle were measured to be 8.91 ± 0.93 GPa and 558 ± 60 MPa, respectively. These results are critical for generating accurate finite element models of the head's mechanical behavior while it undergoes deformation. AbstractThe rostrum is an extension of the cuticle of the head of weevils (Coleoptera: Curculionidae) and is often used to bore holes for oviposition (the process of laying eggs) into host plant tissue where larval development occurs. In members of the genus Curculio Linnaeus, 1758, the rostrum is long, slender, and strongly curved, but is nevertheless used to excavate straight bore-holes in the fruit of various host plants, through significant deformation of this structure. In this study, scanning electron microscopy (SEM) was used to examine the rostrum of Curculio longinasus Chittenden, 1927, leading to a microstructural model that describes its deformation behavior.Specifically, we used the continuous stiffness measurement (CSM) technique in nanoindentation to measure the Young's modulus and hardness of rostrum. The values of Young's modulus and hardness for the endocuticle were measured to be 8.91 ± 0.93 GPa and 558 ± 60 MPa, respectively. These results are critical for generating accurate finite element models of the head's mechanical behavior while it undergoes deformation.

Morphology and nanoindentation properties of mouthparts in Cyrtotrachelus longimanus (Coleoptera: curculionidae)

“…Figure shows the nanoindentation properties of the mouthparts in “dry” and “fresh” conditions. The maximum load for indentation was 10 mN, the load rate was 0.33 mN/s, and holding time was 30 s. A value of 0.3 for Poisson's ratio was used according to the literatures [0.25 for sheep horn (Warburton, ); 0.3 for rostrum of insect (Singh et al, ); 0.3 for elytra (Dai & Yang, )]. The Young's modulus and hardness were observed to be constant with respect to depth (plateau region) between ∼500 and 1000 nm (Figure ).…”

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

“…Despite its documented performance in many Curculio species, it remains unknown how the rostrum of female acorn weevils can withstand the repeated, often extreme bending incurred during the process of egg‐chamber excavation. Previous work in the species Curculio longinasus Chittenden, 1927 revealed modifications to the cuticle of the rostral apex—the region exposed to the strongest degree of bending—that might be linked to the flexibility of the rostrum . This modified composite structure has not been fully characterized—and has not been examined in any other species in this genus—nor has it been demonstrated that the modifications to the cuticle meaningfully alter the mechanical behavior of the snout in the context of bending.…”

“…However, acorn weevils in the genus Curculio Linnaeus, 1758 (Curculionidae in the sense of ref. ) instead exhibit unusual distal flexibility in an elongate extension of the head called the rostrum (snout) . The rostrum is a hollow, strongly curved (over 90° in some species), cylindrical, exoskeletal extension of the otherwise nearly spherical head, which bears at its apex the terminal chewing mouthparts .…”

“…This structure is used by the female to feed on fruit (plant) tissue and to excavate sites for egg‐laying (oviposition, see Figure ). The rostrum can be repeatedly bent, without evident damage, and despite being composed of the same material as other rigid body parts . By maintaining constant upward pressure on the snout and continuously rotating around the bore‐hole, females are able to flex the rostrum into a straightened configuration and produce a linear channel into the fruit.…”

“…The macrofibers are nearly 3–4 orders of magnitude larger than the nanoscale features of the Bouligand structure, and consequently exhibit vastly different mechanical behavior. Chitin macrofibers are orthotropic (axial: E 1 = 8.5 GPa; transverse: E 2 = E 3 = 0.52 GPa; nanoindentation:, constitutive model:), and arranged in unidirectional plies, as depicted in Figures and . Typically, adjacent macrofiber laminae are paired and pseudo‐orthogonal—i.e., angled nearly 90° to each other (see Figure )—with a constant stacking angle between pairs, although other configurations have been observed .…”