Mechanical behavior of the rostrum of Curculio Linnaeus, 1758 (Coleoptera: Curculionidae)

Weevils in the genus Curculio Linnaeus, 1758 engage in a unique drilling behavior during oviposition that causes significant elastic deformation of the rostrum. This deformation occurs as the apex of the rostrum is displaced antero-dorsally about its longitudinal axis to become nearly linear in form. As yet, there is no explanation of how these weevils are able to withstand such bending without apparent harm after multiple oviposition events. Serial thin-sectioning and confocal microscopy have revealed that the ratio of endocuticle to exocuticle increases greatly towards the apex of the rostrum and towards the dorsum between the scrobe channels, with the overall thickness of the cuticle remaining largely the same throughout. We constructed finite element models of the head of Curculio species and compared the effects of varying thicknesses of endocuticle and exocuticle on the mechanical performance of the cuticle during the extension of the rostrum. We found that regions with a relatively thick endocuticle and thin exocuticle were significantly more compliant and thus able to be displaced with smaller forces and with reduced stress upon the material than cuticle with a more typical composite profile. By optimizing the composite structure of the cuticle of the rostrum, Curculio species are able to better tolerate the relatively large strains placed on the rostrum during the preparation of oviposition sites. This research therefore establishes a connection between a mechanical stressor and its impact on the form and function of anatomical structures.