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

• Published in: Advanced materials (Weinheim) Vol. 31; no. 41; pp. e1903526 - n/a
• Main Authors: Jansen, Michael A., Williams, Jason, Chawla, Nikhilesh, Franz, Nico M.
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 01-10-2019
• Subjects: acorn weevils; Animals; Avoidance; Biological Evolution; Biomechanical Phenomena; Biomechanics; Catastrophic events; Computer architecture; Correlation; Crack initiation; Elastic Modulus; Elasticity; Evolution; Exoskeletons; Female; Flexibility; fractography; Fracture toughness; Laminates; Materials science; Mechanical Phenomena; Morphology; Plate theory; Stiffness; Structural damage; Structural failure; Tensile Strength; tensile testing; Thickness; Weevils - anatomy & histology; Weevils - physiology; tensile testing; fractography; acorn weevils; exoskeletons
• DOI: 10.1002/adma.201903526
• EISSN: 1521-4095
• ISSN: 0935-9648

The acorn weevil (Curculio Linnaeus, 1758) rostrum (snout) exhibits remarkable flexibility and toughness derived from the microarchitecture of its exoskeleton. Modifications to the composite profile of the rostral cuticle that simultaneously enhance the flexibility and toughness of the distal portion of the snout are characterized. Using classical laminate plate theory, the effect of these modifications on the elastic behavior of the exoskeleton is estimated. It is shown that the tensile behavior of the rostrum across six Curculio species with high morphological variation correlates with changes in the relative layer thicknesses and orientation angles of layers in the exoskeleton. Accordingly, increased endocuticle thickness is strongly correlated with increased tensile strength. Rostrum stiffness is shown to be inversely correlated with work of fracture; thus allowing a highly curved rostrum to completely straighten without structural damage. Finally, exocuticle rich invaginations of the occipital sutures are identified both as a likely site of crack initiation in tensile failure and as a source of morphological constraint on the evolution of the rostrum in Curculio weevils. It is concluded that avoidance of catastrophic structural failure, as initiated in these sutures under tension, is the driving selective pressure in the evolution of the female Curculio rostrum.
The effect of heterogeneous laminate microstructure on cuticle strength in the acorn weevil rostrum is investigated. The results indicate that increased endocuticle thickness and advantageous fiber orientation near the rostral apex allows the snout to be repeatedly subjected to extreme bending without harm. These modifications mitigate the risk of catastrophic structural failure through a tradeoff between stiffness and toughness of the laminate.