On the form and bio-mechanics of venom-injection elements.

A wide variety of animals-from insects to snakes-crucially depend on their ability to inject venom into their target, be it their prey or their predator. To effectively deliver their venom, venomous animals use a specialized biomechanical element whose tip must penetrate through the integument of the target. During this process, the tip of the venom-injection element (VIE) is subject to local forces, which may deform it and cause considerable structural damage to the VIE, with devastating consequences for the survival of the animal or, in the case of eusocial insects, to the colony. Hence, it is plausible that millions of years of evolution have carefully 'shaped' the architecture of VIEs across different taxa toward a similar mechanical function, namely, to effectively resist the mechanical forces exerted on the tip. The present study aims to identify such a common architecture by analyzing the form-function relationships in various biological VIEs. A universal structural modeling, which quantifies the fundamental geometrical characteristics of a wide range of VIEs is constituted, and a theoretical mechanical framework that analytically correlates these characteristics with the material stress fields is introduced. This investigation reveals that the architecture of biological VIEs reduces the magnitude of applied stresses and confines the maximal stress to the near-tip region of the element. The presented analytical approach and modeling can be straightforwardly applied to various other types of bio-mechanical elements and can potentially be employed for developing a new class of microscopic injection elements for bio-medical and engineering applications. STATEMENT OF SIGNIFICANCE: Venomous animals-both vertebrate and invertebrate-use an extremely wide variety of venom-injection elements to incapacitate their prey or predator. Despite the clear differences in their typical dimensions, shapes, and evolutionary paths, all venom-injection elements have evolved to perform a single mechanical function, namely, to penetrate a target surface. Accordingly, the architecture of many such elements appears to follow similar principles and their material exhibits similar stress characteristics upon biologically relevant mechanical loadings. The current study introduces a theoretical model that draws connections between the 'universal' structural characteristics of such elements and their bio-mechanical functions. It is found that all examined venom-injection elements provide extreme load-bearing capabilities and unusual post-failure functionalities, which are in good agreement with the wide range of numerical and experimental findings from the literature. The emerging theoretical insights from this study thus shed light on the biomechanical origins of the naturally evolved forms of various biological organisms, including bee and wasp stingers, spider and snake fangs, porcupine fish spines, and scorpion stingers.