The shape of Nature's stingers revealed

Stinger - like structures in living organisms evolved convergently across taxa for both defensive and offensive purposes, with the main goal being penetration and damage. Our observations over a broad range of taxa and sizes, from microscopic radiolarians to narwhals, reveal a self- similar geometry of the stinger extremity: the diameter (d) increases along the distance from the tip (x) following a power law x similar to dn, with the tapering exponent varying universally between 2 and 3. We demonstrate, through analytical and experimental mechanics involving three- dimensional (3D) printing, that this geometry optimizes the stinger's performance; it represents a trade - off between the propensity to buckle, for n smaller than 2, and increased penetration force, for n greater than 3. Moreover, we find that this optimal tapering exponent does not depend on stinger size and aspect ratio (base diameter over length). We conclude that for Nature's stingers, composed of biological materials with moduli ranging from hundreds of megapascals to ten gigapascals, the necessity for a power - law contour increases with sharpness to ensure sufficient stability for penetration of skin - like tissues. Our results offer a solution to the puzzle underlying this universal geometric trait of biological stingers and may provide a new strategy to design needle - like structures for engineering or medical applications.