As ubiquitous defense mechanisms in Nature, stinger-like structures cover a size range over six orders of magnitude. While their composition varies, we uncovered a common geometric trait: a non-linear relationship between diameter and distance from the tip, following a power law with an exponent universally between 2 and 3. Through a combination of theoretical mechanics and experiments, we interpret this universal shape to be the result of a competition between penetration and buckling, motivated by the limitations of the mechanical properties of the stinger material. Our study not only resolves a mystery underlying the structural optimization of convergently evolved natural stingers, but also can offer inspiration for efficient needles in technology or biomedicine, made from sustainable non-metallic materials.
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