Functional morphology of the honeybee stinger and its biomechanical significance

The honeybee stinger, as an important organ for self-defense and reproduction, have evolved specific macroscopic morphologies and microscopic structures. Here, we investigated the surface and cross-sectional structures and material composition of the cuticle in the stingers of worker honeybees (Apis mellifera). Except the stinger bulb, the cuticle sclerotization of the stinger is found to be uniformly distributed along its longitudinal axis, i.e., uniform modulus distribution. Based on this, we developed a two-dimensional (2D) model to explore the influence of various modulus distributions of the stinger on its penetration behavior in vertical and oblique penetrations using the finite element method (FEM). It is found that compared with the stinger models with the modulus-gradient distribution (5 GPa at the stinger tip to 15 GPa at its base) and negative modulus-gradient distribution (15 GPa at the tip to 5 GPa at base), the model with uniform modulus distribution (10 GPa) can be more easily inserted into the skin with less maximal penetration force in no matter vertical or oblique penetrations. Therefore, the uniform modulus distribution along the axis as revealed by our confocal laser scanning microscopy is beneficial for the honeybee stinger to achieve its self-defense with fast penetration. This study significantly enriches the understanding of the stinger functionality and inspires us with new avenues for bioinspired microneedles in modern engineering.