Material composition of the endophytic ovipositor in the damselfly Calopteryx splendens (Odonata, Calopterygidae)

Natural selection has favoured the incorporation of ions, including transition metals, in materials of various biological structures susceptible to mechanical fracture to enhance their failure and wear resistance. With regards to insects, only a few taxa have been investigated. The objective of this study was to analyse the biomechanical properties of the ovipositor in the damselfly Calopteryx splendens (Harris, 1780) (Odonata, Zygoptera, Calopterygidae) through nanoindentation and to ascertain the elemental composition gradient within the cuticle using energy-dispersive X-ray spectroscopy. This research represents the first report indicating that the damselfly ovipositor exhibits a gradient in the mechanical properties of the cuticle, with Young's modulus ranging from approximately 3.0 to 7.0 GPa and hardness from 0.1 to 0.3 GPa. These properties highly correlate with the contents of copper and magnesium, both of which increase in the distal direction. The results also suggests that the mechanical properties of the cuticle are significantly influenced by the degree of sclerotization revealed by confocal laser scanning microscopy. These findings propose that the material properties of the ovipositor cuticle in C. splendens may have adapted to enhance piercing capability and to reduce the risk of structural failure during insertion of eggs in plant substrates.