The biomechanical dilemma of phasmid eggs – how do stick insects hatch?

During their development, phasmid eggs with their long developmental periods face numerous biotic and abiotic challenges. These insect eggs are unique for their thick egg capsule and a specialised escape structure called the operculum/lid. This operculum needs to be easy to open from the inside, yet must not be a ‘weak spot’ for potential predators. One possible solution to this biomechanical dilemma could be a change in the operculum's mechanical properties over time. To investigate this hypothesis, we analysed the effect of age and storage conditions on the biomechanical properties of the egg and the operculum. We also performed high-resolution X-ray microscopy (XRM) and synchrotron studies to analyse the ultrastructure of the eggshell and operculum. Our results show that the eggs of Carausius morosus were able to resist a compressive force of up to 2 N. During maturation, the force required to open the operculum was significantly reduced from 0.14 N to 0.09 N. The properties of the eggshell itself, however, were not affected by humidity and did not change during maturation. Interestingly, the egg properties were affected by the mother's age. Our results thus indicate that the operculum is not a primary fracture site for externally applied stress; however, structural changes in the operculum during the developmental process of the embryo facilitate the hatching process.