Mechanical models affecting beetle horn remodeling

Clarifying the mechanisms of shape alteration by insect metamorphosis is important for comprehending exoskeletal morphogenesis. The large horn of the Japanese rhinoceros beetle Trypoxylus dichotomus is the result of drastic metamorphosis, wherein it appears as a rounded shape via pupation and then undergoes remodeling into an angular adult shape. However, the mechanical mechanisms of this remodeling process remain unknown. In this study, we investigated the remodeling mechanisms of the Japanese rhinoceros beetle horn by developing a physical simulation. We identified three factors contributing to remodeling by biological experiments--ventral adhesion, uneven shrinkage, and volume reduction--which were demonstrated to be crucial to the transformation by a physical simulation. We also corroborated our findings by applying the simulation to the stag beetles mandibular remodeling. These results indicate that the physical simulation is applicable to pupal remodeling in other beetles, and the morphogenic mechanisms could explain various exoskeletal shapes. Significance statementThe metamorphosis in insects is a mysterious process. By metamorphosis, insects sometimes change their shape dramatically. The head horn of the Japanese rhinoceros beetle is one of the most famous examples of metamorphosis. In larva-to-pupa molting, the horn appears suddenly, caused by the "furrow formation and unfolding" mechanism. The unfolding process makes the pupal horn rounded. However, pupa-to-adult molting transforms the rounded shape into an angular shape. In this paper, we investigated the mechanisms of the transformation. We extracted factors contributing to it through observations and experiments and developed a physical simulation. It could reproduce the adult shape from the pupal shape and could be a general model for the pupa-adult transformation of beetles.