Most insects have an elongated shape and can be divided into two main geometric sides: the dorsal and the ventral. However, in some insect groups, such as Chrysididae (Hymenoptera), Cybocephalidae, Clambidae (Coleoptera), and Blaberidae (Blattoidea), a similar spherical shape has independently evolved, with the dorsal side becoming the external side. The cuticle is the most important modified part in spherical insects. To investigate the morphological and functional differences in the cuticle between elongate and spherical insects, three beetles from the superfamily Scarabaeoidea were studied: one spherical insect from Ceratocanthinae and two elongate insects from Hybosorinae and Melolonthinae.
The morphological comparison shows that the dorsal cuticles (e.g., pronotum and elytra) are expanded and curved in the spherical beetles, and additional joints between the cuticles help to maintain the basic shape. Uniaxial compression tests indicate that spherical beetles have greater overall defensive strength. Since the defensive strength of the exoskeleton is affected by both the dimensions (material thickness) and the mechanical properties of the material, CT scans and nanoindentation tests were performed to evaluate these two factors. The results showed that the average cuticular thickness of spherical insects was the highest, and the variation in thickness between different parts of the body cuticle was greater than that of the elongated ones. The elastic moduli of the outer cuticle parts (pronotum and elytra) of Ceratocanthinae were significantly higher than those of other beetles.
In conclusion, the findings show that the cuticles of spherical beetles not only changed their shape, but also exhibited higher stiffness and thickness, supporting the overall higher defensive strength. Further studies on the structures of exocuticles and endocuticles could provide additional information about this strategy. This work helps to explain the evolution of spherical insects and may inform the biomimetic design of spherical robots.
