Mechanical properties and cuticle organisation in mandibles are related to the task specialisation in leafcutter ants (Atta laevigata, Attini, Formicidae)

Abstract

Leafcutter ants show a high degree of task division among the workers of different castes. For example, the smallest workers, the minims, care for the brood and the symbiotic fungus, whereas the larger mediae cut and transport plant material. This is reflected in the size and morphology of the mandibles, but also in their mechanical properties as mediae possess the hardest and stiffest cuticle and the minims—the softest and most flexible one. This is directly related to the content of the cross-linking transition metal zinc (Zn). The cuticle microstructure, which can be more or less anisotropic depending on the orientation of cuticle layers, is known to determine the resistance to loads and stresses and thus contributes to the biomechanical behaviour of the structure. To study how the mandible tasks are related to the cuticular organisation, we here documented the microstructure of the mandibles from the mediae and the minims by scanning electron microscopy. Afterwards, the mechanical properties (Youngs' modulus, E, and hardness, H) of the exo-, meso- and endocuticle were identified by nanoindentation. Tests were performed along the longitudinal and the circumferential axes of the mandibles. We found, that the minims possess mandibles, which are more isotropic, whereas the mandibles of the mediae are rather anisotropic. This difference was never determined within one species before and is probably linked to the task of the individual ant. To gain insight into the origins of these properties, we characterized the elemental composition of the different cuticle layers along the circumferential axis, revealing that only the exocuticle of the mandible cutting edge contains Zn. All other mechanical property gradients thus must be the result of the chitin fibre bundle architecture or the properties of the protein matrix, which awaits further investigation.