The flying insect thoracic cuticle is heterogenous in structure and in thickness-dependent modulus gradation.

The thorax is a specialized structure central to insect flight. In the thorax, flight muscles are surrounded by a thin layer of cuticle. The structure, composition, and material properties of this chitinous structure may influence the efficiency of the thorax in flight. However, these properties, as well as their variation throughout the thorax and between insect taxa, are not known. We provide a multi-faceted assessment of thorax cuticle for fliers with asynchronous (honey bee; Apis mellifera) and synchronous (hawkmoth; Manduca sexta) muscles. These muscle types are defined by the relationship between their activation frequency and the insect's wingbeat frequency. We investigated cuticle structure using histology, resilin distribution through confocal laser scanning microscopy, and modulus gradation with nanoindentation. Our results suggest that thorax cuticle properties are highly dependent on anatomical region and species. Modulus gradation, but not mean modulus, differed between the two types of fliers. In some regions, A. mellifera had a positive linear modulus gradient from cuticle interior to exterior of about 2 GPa. In M. sexta, modulus values through cuticle thickness were not well represented by linear fits. We utilized finite element modeling to assess how measured modulus gradients influenced maximum stress in cuticle. Stress was reduced when cuticle with a linear gradient was compressed from the high modulus side. These results support the protective role of the A. mellifera thorax cuticle. Our multi-faceted assessment advances our understanding of thorax cuticle structural and material heterogeneity and the potential benefits of material gradation to flying insects. STATEMENT OF SIGNIFICANCE: The insect thorax is essential for efficient flight but questions remain about the contribution of the exoskeletal cuticle. We investigated the microscale properties of the thorax cuticle, a crucial step to determine its role in flight. Techniques including histology, nanoindentation, and confocal laser scanning microscopy revealed that cuticle properties vary through cuticle thickness, by thorax region, and between species with asynchronous (honey bee; Apis mellifera) and synchronous (hawkmoth; Manduca sexta) muscles. This variation highlights the importance of high resolution cuticle assessment for flying insect lineages and points to factors that may (modulus gradation) and may not (mean modulus) contribute to different flight forms. Understanding material variation in the thorax may inform design of technologies inspired by insects, such as mobile micro robots.