E31 Mechanical properties of the cuticle

Topic

This very insect cuticle—which serves as a model for a system involving mechanosensory zones, thermal emissivity, and elastic energy storage for wing-beating—constitutes a unique hierarchical composite material. Composed primarily of chitin and protein, its mechanical properties vary enormously depending on the specific region of the insect's body.

For instance, the cuticle at an articulation point is soft and flexible, whereas the mandible, elytron, and leg can be as rigid as the bone of a comparable vertebrate. The closer a specific region of the cuticle is to the mandible or leg, the greater its final rigidity becomes.

This variation in rigidity across different regions of the cuticle is governed by both the degree of sclerotization—specifically, protein cross-linking mediated by quinones—and the cuticle's water content. A higher degree of sclerotization in a given region results in greater final rigidity for that part of the cuticle. Conversely, this Kelvin-Voigt viscoelastic behavior describes the cuticle's damped response to a specific load: the cuticle is not perfectly elastic—unlike the elasticity modeled in other contexts, such as wing flapping—but instead dissipates a portion of the energy, thereby serving to absorb impacts experienced by the insect.

In contrast, the von Mises criterion predicts the onset of permanent deformation in the cuticle when the combined stress acting upon it exceeds its specific yield limit.

Consequently, this model—featuring a hierarchical composite material with mechanical properties that vary according to the specific body region—ensures that the cuticle's laminated structure (which is pleated and reinforced with chitin fibers) provides anisotropic strength optimized for each specific region of the insect. Furthermore, reinforcing this laminated structure with a higher density of chitin fibers results in even greater anisotropic strength in the specific cuticle region modeled.

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