Mechanical properties of the unguitractor apodeme of sunny stick insects, Sungaya aeta (Phasmatodea: Heteropterygidae)

An integral component of musculoskeletal systems are elastic elements mechanically in-series with muscle. Although these in-series elastic elements-e. g. tendons in vertebrates, or apodemes in invertebrates-can neither generate force nor do work, they are thought to bring substantial benefits to musculoskeletal performance; the mechanical properties of tendons, crucial determinants of these benefits, have consequently been subject of a large body of work. In sharp contrast, scarce information exists on the mechanical properties of apodemes. The little data that do exist appear to suggest that apodemes differ so substantially from tendons that their functional significance may differ, too. To increase our understanding of apodeme function, we determined the mechanical properties of the unguitractor apodeme (UTA) of Sungaya aeta stick insects. We devised an experimental protocol that permits tensile testing with slippery and brittle apodemes; we derived and validated a mechanical model that extracts the Young's modulus from tensile tests with specimen with varying cross-sectional area, without the need for explicit measurement of the stress or strain distribution; and we interpreted the magnitude of the UTA modulus, strength and spring constant through allometric comparison with data on vertebrate tendons. The UTA modulus exceeds that of vertebrate tendons by almost one order of magnitude, but the size-corrected spring constant is nevertheless comparable and if anything smaller, due to systematic differences in apodeme and tendon shape. This many-to-one mapping suggests that apodemes may well convey the same functional benefits as tendons, and should not be prematurely excluded from invertebrate musculoskeletal models.