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

Abstract Elastic elements that act mechanically in series with muscle are integral components of musculoskeletal systems. Although these elements can neither generate force nor do work, they are thought to bring substantial benefits to organismal performance. The mechanical properties of vertebrate tendons, crucial determinants of these benefits, have consequently been the subject of a large body of work; by contrast, scarce information exists on the mechanical properties of arthropod apodemes. 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 apodemes; we derived and validated a mechanical model that extracts Young’s modulus from tensile tests with specimens with varying cross-sectional area; and we interpreted 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, owing 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—they should not be prematurely excluded from arthropod musculoskeletal models.