Centipedes are terrestrial and predatory arthropods that possess an evolutionary transformed pair of appendages used for venom injection—the forcipules. Many arthropods incorporate reinforcing elements into the cuticle of their piercing or biting structures to enhance hardness, elasticity or resistance to wear and structural failure. Given their frequent exposure to high mechanical stress, we hypothesise that the cuticle of the centipede forcipule might be mechanically reinforced. With a combination of imaging, analytical techniques and mechanical testing, we explore the centipede forcipule in detail to shed light on its morphology and performance. Additionally, we compare these data to characteristics of the locomotory leg to infer evolutionary processes. We examined sclerotization patterns using confocal laser-scanning microscopy based on autofluorescence properties of the cuticle (forcipule and leg) and elemental composition by energy-dispersive X-ray spectroscopy in representative species from all five centipede lineages. These experiments revealed gradually increasing sclerotization towards the forcipular tarsungulum and a stronger sclerotization of joints in taxa with condensed podomeres. Depending on the species, calcium, zinc or chlorine are present with a higher concentration towards the distal tarsungulum. Interestingly, these characteristics are more or less mirrored in the locomotory leg’s pretarsal claw in Epimorpha. To understand how incorporated elements affect mechanical properties, we tested resistance to structural failure, hardness (H) and Young’s modulus (E) in two representative species, one with high zinc and one with high calcium content. Both species, however, exhibit similar properties and no differences in mechanical stress the forcipule can withstand. Our study reveals similarities in the material composition and properties of the forcipules in centipedes. The forcipules transformed from an elongated leg-like appearance into rigid piercing structures. Our data supports their serial homology to the locomotory leg and that the forcipule’s tarsungulum is a fusion of tarsus and pretarsal claw. Calcium or zinc incorporation leads to comparable mechanical properties like in piercing structures of chelicerates and insects, but the elemental incorporation does not increase H and E in centipedes, suggesting that centipedes followed their own pathways in the evolutionary transformation of piercing tools.

中文翻译：

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蜈蚣注射毒肢的材料组成及力学性能


蜈蚣是陆生和掠食性节肢动物，拥有一对用于注射毒液的进化附肢——前肢。许多节肢动物将增强元件纳入其刺穿或咬合结构的角质层中，以增强硬度、弹性或耐磨性和结构破坏性。鉴于它们经常受到高机械应力，我们假设蜈蚣的爪的角质层可能得到了机械强化。通过结合成像、分析技术和机械测试，我们详细探索了蜈蚣的爪，以阐明其形态和性能。此外，我们将这些数据与运动腿的特征进行比较，以推断进化过程。我们使用共焦激光扫描显微镜检查了骨化模式，该模式基于角质层（前肢和腿）的自发荧光特性，并通过能量色散 X 射线光谱法检查了所有五个蜈蚣谱系的代表性物种的元素组成。这些实验揭示了朝向前跗节的硬化逐渐增加，并且具有浓缩足节的类群中关节的硬化逐渐增强。根据物种的不同，远端跗节的钙、锌或氯浓度较高。有趣的是，这些特征或多或少反映在Epimorpha的运动腿的前爪上。为了了解掺入的元素如何影响机械性能，我们测试了两种代表性物种（一种具有高锌含量，一种具有高钙含量）的抗结构破坏性、硬度 (H) 和杨氏模量 (E)。 然而，这两种物种都表现出相似的特性，并且触爪可承受的机械应力没有差异。我们的研究揭示了蜈蚣的爪的材料成分和特性的相似性。爪从细长的腿状外观转变为刚性的刺穿结构。我们的数据支持它们与运动腿的系列同源性，并且爪的跗骨是跗骨和前爪的融合体。钙或锌的掺入会产生类似螯合物和昆虫的穿刺结构的机械性能，但元素的掺入不会增加蜈蚣的 H 和 E，这表明蜈蚣在穿刺工具的进化转变中遵循了自己的途径。