Extending Without Extensors: Morphofunctional Insights and Qualitative Experiments on Myriapod Legs

Until recently, the absence of intrinsic extensor muscles in the distal leg joints of Myriapoda-a condition previously recognized in spiders-was known but scattered across various publications, lacking a consolidated overview. This has led to various hypotheses regarding the mechanisms underlying the extension of these so-called hinge joints. Hemolymph pressure has been proposed as a possible explanation, along with the pressing of the leg tip against the ground and the elasticity of the joints. The aim of this study is to investigate the morpho-functional background responsible for the extension of the joints and whether the absence of intrinsic extensor muscles was a feature present in the ground pattern of the Myriapoda. To this end, we examined the cuticle and muscle anatomy of representatives from the four major taxa-Chilopoda, Symphyla, Pauropoda, and Diplopoda-using confocal laser scanning microscopy, micro-computed tomography, and semi-thin sections. Additionally, extension experiments have been conducted on the legs of freshly euthanized Spirostreptus spec. 1 "Tanzania" (Diplopoda) and Lithobius forficatus (Chilopoda). All intrinsic and extrinsic muscles of the investigated species were described, along with the cuticle anatomy of the joints and their autofluorescent properties. The findings confirm that hinge joints lacking intrinsic extensor muscles were likely already present in the ground pattern of Myriapoda. Furthermore, three main extension mechanisms of the hinge joints are hypothesized. These mechanisms are: (i) the generation of hemolymph pressure within the leg, (ii) elasticity of the joints' cuticular structures, and (iii) pressing the tip of the leg against the ground.