Consequences of the Loss of the Clicking Mechanism: A Study of Thoracic Functional Morphology in Plastocerus thoracicus Fleutiaux, 1918 (Coleoptera: Elateridae)

Elateridae (Coleoptera: Elateroidea) are renowned for their clicking mechanism. However, several lineages exhibit body softening that compromises this mechanism, particularly within Plastocerini, Drilini, and Omalisinae. It remains unclear how this body softening is anatomically achieved and which specific structures are degraded in relation to the loss of clicking function. To elucidate the internal morphological adaptations and distinguish them from hard-bodied clicking elateroids, we employed micro-CT to scan <i>Plastocerus thoracicus</i> and reconstruct its thoracic morphology in 3D and quantified key muscle ratios (e.g., M2/M60, M4/M60). Based on our study of <i>P. thoracicus,</i> a detailed comparison was made with previously reported data on <i>Campsosternus auratus</i> (Elateridae) and <i>Cerophytum lii</i> (Cerophytidae). Three-dimensional reconstructions revealed significant structural divergences in <i>P. thoracicus</i>: (1) the clicking-related muscles M4 are markedly weaker than those in <i>Ca. auratus</i> and <i>Ce. Lii</i>. (2) the prosternal process (PP) is extremely narrow. The posterior part of the pronotum exhibits underdeveloped regions, including the posterodorsal evagination (PdE) and the posteromedial process (PmPr). (3) the mesonotum (i.e., the "biological spring" identified in previous studies) is greatly flattened and weakened. (4) the flight muscles (M60, M64) and walking muscles (M74, M75) exhibited significantly bigger volume than <i>Ca. auratus</i> and <i>Ce. lii</i>. These findings provide critical data for understanding the morphological evolution of Elateridae and offer insights into the functional adaptations of the clicking mechanism through comparative anatomy.