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

## TL;DR

This study uses 3D imaging to explore how the loss of a clicking mechanism in a rare beetle species affects its thoracic structure and function.

## Contribution

The study reveals specific anatomical changes in a soft-bodied beetle linked to the loss of the clicking mechanism through 3D reconstruction and comparative analysis.

## Key findings

- The clicking-related muscles in Plastocerus thoracicus are significantly weaker compared to other species.
- Structures like the prosternal process and mesonotum are underdeveloped or flattened in P. thoracicus.
- Flight and walking muscles in P. thoracicus are larger in volume compared to hard-bodied species.

## Abstract

Chinese Plastocerini (Elateridae: Dendrometrinae) are represented by a single and rare species Plastocerus thoracicus Fleutiaux, 1918, which exhibits cuticle softening and the loss of the functional clicking mechanism. Here, we employed micro-computed tomography (micro-CT) and three-dimensional (3D) reconstruction to elucidate the thoracic morphology of this species. While body softening has been documented in several Elateridae groups, the associated internal structural modifications remained unexplored. Specifically, this study aimed to identify the anatomical changes in the thoracic structures associated with body softening and the loss of the clicking mechanism. By conducting comparative morphological analyses between P. thoracicus and two functionally contrasting species, Campsosternus auratus (Drury) and Cerophytum lii Qiu & Ruan, we provide insights into this question.

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 Plastocerus thoracicus and reconstruct its thoracic morphology in 3D and quantified key muscle ratios (e.g., M2/M60, M4/M60). Based on our study of P. thoracicus, a detailed comparison was made with previously reported data on Campsosternus auratus (Elateridae) and Cerophytum lii (Cerophytidae). Three-dimensional reconstructions revealed significant structural divergences in P. thoracicus: (1) the clicking-related muscles M4 are markedly weaker than those in Ca. auratus and Ce. Lii. (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 Ca. auratus and Ce. lii. 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.