The mesosoma is the power core of the ant, containing critical structural and muscular elements for the movement of the head, legs, and metasoma. It has been hypothesized that adaptation to ground locomotion and the loss of flight led to the substantial rearrangements in the mesosoma in worker ants, and that it is likely the ant mesosoma has undergone functional modifications as ants diversified into different ecological and behavioral niches. Despite this importance, studies on the anatomy of the ant mesosoma are still scarce, and we have little understanding of important variation across the ant phylogeny. Recent advances in imaging techniques have made it possible to digitally dissect small insects, to document the anatomy efficiently and in detail, and to visualize these data in 3D. Here we document the mesosomal skeletomuscular system of workers of the red wood ant Formica rufa Linnaeus, 1761, and use it to establish 3D atlas of ant anatomy that will serve as reference work for further studies. We discuss and illustrate the configuration of the skeletomuscular components and the function of the muscles in interaction with the skeletal elements. This anatomical evaluation of a “generalized” ant provides a template for future studies of the mesosoma across the radiation of Formicidae, with the ultimate objective of synthesizing structural, functional, and transformational information to understand the evolution of a crucial body region of ants.

Most of the data presented in the current study is based on adult workers of Formica rufa preserved in ethanol (collection code FHG01185: Germany, Hessen, Darmstadt, Prinzenberg, 49.836565°, 8.664995°, 240 m, forest edge and open area, 20.IV.2016, leg. F. Hita Garcia). Two F. rufa specimens (unique specimen identifiers: CASENT0741316 and CASENT0741323) were used for µCT-scanning of mesosoma and the middle leg. In order to achieve a higher quality scan, the middle leg was detached from the body and scanned individually. The specimens were stained in iodine solution one week prior to the scanning date. 

The µCT-Scanner used was a Zeiss Xradia 510 Versa 3D X-ray microscope operated with the Zeiss Scout-and-Scan Control System software (version 14.0.14829.38124) at the Okinawa Institute of Science and Technology Graduate University, Japan. The scanning parameters chosen consisted of a 40 kV (75 μA) / 3 W beam strength with 4s exposure time for the thorax and 7s for the leg under a 4x magnification, which resulted in a voxel size of 2.97 μm and 1.40 μm respectively. 3D reconstructions of the resulting scan projection data were done with the Zeiss Scout-and-Scan Control System Reconstructor (version 14.0.14829.38124) and saved in txm file format. Postprocessing of txm raw data was done with Amira 2019.2 software (Visage Imaging GmbH, Berlin, Germany) in order to segment individual structures into discrete materials. The segmented materials were then exported with the plugin script “multiExport” (Engelkes et al. 2018) in Amira 2019.2 as 2D Tiff image stacks. VG-Studio 3.4 (Volume Graphics GmbH, Heidelberg, Germany) was used to create volume renders out of the Tiff image series.

CT scans can be viewed using Fiji (ImageJ) using XRM Reader plug-in.