The Mesosoma of Protanilla (Leptanillinae) and the Groundplan of the Formicidae (Hymenoptera)

The study of ant morphology is advancing through parallel insights provided by phylogenomics—which provides a statistically robust basis for comparison and evolutionary inference—and phenomics via the application of microcomputed tomography (µ-CT) for the efficient and precise documentation of anatomy. The information provided by µ-CT is complex and rich, allowing for the quantification of geometry and biomechanically relevant variables, as well as comparative morphology via 3D rendering. Recently, the complete musculature of the thorax, propodeum, and legs was documented for the first time in an ant (Formica rufa L.). Here, we provide a detailed comparison of those findings for Protanilla lini Terayama, 2009 (Leptanillinae), representing the Leptanillomorpha, a clade putatively sister to all other living ants. Using µ-CT, computer-based reconstruction, and scanning electron microscopy (SEM), we observe a novel series of morphological features that are plausibly part of the groundplan of the Formicidae. For several of these features, we provide new anatomical concepts and terms, with special discussion of the pronotum, promesothoracic articulation, and metapleural gland region. We also observe characters that are likely correlated with specialized subterranean habits, including the increased flexibility of the promesothoracic articulation, the slender shape of the mesosoma, a simplified vestiture of short setae, and depigmentation. Mesosomal skeletomusculature in P. lini appears to be plesiomorphic relative to other Leptanillomorpha, resembling the putative ancestral condition for the Formicidae. An exception lies in the lack of cervical muscle (Idlm1) in Protanilla that is present in almost all other insects for which this character has been sampled. With this study of P. lini, we are one step closer to realizing the complete set of defining features and variation of the ant mesosoma.