The thoracic morphology of the troglobiontic cholevine species Troglocharinus ferreri (Coleoptera, Leiodidae)

Abstract

The thoracic morphology of the troglobiontic leiodid species Troglocharinus ferreri (Cholevinae, Leptodirini) is described and documented in detail. The features are mainly discussed with respect to modifications linked with subterranean habits. Troglocharinus is assigned to the moderately modified pholeuonoid morphotype. The body is elongated and slender compared to epigean leiodids and also cave-dwelling species of Ptomaphagini. The legs are elongated, especially the hindlegs, though to a lesser degree than in the most advanced troglobiontic species. The prothorax is moderately elongated but otherwise largely unmodified. Its muscular system is strongly developed, with more muscle bundles that in free-living staphylinoid or hydrophiloid species. The pterothorax is greatly modified, especially the metathoracic flight apparatus. The meso- and metathoracic elements of the elytral locking device are well-developed, whereas the other notal parts are largely reduced. The mesonotum is simplified, with the triangular scutellar shield as the only distinctly developed part. The mesothoracic musculature is strongly reduced, with only 6 muscles compared to 12 or 13 in free-living staphylinoid or hydrophiloid species. The metanotum is greatly reduced, without a recognizable subdivision into prescutum scutum and scutellum. It is strongly narrowing laterally and lacks notal wing processes and other wing-related elements, but well-developed alacristae are present. The wings are reduced to small membranous flap-like structures inserted at the posterior end of the metanotum. A metapostnotum is not developed. Like in the case of the head, cave dwelling species of the related Ptomaphagini and Leptodirini show different trends of adaptations, with a compact ovoid or navicular body shape in the former, and a distinct trend towards elongation of the body and appendages in the latter tribe. Structural affinities of the thoraces of T. ferreri and the troglobiontic trechine carabid Sinaphaenops wangorum are mainly due to the reduced flight apparatus. The degree of muscle reduction in the pterothorax is very similar in both species.

Keywords: 3D-reconstruction; Micro-CT; Subterranean beetle; Thoracic anatomy; Troglomorphy.