Taniatrechus xeniae sp. nov. (Coleoptera: Carabidae: Trechini), the second species of the most specialised troglobitic carabid genus of the Caucasus

Taniatrechus xeniae sp. nov., the second species of the genus, is described from Prostornaya Cave in the central part of the Bzyb Mountain Range, Abkhazia. A comparative study of all available specimens of the genus is carried out using both linear morphometrics (indices) and geometric morphometrics: generalised Procrustes analysis (GPA), principal component analysis (PCA), and canonical variate analysis (CVA). This study allowed us to estimate infraspecific variation in the external morphology of T. setosus Belousov et Dolzhansky, 1994, on the one hand, and to reveal reliable diagnostic features that distinguish between the two species, on the other. The diagnosis and description of the genus are completed to include the new species and variation found in the type species. Some characters were also described more accurately or corrected based on more abundant material. The major morphological features of Taniatrechus spp. are photographed, their geographic distribution is mapped, and new data on living conditions of the genus are provided. The members of the genus seem to be the most specialised hypogean Trechini in the Caucasus, with the deepest finding ever reported for Coleoptera (one specimen of T. setosus was found at a level of 1,800 m below the cave entrance). In view of recent discoveries in highlands, the subterranean layer suitable for living of hypogean insects may reach more than three km thick in intensively folded karst areas. Thus, the hypogean life is much more widespread than previously thought. Prostornaya Cave, the type locality of T. xeniae sp. nov., is described for the first time.