Antennal troglomorphy in Arrhopalitidae (Collembola: Symphypleona) with the description of two new cave Pygmarrhopalites Vargovitsh, 2009 species from the Arabika Massif in the Caucasus

The Arabika Massif in the West Caucasus is a high-altitude alpine karst region containing numerous caves, including three of the deepest in the world. These subterranean environments harbor unique speleofauna, including four previously described collembolan species from the genera Schaefferia Absolon, Anurida Laboulbène, Deuteraphorura Absolon, and Plutomurus Yosii. In this paper, we describe two new troglobiont species of Pygmarrhopalites Vargovitsh — P. arbaicus sp. nov. and P. trisetosus sp. nov. —collected from four cold caves at elevations near 2,300 m on the Arabika Massif.

Both species exhibit elongated appendages and represent the atmobiont troglomorphic life form. Additionally, we report P. sericus (Gisin, 1947) from the Caucasus for the first time, discovered in three small caves within the massif. The updated list of Caucasian collembolan species of the family Arrhopalitidae now includes 16 species, 14 of which— representing different life forms—have been recorded from caves in the region.

Identification keys to the genera and species of Caucasian Arrhopalitidae are also provided. Antennal elongation, a common adaptation among cave-dwelling Arrhopalitidae, has evolved convergently through various patterns, primarily affecting the fourth antennal segment. These patterns may occur independently or in combination and include an increased number of subsegments, simple elongation of subsegments, the formation of annulations between subsegments, and the appearance of bare intercalary pseudosubsegments.Subsegmentation patterns are always more pronounced in males.The rich diversity and high endemism of Arrhopalitidae in West Caucasian caves, along with other cave-dwelling groups, make this region a biodiversity hotspot and a center of speleofaunal diversity at the Europe-Asia boundary.