Troglomorphic trends of palpigrades in the Eukoenenia mirabilis-berlesei group, and discovery of a new cave-dwelling species from Spain (Palpigradi, Eukoeneniidae)

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Abstract
The arachnid order Palpigradi remains poorly studied, particularly with respect to patterns of morphological adaptation to subterranean environments. Subterranean adaptation in palpigrades and comparison with endogean species has been documented scarcely. In this study, Eukoenenia serrana sp. nov . is described as a new species in the E. mirabilis-berlesei group from the cave Sima del Pla de Llomes in eastern Spain. The new species exhibits pronounced troglomorphic features, including large body size and strong elongation of appendicular elements, particularly leg I, while retaining diagnostic characters of the group like the single lateral prosomal organ. We explored the morphological differentiation between endogean and hypogean species in the E. mirabilis-berlesei group, a lineage traditionally regarded as endogean and only marginally associated with caves. An analysis of elongation patterns, with an emphasis on the palp and legs, showed clear differences between species from different habitats using a set of linear measurements and derived ratios related to body and their appendages size. Hypogean species consistently show marked elongation of tibia I, basitarsus IV, and combined tarsus–tibia lengths, whereas changes in overall body characters are more moderate. In contrast, the basitarsus IV/tibia IV ratio remains largely unchanged, indicating proportional elongation of these segments. Highly cave-adapted species in this group retained a single blade in the lateral organ, suggesting that sensory and appendicular traits may evolve independently. The results reveal a mosaic pattern of subterranean adaptation within the E. mirabilis-berlesei group and by extension in palpigrades.

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