Mandibular morphology in Leptodirini beetles (Leiodidae: Cholevinae) shows habitat-specific disparity across the karstic vertical profile

Abstract Leptodirini is a Leiodidae tribe with its most diverse subterranean representatives in the Western Palaearctic, inhabiting the entire karstic vertical profile. Most are scavengers linked to allochthonous resources and have not received a comparative study of their mouthparts. We analysed mandibular morphology in 16 Leptodirini and two Catopini species that inhabit leaf litter, epikarst, deep terrestrial subterranean habitat, and the subterranean hygropetric. We confirmed that, on the one hand, there is a directional differentiation of mandible shape and size in habitats across the profile, and on the other hand, there are distinctive similarities among representatives inhabiting the same habitat type. Most obvious changes refer to gradual elongation and slenderization of the mandible with increasing habitat depth, which might improve food-collecting effectiveness. Similarities refer to mandible structure, such as a relatively straight mandibular tip and the presence of the prostheca brush in terrestrial species, in contrast to a perpendicularly medially bent tip and the prostheca comb in semiaquatic species. Accordingly, the litter-, epikarst-, and deep terrestrial subterranean-dwelling guilds can be identified among terrestrial representatives, and the moonmilk-browsing and water-filtering guilds among semiaquatic representatives. We conclude that the mandible shape is environmentally driven, with organic matter being the most important driver of adaptation.