Re-establishment of biting mouthparts in desert-living dung beetles (Scarabaeidae : Scarabaeinae) feeding on plant litter-old structures reacquired or new ones evolved?

Abstract

Evolution of dung beetle (Scarabaeidae: Scarabaeinae) mouthparts for eating moist, fresh dung has led to a loss of any ability to chew. However, the desert-living genus Pachysoma, probably evolved from a wet-dung feeding, Scarabaeus-like ancestor, has switched to dry faecal pellets (of rodents or small ruminants) and plant litter that might require re-establishment of chewing. Indeed, gut contents of a litter-feeding Pachysoma species indicate efficient food comminution prior to ingestion. Cutting and grinding mouthpart structures in six species, of two lineages and with different food preferences, are described and compared with homologous structures in wet-dung feeding Scarabaeus species. In Pachysoma, cutting and breaking of large food items is performed by a clypeal scraper, a prominent epipharyngeal tooth and large maxillary galeal hooks. Further comminution is achieved by a large, grinding area evolved on the mandibular molae. Interspecific differences and the probable function and evolution of these structures are discussed. Particularly the unique tools for cutting/breaking are novel structures and not re-acquired normal biting mouthparts.