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.