Evidence for the independent evolution of a rectal complex within the beetle superfamily Scarabaeoidea

Rectal or cryptonephridial complexes have evolved repeatedly in arthropods, including in beetles where they occur in ∼190,000 species of Cucujiformia + Bostrichoidea, and Lepidoptera where they occur in ∼160,000 species. Sections of the Malpighian/renal tubules coat the outer surface of the rectum, acting as powerful recycling systems of the gut contents, recovering water and specific solutes. There are hints that a rectal complex evolved independently within another beetle group, Scarabaeoidea. Here we report our observations of rectal complexes in Scarabaeoidea, which support this view. We did not find a rectal complex in the related group, Staphylinoidea, or in Lucanidae, a basal group of Scarabaeoidea. We did observe rectal complexes in Melolontha melolontha (Melolonthini), Pachnoda marginata and Cetonia aurata (Cetoniinae), consistent with previous reports from these groups. Intriguingly we found that rectal complexes occur in adult, but not M. melolontha larvae, and larvae but not adults within Cetoniinae, indicating dramatic pupal remodelling of these organ systems. Insights into the structure of the rectal complexes of Scarabaeoidea are compared with the well-studied rectal complexes of Cucujiformia. Finally, we discuss possible functions of the rectal complexes of beetles within Scarabaeoidea, and future approaches to address this question.