Mushroom bodies are in general similarly developed in most taxons studied. The calyx region appears as a single structure, and its dual nature is not yet realized. An anterio-posterior asymmetry of the calyx region with Kenyon cell processes running mostly behind the glomerular neuropil of the calyx is characteristic of all the species studied. In this respect, the calyx region of basal Scarabaeoidea resembles greatly the calyx of many dipterans. Lobe compartmentalization occurs at the initial stage. The passalid beetles represent an exception, as their mushroom bodies are much more developed than in related families. This may be connected with the complicated social behavior of Passalidae.