A simple mushroom body in an African scarabid beetle

This account describes novel mushroom body organization in a coleopteran insect, the African fruit chafer Pachnoda marginata. Each of its prominent mushroom bodies possesses a pair of simple calyces comprising two populations of Kenyon cells, the dendrites of which are organized into a central and an annular zone. Kenyon cells of the central zone extend their dendrites downward and toward the perimeter of the calyx. Their axon-like processes in the pedunculus are densely packed to make up a distinctive shaft of neuropil. Toward the front of the brain, the shafts, one from each calyx, bifurcate to provide a pair of subdivisions in the medial and vertical lobes. Dendrites of Kenyon cells supplying the annular zone extend from the calyx perimeter toward its center. Axons from the annular zones of both calyces together provide a sleeve of axons that ensheaths the two shafts. Sleeve axons bifurcate to provide a second pair of divisions in each of the lobes. These arrangements provide each lobe with a discrete representation of the two Kenyon cell populations of the two calyces. Kenyon cells supplying the central zone have dendritic morphologies reminiscent of class II clawed Kenyon cells that supply the gamma lobes in other taxa. Kenyon cells supplying axons to the sleeve are suggestive of class III Kenyon cell morphologies described from cockroaches and termites. Elaborate intrinsic neurons, comparable to exotic intrinsic neurons in the honey bee gamma lobes, have processes that interact with shaft axons. The present observations suggest that mushroom bodies of Pachnoda represent either a basal organization entirely lacking class I Kenyon cells or an evolutionary modification in which there is no clear morphological distinction of class I and II Kenyon cells. In either case, cellular organization in Pachnoda's mushroom body is simple compared with that of other taxa.