The largest synaptic neuropil domain of the adult central complex, covered by an extensive glial sheath. It is a regular structure of horizontal layers and 16 vertical segments (sometimes called staves or columns), 8 per hemisphere - arranged in 4 closely associated pairs. Its inferior part is much narrower than its superior part, forming a fan shape. Fibers emerging from the basal-most area of the fan project to the noduli. Rostro-caudally, it can be divided into 4 shells on the basis of the extent and position of small field arborizations.