“…The greatest concentration of neuronal cells in the larval nervous system is found in the apical organ, a U-shaped structure containing at least four different neuronal cell types, all of which send axonal fi bers into a central neuropil (Hay-Schmidt 1989Lacalli 1990 ;Santagata 2002 ;Temereva and Wanninger 2012 ;Sonnleitner et al 2013 ;Temereva and Tsitrin 2014 ). Cellular domains within the apical organ include groups of cells that express serotonin (Hay-Schmidt 1990 ;Santagata 2002 ;Temereva and Wanninger 2012 ;Sonnleitner et al 2013), catecholamines (Hay-Schmidt 1990Santagata 2002), FMRFamide (Hay-Schmidt 1990Sonnleitner et al 2013 ), and a cardioactive B-like peptide (Sonnleitner et al 2013 ). Serotonin-like immunoreactive cells include numerous fl ask-shaped cells that overlie other bipolar or multipolar neuronal cells (HaySchmidt 1990 ;Santagata 2002 ;Temereva and Wanninger 2012 ;Sonnleitner et al 2013 ).…”