AbstractHistological and immunohistochemical observations showed that four pairs of neurosecretory A cells coincide with the bombyxin neuron which produces the 4K‐prothoracicotropic hormone (4K‐PTTH). The A cells are characterized by a large cytoplasmic vacuole containing homogeneous granular materials, but the vacuole does not contain the bombyxin. Bombyxin axons from the A cells are distributed on the surface of the corpus allatum (CA), and neurosecretory granules (NSGs) are released by exocytosis at earlier stages than those in other neurosecretory axons (NSAs). Embryonic bombyxin neurons show characteristics of secretory function after the bristle formation stage (156 h after egg laying) and become the neurosecretory A cells in the larval stage. © 1996 Wiley‐Lis