Abstract1InOncopeltus fasciatus, the A‐cells of the pars intercerebralis and their tracts are stainablein situwith the performic acid‐victoria blue (PAVB) method. The axons from these cells, after traversing the corpus cardiacum, terminate in the anterior part of the aorta which thus serves as the neurohemal organ.2Ultrastructurally, four types of secretory neurons are distinguishable in the pars intercerebralis region: pic‐I with granules measuring 1000–3000 Å in diameter; pic‐II with granules of irregular size and shape, the elongate ones showing mean dimensions of 2400 × 1400 Å; pic‐III with less electron‐dense granules measuring 1000–2700 Å in diameter; pic‐IV, present not only in the pars intercerebralis but also in adjacent regions of the brain, with variable proportions of granules measuring 700–1800 A and dense‐cored vesicles measuring 1000–2400 Å.3The nervi corporis cardiaci contain at least three types of neurosecretory axons, based on granule content, presumably representing pic‐I, pic‐II and pic‐III neurons.4The wall of the aorta contains endings of at least three distinct types, again representing pic‐I, pic‐II and pic‐III neurons, and thus provides the neurohemal site for these three types of protocerebral neurosecretory cells. Axons of pic‐IV neurons appear to enter the cerebral neuropil.5The corpus cardiacum is composed of two types of parenchymal secretory cells, with electron‐dense granules measuring 1300–3000 Å and 1000–2300 Å in diameter, respectively. The corpus cardiacum also contains interstitial cells and some axons of extrinsic origin, with and without granules.6The corpus allatum may be paired or median, and receives a small number of at least two types of axons. The corpora allata of some reproducing females show a large number of PAVB‐stainable inclusions which appear to be modified cytoplasmic organelles, but are definitely not neurosecretory material.7The hypocerebral ganglion is composed of two types of secretory‐appearing neurons and glial cells. The two neuronal types contain secretory granules, 1000–3000 Å and 900–2100 Å in diameter, respectively. Axons of the recurrent nerve also may contain occasional granules.8In this heteropteran insect, the two principal functions of the corpus cardiacum appear to be spatially separated: the neurohemal function is subserved by the aortic wall, which permits release of material into both the aortic lumen and the hemocoel, and the intrin