Corpus allatum regulation during the metamorphosis of Periplaneta americana: Axon pathways

The anatomy of the retrocerebral complex was studied after supravital staining with methylene blue, and axonal tracts within the corpora allata (CA) were traced after applying the CoCl₂ technique together with Timm's sulfide-silver enhancement. Cobalt chloride fills of the nerves to and from the CA revealed two major sources of innervation: the brain and the subesophageal ganglion. Three cell clusters in the brain contribute axons that reach each nervus corporis allati I (NCA I) and, apparently, pass to or beyond the CA. These are: a cluster of 8 to 12 cells in the contralateral pars lateralis, a cluster of 16 to 20 cells in the ipsilateral pars lateralis, and a cluster of 50 to 60 cells in the contralateral pars intercerebralis. PAF-stained sections of other brains revealed a corresponding number of PAF-positive cells in these same regions. The medial and lateral neurons arborize in the neuropile adjacent to the pars intercerebralis, and may associate there. The lateral group also arborizes extensively in the neuropile surrounding the pedunculus of the mushroom body. At least four cell bodies located antero-ventrad in the subesophageal ganglion send axons to the CA via each nervus corporis allati II (NCA II).To determine possible inhibitory pathways to the CA, the NCA I, NCA II, and postallatal nerves of last instar larvae were severed; either singly, or in combination. Additional experiments were performed on last instar larvae to substantiate that superlarvae were a direct result of an enhanced or sustained juvenile hormone titre. These experiments included: implanting two or more CA, extirpating one CA, or applying 100 μg of Altosid topically onto allatectomized larvae. The experiments indicated that only NCA I is an inhibitory pathway and that superlarvae were a direct consequence of CA activation. NCA II does not seem to provide the CA with an essential excitatory innervation; when it and NCA I are severed a supernumerary apolysis will still result. Some of the cells in the brain stainable by the CoCl₂ method are most probably identical to those that are PAF-positive. These cells may inhibit the CA in last instar larvae via neurosecretomotor junctions.