A neuroendocrine factor essential for cuticular melanization in the tobacco hornworm, Manduca sexta

Abstract

The decline in juvenile hormone before the head capsule slippage stage during the moult is essential for cuticular melanization in Manduca sexta larvae. Yet when larvae were allatectomized 15–16 h before head capsule slippage, then 10 h later ligated anterior to the 5th abdominal segment, only the posterior portion melanized. The ligation had no effect on dopa decarboxylase activity in the epidermis in either portion. By contrast, no premelanin granules containing phenoloxidase were deposited in the anterior portion. Injection of an extract of the ventral nerve cord prior to head capsule slippage induced the deposition of these granules followed by melanization, but purified melanization and reddish colouration hormone (MRCH) was not effective. When the integuments of larvae at the head capsule slippage stage were cultured in vitro, no deposition of premelanin granules occurred, although the epidermis synthesized the granular phenoloxidase. Addition of either a nerve cord extract and/or haemolymph was ineffective. Implantation of the integument into allatectomized larvae, however, induced melanization of the implants. Thus, both a factor from the ventral nerve cord and another unidentified humoral factor are important for the deposition of premelanin granules.