Cuticular melanization in the tobacco hornworm larva

Abstract

The pigment in the black cuticle of allatectomized and of black mutant Manduca sexta larvae was isolated and shown to be melanin by its chemical solubility properties. Labelled tyrosine was rapidly incorporated into this cuticular melanin when injected 4 hr before the onset of melanization. By contrast, labelled DOPA or dopamine was only incorporated into the head cap, the prothoracic and anal terga, and spiracles when injected 7 hr before melanization. When either [ 14C]DOPA or [ 14C]dopamine was injected at the onset of melanization, it was incorporated into the melanin fraction. When juvenile hormone (JH) was present at the time of head cap apolysis, less tyrosine or DOPA was incorporated into citucular melanin and more into the acid-hydrolyzable fraction of the cuticle. By contrast, dopamine was incorporated equally into the acid-soluble fraction of the cuticle in both wild-type and black mutant larvae. An inhibitor of DOPA decarboxylase activity, carbidopa (α-MDH), inhibited melanization and caused an accumulation of labelled DOPA in both the haemolymph and fat body after the injection of labelled tyrosine. In Manduca larval cuticle therefore both DOPA and dopamine are utilized in melanin formation as well as in cuticular stabilization.