DETERMINANTS OF LARVAL MOLT INITIATION IN THE TOBACCO HORNWORM, MANDUCA SEXTA

Hornworm larvae adhere to Dyar's rule under normal growth conditions, increasing their live weight by an average of 5- to 6-fold from the outset of one instar to the next. This adherence to Dyar's rule is largely maintained even in instars subsequent to those in which larvae have been severely malnourished. The importance of relative weight gain for the normal onset of larval molting contrasts with the requirement for attainment of an absolute body weight prior to pupation. But body size is by no means the sole cue for the initiation of a larval molt. Thus, under conditions of malnutrition larvae can initiate a molt at any weight and can do so even in the absence of any weight gain in an instar: in these circumstances the duration of an instar is inversely related to the weight at the outset of malnutrition. In larvae fed ad lib as well as in malnourished larvae the initiation of a molt appears to be limited to a discrete phase of the photocycle. The failure of larvae to molt on schedule at body weights lower than those predicted by Dyar's rule cannot be attributed to an inability of the brain to stimulate ecdysone-dependent development as is the case in diapausing pupae. Though body size, instar duration, and photocycle all interact to determine the onset of the molt, the role of the brain remains ill-defined.