In Heliconius melpomene, ecdysis to each stadium is asynchronous in continuous light, but is synchronized with respect to the time of day in an LD 12:12 photoperiod regime-generally occurring during the early part of the photophase. This is a consequence of synchronism in the time of day of onset of the moulting cycle, which occurs approximately 24 hours before ecdysis. There is variation in the number of days taken to complete an instar, however. Onset of the moulting cycle is gated by photoperiod, and occurs at the first gate following attainment by the larvae of a certain threshold size, indicated by weight. By depriving larvae of food, the time at which threshold size is obtained can be delayed; this results in a delay in the time of onset of the moulting cycle. Possible biological advantages in the wild of the existence of a threshold size for onset of the moulting cycle, and of synchronism in the time of day of ecdysis, have been suggested.