MOULTING in insects is controlled by the prothoracic glands after they have been activated by the brain1–6. α-Ecdysone and 20-hydroxyecdysone stimulate moulting and are thought to be the principal moulting hormones7–11. It is common to assume that the prothoracic glands control moulting by synthesizing ecdysone, even though there is no direct evidence for this12,13. The experiments reported here show that the head and thorax of Calpodes larvae isolated with intact activated prothoracic glands do not moult; they can only do so if moulting hormone is injected or if at least part of the abdomen is present. This suggests that more than an activated prothoracic gland is needed for moulting.