The five nymphal stages of Rhodnius prolixus are more or less alike. The adult differs markedly from the nymphs. There are thus two phenomena to be considered: simple moulting and moulting coupled with metamorphosis. 1. Causation of Moulting. Moulting occurs at a definite interval after feeding, only one meal being necessary in each stage. There is a ‘critical period’ in the moulting cycle (about 7 days after feeding in the fifth nymph, about 4 days in the earlier nymphs) and removal of the head of the insect before this period prevents moulting. The critical period corresponds with the time when mitotic divisions in the epidermis begin. The blood of insects that have passed the critical period contains a factor or hormone which will induce moulting in insects decapitated soon after feeding. It is suggested that this moulting hormone may be secreted by the corpus allatum, since the cells of this gland show signs of greatest secretory activity during the critical period. Stretching of the abdominal wall provides the stimulus which causes secretion of the moulting hormone. This stimulus is conveyed by nerves to the brain: moulting is prevented by section of the nerve-cord in the prothorax. Section of the nerves between the brain and the corpus allatum appears to prevent moulting; but these experiments were inconclusive. Insects sharing the same blood moult simultaneously. The whole process of growth must therefore be co-ordinated by chemical means; the factors concerned being produced presumably by the growing cells themselves. 2. Causation of Metamorphosis. If fourth or even first nymphs, decapitated soon after feeding, receive the blood from moulting fifth nymphs, they suffer a precocious metamorphosis and develop adult characters. Metamorphosis is therefore brought about by chemical differences in the blood. If fifth nymphs decapitated soon after feeding receive blood from moulting fourth nymphs, they also moult; showing that the moulting factor is the same at all stages. The absence of metamorphosis in normal nymphs before the fifth stage must therefore be due to an inhibitory factor or hormone in the blood. This is proved by the fact that if a fifth nymph decapitated soon after feeding receives the blood from a moulting fourth nymph (not deprived of its head) it develops characters much more like those of a nymph than an adult. The inhibitory hormone is normally produced in such small quantities that simple dilution of the blood of a moulting fourth nymph with that of another fourth nymph (decapitated soon after feeding) causes them both to suffer metamorphosis. The head is necessary for the secretion of the inhibitory hormone. This hormone seems to be secreted after the moulting hormone. Thus if series of fourth, third, second, or first nymphs are decapitated around the critical period, some of them show more or less complete metamorphosis. Others show characters intermediate between those of nymphs and adults. The bearing of these results on the phenomena of diapause and prothetely is discussed.