1. As in Rhodnius, the larval Oncopeltus has bristles which are supplemented at each moult. However, at metamorphosis a dense population of non-innervated hairs develops.
2. Implantation of corpora allata into 5th-stage larvae showed that the development of these hairs can be inhibited universally or locally by the juvenile hormone (JH).
3. Transplantations of integument between 5th-stage larvae of different stages in the moult cycle gave some information about the power of the host to synchronize the graft to its own moult cycle.
4. Transplantations between different larval stages showed that the grafted in tegument responded to the hormonal milieu of the host.
5. Adult integument was transplanted onto larvae to study the reversal of metamorphosis. It was found that the development of a supernumerary population of hairs depended on the integument passing through a moult cycle in the presence of JH.
After two moults in the presence of JH, reversal of metamorphosis was found to vary over the surface of the transplant, being further advanced at the margin. At the edge of the graft properly formed larval bristles developed, while at the centre adult hairs were formed in adult cuticle. Intermediately formed bristles were found in the intervening areas. It is suggested that reactions associated with wounding are the cause of this heterogeneous result.
6. The significance of these results in relation to other work and to theories concerning the mode of action of the juvenile hormone is discussed.