The embryo of Toxoneuron nigriceps (Hymenoptera, Braconidae) is surrounded by an extraembryonic membrane, which, at hatching, releases teratocytes and gives rise to a cell layer embedding the body of the 1st instar larva. This cell layer was studied at different developmental times, from soon after hatching up to the first larval moult, in order to elucidate its ultrastructural, immunocytochemical and physiological function. The persisting "larval serosa" shows a striking structural and functional complexity: it is a multifunctional barrier with protective properties, limits the passage of macromolecules and it is actively involved in the enzymatic processing and uptake of nutrients. The reported results emphasizes the important role that the embryo-derived host regulation factors may have in parasitism success in Hymenoptera koinobionts.