Mature larvae of the gregarious endoparasitoid Cotesia kariyai construct cocoons for pupation approximately 10 days after parasitization and emerge from their host Pseudaletia separata under a long day photo-regime (16L8D) at 25 ± 1°C. The parasitoid larvae make capsules in the host hemocoel just prior to their emergence. These capsules function as anchors, which enable them to press against the host integument from inside the host. It was predicted that this anchor might be composed of silk proteins secreted from the parasitoid larvae, because a previous study showed that the anchor was made up of a glycoprotein and that the silk gland of parasitoid larvae developed from 2nd larval stage. Fibroin-like proteins in C. kariyai larva mainly consist of two proteins with molecular masses of the 300.6 and 46.7 kDa estimated by SDS-PAGE. The fibroin-like proteins with the same molecular mass were detected from the anchor proteins just prior to parasitoid emergence. These results indicate that the anchor was assembled with fibrion-like proteins and was formed just before parasitoid emergence while in the host body cavity. Injection of bovine pancreatic trypsin inhibited the emergence of parasitoid larvae from the host because the anchor was decomposed by trypsin. Trypsin activity in the parasitized host hemolymph increased only after parasitoid emergence.