Abstract: The shape of newly laid eggs of the wasps Microplitis mediator is different from that of the eggs in the calyx region, due to mechanical press while they pass through the ovipositors, which initiates the development of the embryos. Six hours after oviposition, the embryo attains the germ band stage, and a serosal membrane is observed. Hatching occurs at 32～34 h. Immediately after hatching, the cells from the upper serosal membrane of the embryo disperse to form teratocytes in the haemocoele of the host, while those from the lower part are released gradually. The 1st instar larva has a pair of sclerotized mandibles, which are used as a weapon to fight with its conspecies competitors. A caudal sac is well developed after the larva enters its 2nd instar at 108～113 h.The surface of a mature teratocyte (6 days after parasitization) is covered with numerous microvilli. A vesiculated endoplasmic reticulum system disperses throughout the cytoplasm in which many Golgi complexes exist too. The nucleus is extensively ramified.

Key words: Microplitis mediator, embryogenesis, teratocyte, ultrastructure