Life cycle of Xenos vesparum within its paper wasp host, Polistes dominula.
A free-living male (top left) mates with the endoparasitic neotenic female via her extruded cephalothorax, which protrudes through the host cuticle (top right). The female produces first instar larvae that exit through the brood canal in the extruded cephalothorax. The free-living 1st instar larvae (center) seek a host within paper wasp nests. Upon entering a host, the 1st instars molt to apodous endoparasitic second instar larvae with a soft cuticle. The second instar larvae molt two additional times. Male 4th instar larvae (bottom left) molt to form a pupa, which extrudes from the abdomen of the host cuticle. At the end of pupal and adult development, the free-living male emerges from the ecdysed cuticles and from its host. Female 4th instar larvae (bottom right) do not undergo additional molts, but instead develop a cephalothorax, which is extruded through the host cuticle for the purposes of mating and release of 1st larvae.