Xenos vesparum (Strepsiptera: Xenidae)—A New Insect Model and Its Endoparasitic Secondary Larva

Abstract Xenos vesparum of the highly specialized Strepsiptera is a new insect model in the context of host-parasite relationships.The endoparasitic female and male secondary larvae were studied using µCT, 3D-reconstructions, histology, and photomicrography.The infectious primary larva is followed by a trophic and endoparasitic secondary larval stage. In contrast to immature stages of other holometabolous groups, the second instar increases dramatically in size. Compound eyes and external wing anlagen are present in male larvae before the pupal stage. In contrast to the females, the brain of males bears well-developed optic neuropils and retinula cells are present.The cephalothorax is comparatively simple in the female larvae, yet distinctly more complex than in the adult, where most muscles are reduced. Large testes are present in male larvae and numerous oocytes in the females, but they are still immature.The larval features are discussed in the context of holometabolous development and heterochronic processes. Unique features of Strepsiptera are the early differentiation of the sexes and the occurrence of compound eyes and external wing anlagen in male secondary larvae. The phylogenetic position of Strepsiptera suggests that this is a secondary feature and thus an autapomorphy. To address mature females of Stylopidia as neotenic adults is an oversimplification.They display a mosaic pattern of paedomorphic characters such as features of the nervous system and the presence of stemmata, but also non-paedomorphic structures, such as the spiracles.