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.

The morphology of the free‐living females of Strepsiptera (Insecta)

“…In the following, external and internal morphological features of females of Mengenillidae are compared with the character states found in those of Corioxenidae (Pohl & Beutel, 2008), Stylops ovinae Noskiewicz & Poluszyński, 1928 (Löwe et al, 2016) and Xenos vesparum Rossi, 1793(Richter et al, 2017). The musculature is also compared to that of female secondary larvae of X. vesparum (Weingardt et al, 2023), as it is very strongly reduced in adult females of Stylopidia. Furthermore, the morphology of males of E. laboulbenei and M. moldrzyki , and of the male tertiary larva of E. laboulbenei and a female late instar of the extinct † Mengea sp.…”

“…(cited after Kinzelbach, 1971). However, Malpighian tubules were not found in the late female larvae of X. vesparum (Weingardt et al, 2023). A lower number or absence of Malpighian tubules is a possible apomorphy of females of Stylopidia or Stylopiformia.…”

“…Ovaries and oviducts are absent in all known adult females of Strepsiptera, and the eggs float freely in the hemolymph. Weingardt et al (2023) showed that the ovaries are already absent in the late secondary larvae of X. vesparum . However, the oocytes are still regularly arranged in two longitudinal series.…”

“…Interestingly, some features like optic lobes, and nerves of the antennae and legs are still retained in stylopidian females, although the corresponding appendages are vestigial or completely reduced. Individual abdominal ganglia observed by Weingardt et al (2023) in secondary larvae of X. vesparum are still present in adult females of Stylopidia (Lauterbach, 1954). They are clearly absent in Mengenillidae.…”

Diversity and Biology of the Indian Strepsiptera with a Pictorial Key

“…The females are strongly morphologically simplified and their legs, wings, and external genitalia are completely absent. They exhibit a mosaic pattern of paedomorphic characters such as features of the nervous system and the presence of stemmata (the single-lensed eyes positioned bilaterally on the female head) but also non-paedomorphic structures such as the spiracles (Weingardt et al, 2023). Endoparasitic existence of female depends exclusively on the insect hosts throughout the course of their lives (except, the Mengenillidae).…”