The female cephalothorax of Xenos vesparum Rossi, 1793 (Strepsiptera: Xenidae)

The female cephalothorax of Xenos vesparum (Strepsiptera, Xenidae) is described and documented in detail. The female is enclosed by exuvia of the secondary and tertiary larval stages and forms a functional unit with them. Only the cephalothorax is protruding from the host's abdomen. The cephalothorax comprises the head and thorax, and the anterior half of the first abdominal segment. Adult females and the exuvia of the secondary larva display mandibles, vestigial antennae, a labral field, and a mouth opening. Vestiges of maxillae are also recognizable on the exuvia but almost completely reduced in the adult female. A birth opening is located between the head and prosternum of the exuvia of the secondary larva. A pair of spiracles is present in the posterolateral region of the cephalothorax. The musculature of the female cephalothorax is strongly reduced. Only muscles of the mandibles, foregut and a pair of longitudinal muscles are present. The nervous system is strongly flattened dorsoventrally. The brain is shifted to the prothoracic region together with the frontal ganglion. Well-developed optic nerves are present and vestiges of stemmata. The suboesophageal ganglion is fused with the thoracic and abdominal ganglia thus forming a compact undivided ganglionic mass. The dorsal vessel forms a ring-shaped structure around the brain. A valvula cardiaca is present between the posterior foregut and midgut. The midgut is strongly bloated and probably involved in inflating the cephalothorax during pheromone release of the female. The Nassonov's glands are located on the ventral side of the cephalothorax. Structural features of the females of X. vesparum are compared to conditions found in the head and thorax of the free-living females of Mengenillidae and cephalothoracic characters of Stylops ovinae (Stylopidae). The highly modified morphology of female Stylopidia is dicussed with respect to their permanent endoparasitism and also with their neotenous development.