The female cephalothorax of stylops ovinae Noskiewicz & Poluszyński, 1928 (Strepsiptera: Stylopidae)

Abstract

External and internal features of the female cephalothorax of the strepsipteran species Stylops ovinae (Stylopidae) are described in detail. Many derived features are closely related with the obligatory endoparasitism. A conspicuous characteristic is the secondary tagmosis with an anterior cephalothorax and a large, sack-shaped posterior body region. The cephalothorax comprises the head, thorax and anterior half of abdominal segment I and protrudes from the host's abdomen. It contains the mouth and birth opening, vestiges of cephalic appendages, and most internal organs including the large Nassonov's glands. The posterior body part contains mainly the reproductive organs. The constriction of abdominal segment I probably prevents the exposed anterior body from slipping back into host's body cavity. The narrow mouth opening is used for the uptake of the host's hemolymph by the secondary larval stage and is maintained as a non-functional structure by the adult females. The tentorium is absent. The mandibles are immobilized in the adult female. They are used for penetrating the host's body wall by the second larval instar. The brood canal formed by the larval and pupal exuvia is an autapomorphy of Stylopidia, the birth opening where the first instars are released from the female an autapomorphy of Stylopiformia, and the cephalothoracic invagination an autapomorphy of Stylops. A single pair of functional spiracles is laterally placed on abdominal segment I. An unusual circular hemolymph vessel is present in the pharyngeal region. The brain is shifted to the thorax. The suboesophageal complex and the thoracic and abdominal ganglia form a single ganglionic mass in the thoracic region. The musculature of the cephalothorax is extremely reduced. Nassonov's glands, which produce sex pheromones, are the largest internal structures in the cephalothorax. Despite of the loss of the digestive function, the digestive tract is not degenerated including a foregut with well-developed muscles.