Serial block-face scanning electron microscopy sheds new light on the head anatomy of an extremely miniaturized insect larva (Strepsiptera)

Abstract

Serial cross sections (80 nm) and an image stack (353 sections) of the head of the extremely miniaturized first instar larvae of Stylops ovinae (Stylopidae) were obtained with serial block-face scanning electron microscopy. This approach made it possible for the first time to reconstruct the head anatomy of a strepsipteran larva precisely, partly to cell level. The cephalic anatomy is described in detail, based on a 3D-reconstruction. It was possible to show the proportions of the exo- and endocuticle and the epidermis. Endoskeletal structures are the pharyngeal skeleton and the anterior tentorial arms, which form an articulation with the mandibles. The dorsoventrally flattened mandibles are the only functional mouthparts and used for penetrating the host's cuticle. The maxillae are fused with the head capsule to form the ventral plate, and the strongly modified and reduced labium functions as a closure of the ventral head opening. The nerves ending at the anterior head margin were reconstructed. The innervation of setae, olfactory pits and the vestigial maxillary palps are also documented for the first time. Five fat body cells are located in the head. Antennae, hypopharynx and labrum are not present as defined structures. A structure resembling a labrum is a secondary formation. Only nine cephalic muscles are present. Muscles of the antennae, hypopharynx, labrum and maxillae are missing. Some of the muscles originate in the thorax. Homology issues, possible phylogenetic implications, functional aspects of head structures, and also possible correlations of structural features with miniaturization and parasitism are discussed.