Morphology of the head of Mydas clavatus (Diptera: Mydidae)

The external and internal morphology of the head capsule and mouthparts of M. clavatus (Diptera : Mydaidae) are described by light and scanning electron microscopy. The face bears a prominent postclypeal region, and a conspicuous anteclypeus. Large air sacs occupy a major portion of the internal head volume. Internally, there is a welldeveloped tentorium. Paired labellar glands are found within the head capsule. Three major head pumps are present. The mouthparts consist of the labrum, reduced maxillae, and labium. Labellar lobes are well developed and bear rows of pseudotracheae. The fly feeds by extending the mouthparts by means of muscular contractions, and spreading the large furca to expose the pseudotracheae to the substrate. Previous authors assumed that Mydaidae was a predaceous family. The structure of the mouthparts of M. clavatus is indicative of nectar-feeding habits. M. clavatus has a fragile hypopharynx and both labrum and hypopharynx are incapable of being manoeuvred, ruling out the possible role of predation. In the field, flies were observed to feed only on nectar. M. clavatus lacks mandibles in both males and females and has a movable hinged fulcrum similar to the cyclorraphous Diptera. The labella show a different organization than that found in blood-sucking and nectar-feeding Tabanoidea as well as Cyclorrapha. M. clavatus on the other hand has a more primitive arrangement of the labial muscles and sclerites. M. clavatus is unique in having long internal pseudotracheal tubes, an esophageal dilator originating on an air sac, and cibarial protractors that originate on the subgenae.