Clypeo-cibarial apparatus, proboscis mobility, and cephalic foregut structure of Bombyliidae (Diptera)

The clypeus of Bombyliidae is subdivided into five morpho-functional parts: median centroclypeus, two lateral “membranes,” and two sclerotized zones. The centroclypeus and lateral membranes are formed by solid sclerotized cuticle and are apparently non-extensible. The dorsal hinge, around which the centroclypeus turns with respect to the head capsule, is an intraclypeal structure formed by rigid cuticle. The true membranes occur only in the distal portion of the clypeus and also at the bases of the labrum and labium. In the long-proboscid bombyliids, the labial base lies deep in the head cavity when the proboscis is in the resting position. As in the previously studied members of Stratiomyidae, the precerebral portion of bombyliid foregut shifts over a considerable distance during the movements of the proboscis. This makes the functioning of the radial pharyngeal dilators impossible, and these muscles disappear, while the typical sclerotized pharynx is modified into an oesophagus-like tube. The formation of the fulcrum is usually related to the development of the clypeus mobility. This process occurred repeatedly in several taxonomically distant dipteran groups.