Histological structure of clypeo-cibarial and labral skeleton and musculature in Nematocera (Diptera). 3. Bibionomorpha

The labrum in the Bibionomorpha studied is developed to a different extent. It is elongated in Bolitophila, Sylvicola, and Bibio, relatively short in many mycetophilids, and very short in Keroplatus. Its outer wall is usually sclerotized and rarely membranous (Keroplatus, Scatopse, and Sylvicola). The labro-epipharyngeal muscle fibers are found in the majority of species studied. They are mostly located at the labral base and in Bolitophila, through the whole labrum. Except for Keroplatus, there exists a clypeo-labral muscle acting perhaps as a labral levator. The clypeus either has a form of completely sclerotized shield strongly fused with the frons and genae (Bolitophila, Macrocera, Scatopse); or it consists of a sclerotized shield-like centroclypeus surrounded with its membranous zone (Mycetophillidae, Bradysia); or its sclerotized portion is horseshoe-shaped and also surrounded with a membrane, except for a narrow sclerotized stripe connecting it with the frons (Bibio, Sylvicola). Correspondingly, there are three types of clypeo-cibarial junction: distal rod-shaped, central lamellar, and central rod-shaped, the latter being similar to the fulcrum of brachyceran Diptera. Epithelial clypeo-cibarial junction is absent. There is a tentorio-cibarial muscle in all the studied species of Bibionomorpha, whereas a very feeble cranio-cibarial muscle was found only in Boletina and Scartopse.