part of labral segment pharyngeal sclerite median tooth unpaired labrum FBbt:00001861 A grey, shovel-like structure that marks the site where the posterior roof of the atrium merges with the inner layer of the dorsal pouch. It is on the midline of the atrium, at the level of the anterior processes of the H-piece (hypostomal sclerite). It is characteristic of the first larval stage. fly_anatomy.ontology larval labrum cuticular specialization