develops into cuboidal/columnar epithelium dorsal air sac dorsal air sac primordium Primordium formed by tracheoblasts that bud from the tracheal branch which adheres to the wing disc during the late larval stage. During pupal stages this primordium forms an epithelial sac. Cells at the tip of this primordium show extensive protrusive activity - producing filopodia and lamellopodia - while maintaining epithelial integrity (including adherens junctions with their neighbors). fly_anatomy.ontology FBbt:00007135 air sac primordium