overlaps develops from mesothoracic segment mesothoracic tracheal primordium tracheal metamere 1 Note that terminal tracheation by each tracheal metamere extends beyond its segment of origin. According to Whitten (1980), the most anterior tracheal metamere corresponds to the prothoracic segment. tracheal hemisegment Tr1 fly_anatomy.ontology Metameric subdivision of the tracheal system that originates in the mesothoracic segment (T2). As well as tracheating the mesothoracic segment, the major trachea of this metamere extends into all segments anterior to T2. FBbt:00005026 dorsal ectoderm derivative thoracic tracheal metamere