part of continuous with adult tracheal lateral trunk adult spiracular tracheal branch 1 prothoracic leg tracheal branch adult tracheal air sac propleural air sac A ventrally located air sac of the prothorax. It arises from the lateral trunk and extends ventrally just posterior to the first leg. The anterior thoracic spiracle opens into this air sac. FBbt:00003113 fly_anatomy.ontology