part of continuous with adult tracheal lateral trunk mesothoracic leg tracheal branch adult mediosternal tracheal branch adult tracheal air sac sternopleural air sac A ventrally located air sac of the adult mesothorax, lying above the base of the mesothoracic leg. It arises from the lateral trunk and extends into the base of the mesothoracic leg. A trachea that extends into the mediosternal region arises from the sternopleural air sac. FBbt:00003117 fly_anatomy.ontology