part of thoracic segment adult thoracic segment adult spiracle adult thoracic spiracle Manning and Krasnow (1993) assign the anterior spiracle to the mesothoracic segment, as we have done here, despite recording its origin as being from tracheoblasts associated with the humeral (dorsal prothoracic) disc. A spiracle of the adult thorax. There are 2 pairs of these, in segments T2 and T3. Each has a two lipped external rim that can be closed by the action of an associated spiracular occlusor muscle (FBbt:00013335). fly_anatomy.ontology FBbt:00014814