Inner view of ventral area of a single segment of "Julus", much enlarged to show the structure and arrangement of the tracheal organs. The two pairs of tracheae are seen in situ, the posterior pair overlapping the anterior. *"h", Posterior margin of the body- ring (tergum). *"r", Anterior border *"st", Tubular chamber of tracheae *"t", Fine tracheae given off from it. *"ms", Respiratory muscle attached to tracheal sac. *"m", Ventral body muscle