Fig. 6. Sequential control of lumen expansion and lumen clearance in the Drosophila tracheal system. At stage 14 of embryonic development, chitin (red) coordinates the radial expansion of the lumen by forming, upon secretion, a transient fibrillar chitin-based matrix (red lines). These chitin cables are also required for the normal organization of the apical β_H spectrin cytoskeleton (blue). At stage 16 of embryonic development, septate junction proteins are required for the apical secretion of Verm, which, together with Serp, is required for modifying the chitin matrix to prevent the tracheal tubes from becoming too long (Wang et al., 2006). At stage 17 of embryonic development, Wurst is essential for remodeling the chitin matrix and for clearing the tracheal airway.