Schematic of tracheal development. (a) At embryonic stage 11, the placodes have invaginated and are ready to extend stereotypically. (b) At stage 12, the branches begin extension. (c) At stage 13, the branches have fully extended and begin to fuse. (d) By stage 16, the tracheal fusion is complete. Progenitor cells called tracheoblasts that become activated during metamorphosis remodeling are located in the spiracular branch (SB). (e) Schematic of typical tracheal branch development from embryonic stage 12 to stage 13 to stage 16, after the completion of tracheal branching and fusion and to the completion of metamorphosis remodeling. DB, dorsal branch; DT, dorsal trunk; GB, ganglionic branch; LT, lateral trunk; TC, transverse connective; VB, visceral branch.