Fig. 1. Drosophila tracheal and leg primordia derive from closely associated populations of cells. (A) The Drosophila tracheal system visualized by the 2A12 antibody. (B) The tracheal system arises from 10 tracheal placodes (arrowheads on first and last placode) that express the trh gene; trh is also expressed in the salivary glands (sg). (C) Staining for trh in the tracheal placodes, and Dll in cephalic structures and leg primordia (arrows) in a stage 11 embryo. (D) Higher magnification view, showing the proximity of thoracic tracheal placodes and leg primordia. (E) Staining for trh expression (by means of an enhancer trap insertion in the gene) and btd expression in a stage 9/10 embryo. (F) Higher magnification view, showing the close apposition of trh- and btd-expressing cells.