Fig. 2. Requirement of Wg, Hh and Egfr signalling for posterior spiracle formation. (A) Cuticle of a wild-type larva. (B) Magnified view of the posterior spiracle. The spiracular chamber cells are differentiated into filzkörpers (white arrow), which are located within a protruding dome-shaped structure, the stigmatophore. (C,E,G) No posterior spiracles are present in wg^CX4 (C), hh^IJ35 (E) and Egfr^f2 (G) mutants. (D,F,H) Cuticles of thermosensitive alleles of wg, hh, Egfr shifted to restrictive temperature from 5-8 hours of development. Differentiated filzkörpers are present in wg^IL114 embryos (D), stigmatophores are present in hh^ts2 embryos (F) and complete posterior spiracles are present in Egfr^tsla embryos (H), despite severe segment polarity defects. In wg^IL114 embryos, filzkörpers are often at abnormal positions, which reflects the absence of attachment to the disorganised mutant tracheal network.