Insects breathe through a network of epithelial tubes, called trachea, that transport gases around their bodies. In Drosophila, the uniform expansion of these tubes during development requires the assembly of a transient intraluminal chitin matrix. Now, on p. 163, Moussian et al. report that chitin filament assembly depends on Knickkopf (Knk) and Retroactive (Rtv), proteins that are also involved in the formation of the fly cuticle. The researchers describe how knk and rtv mutants develop severe tracheal tube size defects similar to those seen in chitin-deficient embryos, and show that Knk, an apical GPI-linked protein, is mislocalized in tube expansion mutants in which septate junction proteins are disrupted. The researchers propose that septate junctions, which resemble vertebrate tight junctions, ensure the correct distribution of the components needed for chitin filament assembly and so ensure the uniform expansion of the trachea.