mmy^LM1 mutants display severe disruption of tracheal tube morphology, with constrictions and cyst-like expansions along the tracheal tubes in homozygous embryos. Early events in tracheal development, including tracheal sac formation and primary branch budding and outgrowth, appear normal in mmy^LM1/mmy^k13717b mutants. Defects are detected at late stage 14, when the dorsal trunk begins to dilate, where in the mutant, expansion occurs unevenly. Portions of the tube dilate poorly, whereas other regions expand too much. As expansion proceeds, the dorsal trunk becomes progressively more mis-shaped, with constricted and distended regions all along its length. In mmy^LM1 mutants, the thin matrix forms on the aplical surface of the tracheal epithelium just before expansion, as in wild-type, but small ridges, which contribute to the expansion, do not, with the mature tracheal epithelium having a thin epicuticle but virtually no procuticle or taenidia.