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.

Unlike wild-type, post stage 15 trachea in mmy^k13717b homozygotes are unable to exclude from their lumens, a fluorescently labelled 10kDa dextran injected into the body cavity. This is consistent with these trachea lacking a functioning septate junction barrier. The primary tracheal branches of these embryos exhibit dramatic diameter expansions and constrictions, but their length is normal.

Early tracheal development, including primary branch budding and outgrowth and lumen formation and morphology occur normally during stages 11-14. Stage 16 embryos show a number of tracheal defects. These include a reduced length of the transverse connective compared to wild type and severe defects in the diameter of the dorsal trunk and other primary branches. Irregular and variable tracheal tube morphologies, with local constrictions and/or dilations are seen in all major branches. Regions of tracheal tubes with more than twice the normal diameter alternate with severe constrictions. The defects are first seen at stage 14.