In stage 16 mmy^unspecified embryos, the trachea is aberrantly permeable to a fluorescently labelled 10kDa dextran in some stretches of the dorsal trunk, but not in others. At the end of embryogenesis, the dorsal trunk of mmy^unspecified mutants has a highly disorganized structure and displays constrictions and dilations. The luminal envelope is formed but there are no taenidial folds. Microvillae are still present, while such structures are undetectable by this stage in wild-type trachea. The tracheal lumen shows a detachment between the cell membranes and the envelope. mmy^unspecified embryos have no recognizable cuticular structures and no denticle belts or hairs. No chitin or N-acetylglucosamine is detectable in the epidermis of mmy^unspecified embryos. The procuticle, that is composed of stacks of highly organized chitin laminae, is missing and underlying epithelial cells appear detached in many places. However, the envelope and the epicuticle, which are chitin-free, maintain remnants of their structure.

Mutant embryos show a clear fusion of commissures and frequent disruption of longitudinal connectives. Midline glial cells are either reduce in number or fail to properly migrate in between anterior and posterior commissures. General embryonic morphology is relatively normal, though the PNS shows a slight reduction in sensory neurons and defects in the trajectories of motoneurons.

Fas2-positive nerve bundles frequently cross the midline in mutant embryos, in contrast to wild-type.

mmy^unspecified mutant embryos have poorly differentiated mouthparts and denticles.