The bs^ex84 mutant affects the outgrowth of the first terminal branches of dorsal branches that form at the end of embryogenesis.

The size of the primary branches of the tracheal system is largely unaffected in mutant embryos.

Severe tracheal phenotype. Major tracheal branches and connections between branches are normal, but terminal branches were truncated or absent. In larvae, the parental branches end abruptly without ramification. Surviving larvae can move about but are sluggish. Many contain dark tissue masses typically associated with the fat body, possibly due to necrosis of the fat body caused by inadequate oxygen supply. The terminal cells of mutant embryos do not have long projections extending to their targets. No muscle defects can be detected in mutant embryos.