In Cht2Scer\UAS.cTa; Scer\GAL4btl.PS embryos, the lumen of the dorsal trunk of the tracheal system fails to expand uniformly during stage 15, remaining constricted at branch fusions, and the tube becomes excessively overgrown between fusion junctions. This overgrowth continues through stages 15 and 16 resulting in dilated, elongated and irregularly shaped dorsal trunk.