Flies carrying Hsap\ACTBAct88F.PB in an Act88F6 background are unable to fly. Sarcomeric organisation including Z-discs is completely missing in the indirect flight muscles (IFMs) and thin filaments appear to be absent; the IFMs have randomly distributed myosin thick filaments with no apparent hexagonal array of actin containing thin filaments surrounding them. Occasionally, indications of an M-line-like structure can be discerned.