Flies expressing Hsap\ACTB^Act88F.PB in an Act88F⁶ background have thin and thick filaments which are bundled together into myofibrillar-like structures in their indirect flight muscles (IFMs). However, they are not marked by regular sarcomeric repeats (Z-discs and M-lines) and frequently split or taper. The thin filaments interdigitate with thick filaments in an orderly fashion and "rigor chevrons" comparable to wild type are seen on the thin filaments in rigor. The polarity of the chevrons is uniform along the entire length of a particular thin filament, however, along the adjacent thin filament, the chevrons may be oriented in the opposite direction. Amorphous, dense "Z-bodies" are seen instead of Z-discs. In some areas, the Z-bodies are not integrated into myofibrils and occur as isolated dense bodies with tangles of short thin filaments emerging from them. Some other Z-bodies are narrow and extend axially over 200-300nm, while others are so well aligned laterally that they appear to be a Z-disc. However, unlike in wild type, only a few thin filaments appear anchored to each Z-body, which is separated from the neighbouring one by a gap. In a few cases, axially repeated Z-discs delineate a rudimentary sarcomere. These sarcomeres are on average 2μm in length. The myofibrils have no regular boundary in cross-section; central areas showing hexagonal packing of the thin and thick filaments are seen surrounded by a disorganised packing of the filaments. The modified terminal Z-discs (MT-Zs) of the myotendon junction are absent, leaving the myofibrils unconnected to the tendon cell and therefore, the cuticle. Dense junctions are seen between the membranes of the muscle and tendon cells and the tendon cell morphology is almost normal.