Mutant embryos show grossly normal morphology, normal segmentation, normal epidermal denticle belts and normal patterning of the neuromusculature. They show greatly reduced movement compared to wild-type. Mature embryos show striking defects in individual muscle fibre morphology. The muscles are smaller and thinner than normal and spindly with irregular edges and bulges at the sites of the muscle nuclei. The location and width of the muscle attachments is normal, even in cases where the morphology is otherwise severely altered. The number of muscle nuclei is reduced compared to wild type. All muscles show a similar quantitative reduction in nuclei numbers. The ordered sarcomere structure seen in wild-type muscles is totally disrupted.