The pattern of somatic ventral muscles is severely disrupted in mutant embryos, and muscles 25 and 29 are completely absent. The syncytial fibres at the normal positions of muscles 26 and 27 have aberrant and variable morphologies, although their insertion sites and sizes are still reminiscent of the two muscles. It cannot be determined whether they form two separate or one large syncytium in each segment. The progenitors of muscles 26 and 27 appear to form normally. Disruptions are also seen in lateral and dorsolateral areas of the body wall musculature; muscle 5 is absent, whereas muscle 8 appears to be duplicated in each of the segments. At the normal positions of muscles 11 and 18, there is one syncytium (or perhaps two closely associated ones) with a morphology that does not resemble either of these two dorsolateral muscles, although it shares their ventral attachment site. The pattern of dorsal somatic muscles is normal. Additional lateral and ventral adult muscle precursor cells are formed. Most homozygotes or hemizygotes die during different stages of development, but approximately 10% survive to adulthood under favourable conditions. These escapers are very weak, unable to reproduce and have a short life span. The flies show sluggish movements and the majority have outspread, upheld or drooping wing postures. Freshly eclosed adult escapers have a duplicated muscle 8 in every abdominal hemisegment. The ventral and lateral adult muscle groups have essentially the same number of muscle fibres as in wild-type flies (despite the supernumerary lateral and ventral adult muscle precursor cells that are formed in embryos). The flies have a full complement of direct and indirect flight muscles and their overall morphology closely resembles that of wild-type flies.