The ventral nerve cord of homozygous embryos appears wild type at the permissive temperature (17^oC) and homozygotes eclose. At the restrictive temperature (29^oC), homozygous embryos have a strong midline defect; the commissures appear fused and the connectives are closer to the midline than normal. sim^J1-47/sim² embryos raised at 17^oC show a mild fused commissure phenotype in the ventral nerve cord, while at 29^oC they show a complete collapse of the nervous system. Homozygous larvae raised at the permissive temperature (17^oC) have a normal sized anal pad and anal slit. Homozygous larvae raised at the restrictive temperature have a greatly reduced anal slit. The anal slit is shortened in sim^J1-47/sim² larvae raised at the permissive temperature (17^oC), while at the restrictive temperature (29^oC) the anal slit is completely missing. The anal pad cannot be seen in homozygous adult females (raised at the permissive temperature). The hindgut is not connected to the rectum and is frequently swollen. The thorn bristles and vulva appear to be missing. The ovaries are present but poorly developed. Oogenesis appears to start normally but stops prematurely. The anal plate, clasper and penis are missing in homozygous adult males (raised at the permissive temperature). Testis development is severely impaired. The hindgut is not properly connected to the rectum. The Malpighian tubules form normally. At 29^oC, sim^J1-47/sim² adults are not recovered. At 17^oC, rare escapers (0.1% of the expected number of flies) are seen. These flies are sterile and frequently lack external genitalia and the anal plate. Of flies without external defects, sim^J1-47/sim² males do not perform normal courtship behaviour and sim^J1-47/sim² females ignore wild-type males. Most sim^J1-47/sim² adults only walk in circles and they show abnormal responses in Buridan's paradigm; they show a markedly reduced walking speed, the activity period is usually shorter and they show no measurable influence from visual landmarks on their orientation behaviour. Optomotor tests indicate that the flies are not entirely blind. Leg movement appears to be coordinated normally, although step length differs between the right and left body side. The flies show defects associated with the inner chiasm of the optic lobe and in the central complex. In the central complex, the protocerebral bridge is thinner and less compact than normal in 70% of brains and the fan-shaped body is divided posterior-sagittally in 15% of brains. Ectopic fibre bundles cut into the lobula either unilaterally or bilaterally in 40% of optic lobes.