Homozygous embryos appear morphologically normal, but fail to initiate peristaltic movements at the end of embryogenesis and therefore fail to hatch (this homozygous embryonic lethality may be due to second site lethal mutations, rather than the nab^A23 lesion). nab^A23/Df(3L)GN19 animals die during the first to second larval instars. The larvae initiate fewer movements than normal and have slow muscle contractions. nab^e310/nab^A23 and nab^G26/nab^A23 animals survive larval development, and most of them pupate. Most of the pupae eclose, although about 20% are unable to completely escape the pupal case and become trapped. The eclosed adults appear morphologically normal and show grooming behaviour. However, when they try to walk they are extremely uncoordinated and fall often, and if they land on their backs, a few of the adults are able to right themselves with considerable difficulty. The adults do not fly or jump and die within 24-48 hours of eclosion, presumably from an inability to feed. Homozygous embryos lack EL (eve lateral) neurons in many abdominal segments. The mean number of EL neurons per abdominal hemisegment is 4.6 in homozygous embryos compared to 8.1 in wild-type embryos, and the number of abdominal hemisegments with less than 7 EL neurons is 92.50% in homozygous embryos, compared to 7.68% in wild-type embryos.