Many hemisegments show severe reduction in the number of serotonin neuron: mutants exhibit a single cell instead of the normal mitotic pair in thoracic and abdominal segments. nub and zfh2 staining reveals the more medial serotonin cell of the mitotic pair is lost: the EW1 neuron. 15% pupae survive to become viable adults, adults exhibit reduced number of serotonin neurons in the ventral cord. Both larvae and flies appear normal for phototaxis and geotaxis. The adults are much less active in locomotion. Flight is impossible due to the extended wings. Adults also have difficulty holding onto glass surfaces when climbing vertically or when rotated upside down, flies have no obvious morphological defects in the legs or tarsi.

Homozygotes have a 'wings held out' phenotype. Interneuronal projections derived from the NB 7-3 neuroblast show severe pathfinding defects, failing to cross the midline and showing variable orientations. The motorneuron derived from NB 7-3 appears to have a normal axonal projection. NB 7-3 clones contain more cells than normal in the thorax. Serotonergic cells are almost entirely lacking in the central nervous system of eg^P289 first instar larvae.