In the ventral nerve cords of stage 16 eg18B mutant embryos, there is a loss of ability to take up serotonin in approximately 70% of hemisegments.
Lethality is during late embryonic and early larval stages. Many hemisegments show loss of serotonin neurons: mutants exhibit a single cell instead of the normal mitotic pair. nub and zfh2 staining reveals the more medial serotonin cell of the mitotic pair is lost: the EW1 neuron.
Serotonergic cells are almost entirely lacking in the central nervous system of eg18B first instar larvae.