HORMONALLY mediated changes in behaviour patterns are found in many animal species, including adult insects(1-4), raising the possibility that the neurological modifications linked with such changes could be mediated by morphogenetic hormones. Neuronal plasticity and neurogenesis in the adult brain have rarely been reported in vertebrates(5-10), and in adult insects only modifications of mushroom body volumes or fibres have been described(11-14). Here we report the presence of undifferentiated cells, located dorsally in mushroom bodies of the house cricket Acheta domesticus, that persist, divide and give rise to cortical interneurons during adult life. Furthermore, juvenile hormone, which induces oviposition behaviour in adult crickets(2,3), acts on neurogenesis in this cell cluster. Females deprived of juvenile hormone showed a significant decrease in cell division, which was reversed by hormone injection. Production of juvenile hormone therefore appears to be a major factor in the adaptation of the brain to concomitant physiological and behavioural changes in adult insects.