THE incidence of cell death among differentiating cells of the embryonic vertebrate central nervous system is well documented1–3, and there is evidence that in some cases its occurrence is influenced by the peripheral load encountered by the outgrowing fibres1–5. In vertebrate neural ontogeny both morphogenetic and histogenetic cell death have been observed6,7. Histogenetic cell death—which is involved in the differentiation of organs and tissues—has also been reported in the embryonic nervous systems of insects, for example, Tenebrio8 and the gall midge, Miastor (personal communication from S. Counce). It is reported here in the post-embryonically developing optic lobes of the larval and pupal brains of the endopterygote Danaus plexippus L. (Lept.) and is of special interest because it is possible to determine the age of a particular cell in this region and to observe cell and fibre areas of a wide range of ages within the same individual (our unpublished results).