Printed in Great Britain © Company of Biologists Limited 1981 Projections from sensory neurons developing at ectopic sites in insects

This paper reviews recent experiments which attempt to gain more understanding about the recognition processes involved in the formation of neuronal connexions by studying the degree of specificity with which sensory neurons form their central connexions. This is done by generating ectopic neurons (either by transplantation or by genetic mutation) whose axons grow into novel regions of the central nervous system, and then examining their projections and synapses. The sensory systems reviewed are: the Antennapedia, spineless-aristapedia, proboscipedia, and bithorax homeotic mutants of Drosphila melanogaster; the cercus-to-giant interneuron system of crickets, and the wind-sensitive hair system of locusts. The results show that ectopic neurons form projections that are discrete and characteristic, not random and chaotic. In those cases where single classes of sensilla have been studied, they follow either their normal CNS pathways or those pathways normally used by their segmental homologues. Ectopic sensory neurons can also form appropriate functional connexions in some cases but not in others. Possible reasons are-discussed, but detailed understanding of the underlying events requires further experimentation