Preimaginal conditioning in Drosophila revisited

During metamorphosis, the nervous system of a holometabolous insect changes significantly. Attempts to demonstrate preimaginal conditioning, here taken to mean the retention of learning through metamorphosis, have given mixed results. We used two behavioural assays (the T maze and trap assay) to see whether a change in adult responsiveness could be induced by exposing Drosophila melanogaster larvae to a conditioning stimulus. There was no evidence for preimaginal conditioning from either assay, but the trap assay demonstrated that menthol contamination from the larval environment on the puparial surface could induce a change in adult behaviour. Exposure of adult insects to this contamination could give the appearance of preimaginal conditioning, when in fact the behavioural induction occurred during the adult stage. Young flies responded less strongly than older flies to the odour cues in both assays. This may explain the apparently contradictory findings of some earlier studies of preimaginal conditioning.