Humidity as an aversive stimulus in learning in Drosophila melanogaster.

The learned suppression of photopositive tendencies was studied at the individual level in young flies of both sexes. In a T-maze, flies had to choose between an arm leading to a lighted vial associated with an aversive stimulus (i.e., a solution of quinine chlorhydrate deposited on a filter paper in the vial) and another arm leading to a darkened vial free of quinine. The present experiments were carried out to determine the roles of quinine and relative humidity in the maze. The flies avoided the lighted vial containing quinine even if they had no tarsal contact with quinine, and this result was not due to any odor of quinine. Subsequent experiments showed that relative humidity in the lighted vial, and probably in the arm leading to it, was an aversive stimulus, which partly explains why the flies avoided the lighted vial. However, in conditions in which the flies had tarsal contact with water or quinine it was confirmed that flies trained with quinine have higher avoidance scores than those trained with water only. Moreover, individual aversion to humidity was not correlated with the individual avoidance score: At similar levels of motivation (i.e., similar levels of aversion to humidity), some flies learn to avoid the lighted vial containing quinine whereas others do not. All these results show that, in addition to quinine, humidity is an unconditioned aversive stimulus in our paradigm and thus needs to be tightly controlled in experiments of conditioned avoidance.