Combinations of unconditioned (US) and conditioned (CS) stimuli experienced by the bees during training. Each rectangular box indicates the contents of the flight arena at the beginning of each trial, for bees in the different treatments (represented by columns) in each experimental phase (top and bottom rows for phases 1 and 2, respectively). We studied the effect on learning of four reinforcement schedules, characterized by different choices of the US-. The US- could be water (W) or quinine (Q) during the whole experiment, or change from one to the other halfway through the experiment, accounting for the four experimental treatments: WW, WQ, QW and QQ. Regardless of the treatment, bees entering the arena encountered eight distractor flowers (US- column in each rectangular box), each one containing ca. 25 µl of the US- (represented by “filled cups”), and eight target flowers (US+ column). Four target flowers were empty (empty cups) and the other four contained ca. 25 µl of the US+ (sucrose solution – filled cups). Note that, in the experiment, distractor and target flowers were haphazardly distributed throughout the arena. Distractor and target flowers were identified by the colour squares (CS+ and CS-) on which they were set (represented by the cream and reddish parallelograms in the figure). The squares were cut from Canson Mi-Teintes cardboard (refs. 133 and 336). Forty bees were allocated to the different treatments, 10 bees per treatment. Within each experimental group, colour #133 was the CS+ for five of the bees and the CS- for the other five.