Fig. 4. Choice data, expressed as a proportion, for each of 18 successive choice-trials. Naïve locusts were allowed to choose between two scented arms (lemon grass or peppermint), one containing four pieces of wheat, the other containing one piece of wheat. A total of 12 locusts was observed, and for each locust the odour/reward combination remained constant for the first nine trials (six locusts had lemon grass paired with the high reward; six had peppermint paired with the high reward). At the 10th trial, the odour/reward combinations were reversed. Data are also presented for the positive rewards combined. For choice-trials 10-18, choosing correctly would be indicated by small values. The dotted line indicates indifference to the positive and negative rewards.