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First published online September 9, 2005
Journal of Experimental Biology 208, 3463-3473 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01767

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Variable rewards and discrimination ability in an insect herbivore: what and how does a hungry locust learn?

Spencer T. Behmer^1,2,*, Corlisa E. Belt² and Martin S. Shapiro³

¹ Department of Entomology, Texas A&M University, College Station, TX 77843-2475, USA
² Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK
³ Department of Psychology, California State University-Fresno, Fresno, CA 93740-8019, USA

View larger version (28K): [in a new window]   Fig. 1. Diagram of the two-sided Y-maze. Moveable gates separate area X from areas A and B. Gates also separate areas A and B from the removable arms.

View larger version (19K): [in a new window]   Fig. 2. Choice data, expressed as a proportion, for each of nine successive choice-trials. Naïve locusts were allowed to choose between two coloured arms (green or yellow), one containing four pieces of wheat, the other containing one piece of wheat. A total of 12 locusts were observed, and for each locust the colour/reward combination remained constant over the entire experiment (six locusts had the high reward paired with green, while six had the high reward paired with yellow). Data are also presented for the positive rewards combined. The dotted line indicates indifference to the positive and negative rewards.

View larger version (15K): [in a new window]   Fig. 3. The mean (± S.E.M.) natural log latency of response for the positive and negative colours over the course of training. Three measurements were recorded: (A) enter time, (B) approach time and (C) exit time.

View larger version (13K): [in a new window]   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.

View larger version (22K): [in a new window]   Fig. 5. The mean (± S.E.M.) natural log latency of response for the positive and negative odours over the course of training. At the 10th trial, the odour/reward combinations were reversed. Three measurements were recorded: (A) enter time, (B) approach time and (C) exit time.

View larger version (19K): [in a new window]   Fig. 6. Choice data, expressed as a proportion, for each of nine successive choice-trials. Naïve locusts were allowed to choose between two scented arms (lemon grass or peppermint), one containing p21:c21 food, the other containing p7:c7 food (see the Methods and Results section for a full description of these foods). A total of 12 locusts was observed, and for each locust the odour/reward combination remained constant over the entire experiment (six locusts had the high reward paired with green, while six had the high reward paired with yellow). Data are also presented for the positive rewards combined. The dotted line indicates indifference to the positive and negative rewards.

View larger version (14K): [in a new window]   Fig. 7. The mean (± S.E.M.) natural log latency of response for the positive and negative odours, in response to synthetic foods, over the course of training. Three measurements were recorded: (A) enter time, (B) approach time and (C) exit time.