Abstract 1. 1. Using hoppers of the two locust species, Locusta migratoria migratorioides (R. & F.), and Schistocerca gregaria Forsk., an investigation was carried out of the rate at which the ability to group with other hoppers in a uniform environment (social aggregation) was learned and the rate at which it was lost when rearing in isolation followed training in crowds. 2. 2. Individual fourth-instar hoppers were observed in an evenly heated and lighted arena that contained two groups of tethered decoy hoppers, of the same species as the one being tested. The time spent near the decoys and the types of reaction shown by the test hopper as the result of contact with the decoys was measured. 3. 3. For training, a number of formerly isolated hoppers were placed in a cage with one part of the floor heated. Under these conditions the hoppers crowded on to the warmed spot and so received training in social aggregation. 4. 4. When trained for 4 hours or 24 hours the measured aspects of social aggregation in Locusta hoppers increased, but not to the levels of typical gregarious hoppers. Schistocerca hoppers trained for 4 hours behaved like gregarious hoppers. 5. 5. When a period of isolation followed training, behaviour reverted part way, but never completely towards that of solitary hoppers. The reversion was more pronounced in Locusta than Schistocerca, where it was negligible. 6. 6. The implications of the irreversible nature of these aspects of social aggregation in Schistocerca are discussed.
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