Learning and social aggregation in locust hoppers

Abstract

1. 1. When locusts swarm, the hoppers (nymphs) live together in bands that depend on the individuals being attracted towards one another (phase gregaria, obtained in the laboratory by rearing hoppers together in crowds). They are then said to aggregate socially. This behaviour pattern can be measured in the laboratory in a cage that provides a physically uniform environment. In non-swarming periods, the scattered hoppers (phase solitaria, obtained in the laboratory by rearing hoppers in isolation) are not attracted towards one another when first confined together, If hoppers that have been reared in isolation are forcibly crowded together they learn to aggregate socially within a few hours. The types of stimuli between hoppers that are of importance during the process of learning to aggregate socially were studied in the two locust species Locust migratoria migratorioides (R. & F.) and Schistocerca gregaria (Forsk.). 2. 2. Single hoppers reared with a crowd of non-gregarious grasshoppers or woodlice (Isopods) aggregated socially when first placed together. 3. 3. By rearing single hoppers in transparent boxes placed in a cage full of other hoppers, direct tactile contact with other hoppers was eliminated, but visual, auditory and olfactory stimuli were received from the others. In such box-reared hoppers, social aggregation was increased a little in Locusta, but not at all in Schistocerca. 4. 4. Previously isolated hoppers showed increased grouping after being crowded with other hoppers for only seven hours. Crowding in the dark produced similar results to crowding in the light. Social aggregation was also increased in hoppers that were reared isolated but touched with fine wires for a seven-hour training period before being tested; they grouped almost as well as hoppers that had been crowded with other locusts. 5. 5. The behaviour patterns shown by crowded and by previously isolated hoppers when they first meet others are described. Both types of hopper examined objects, including other locusts, with the antennae and palps, but they differed in behaviour when they were themselves touched. Previously isolated hoppers moved away; crowded hoppers remained where they were and kicked with the hind legs or twirled the antennae. During the process of learning to aggregate socially, previously isolated hoppers appear first to become habituated to being touched. This is followed, after further contacts, by hind-leg kicking and antenna twirling, and these probably play a positive role in keeping the hoppers together in groups. The complete learning process can be brought about by touching isolated hoppers repeatedly with fine wires before they are put with other locusts. 6. 6. In contrast to locusts, grasshoppers do not aggregate socially even when reared in crowds. 7. 7. Records of colour changes were kept during the experiments. Phase gregaria hoppers are typically black and orange or yellow, whilst phase solitaria hoppers are mainly green or fawn. The various treatments suggested that phase gregaria colouring depended on a complex of stimuli between hoppers, through the auditory, visual, olfactory and tactile sense organs. 8. 8. On the whole, the two species of locust gave similar results. The process of learning to aggregate socially was a little more rapid in Schistocerca than in Locusta.