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Social Aggregation and Gregarious Behaviour in Hoppers of Locusta Migratoria Migratorioides (R. & F.) 1)

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Under field conditions bands of Locststa migratoria migratorioides (R. & F.) nymphs (hoppers) carry out a daytime wandering called marching, which is preceded by a period of basking in groups. The formation of these basking groups may depend entirely upon the hoppers reacting in a similar way to such environmental conditions as sunny and wind-sheltered spots, although many field workers have expressed the view that the hoppers also react positively to one another. The coming together of the hoppers as the result of interactions between the individuals is called social aggregation. In the field, it is difficult to separate social aggregation from aggregation due to a patchy environment, but in the laboratory such separation is possible. Laboratory hoppers spend some time resting in groups before marching begins and social aggregation was studied during this pre-marching resting period. When locust populations are low, individual hoppers rarely meet other individuals of their species. It has been suggested that such isolated hoppers have no strong attraction for others of their species, although such attraction develops if the hoppers are forcibly crowded. During this study therefore, the behaviour of hoppers reared in crowds was compared with that of hoppers reared in isolation. Two experimental methods were used. In the first a ring-like cage, lighted from above was used. The physical conditions within this cage were reasonably uniform over its whole area. The floor was divided into a number of equal divisions, by pencil marks, and some 15 hoppers were tested together in the cage. After settling down, the number of hoppers per cage division were counted and if the hoppers were distributed at random, then the number of areas containing 0, 1, 2, 3 and more hoppers would follow a POISSON series. The actual results were compared with the POISSON: in addition, the numbers of hoppers in groups (as estimated by the numbers that rested in cage division containing 2 and more hoppers) were analysed in the usual way. Using this technique, it was found that the proportion of test hoppers which aggregated socially depended upon rearing conditions. Significantly more 2nd instar hoppers that had been reared crowded, aggregated socially, than similar hoppers reared isolated from hatching (comparative figures were 50-75 % of crowded hoppers aggregated, but only 30-45 % of isolated hoppers did so). Even hoppers reared in isolation aggregated more with one another than the nymphs of the 2 grasshopper species Anacridium aegyptium and Cyrtacanthacris tatarica (20-45 % aggregated). Social aggregation would therefore appear to be a specific characteristic of Locusta migratoria migratorioides, which becomes greatly intensified as the result of the hoppers being crowded. In 2nd instar hoppers the colour of the individuals at hatching and the parental rearing conditions did not appear to affect the proportion showing social aggregation, although there was some evidence that these factors had some effect on the behaviour of 1st instar hoppers. When formerly isolated hoppers were crowded, a considerable increase in the numbers showing social aggregation took place after 24 hrs, whilst after 4 days, social aggregation was the same in these hoppers as in others that had been reared crowded from hatching. On the other hand, when formerly crowded hoppers were isolated for as long as 8 days, only a small reduction in the number aggregating socially took place. This means therefore that Locusta hoppers rapidly become conditioned to one another as the result of forced crowding but such conditioning is less easily lost. Newly hatched hoppers did not aggregate with one another, even when crowded. Behaviour changed very suddenly, suggesting that very young hoppers had to await the maturation of sense organs or parts of the nervous system involved in hopper interactions, before showing social aggregation. This indicates the importance of hopper reactions to a patchy environment, which results in the forced crowding of hoppers less than 3 days old and in hoppers that have formerly lived in isolation. Social aggregation, even in crowded hoppers was sometimes influenced by environmental factors. The hopper interactions leading to aggregation were equally well shown at air temperatures from 24-35° C, but were reduced at very high temperatures, in very dry air, in the absence of radiant heat or in the absence of light. In the second experimental method, individual hoppers were offered a choice of 3 decoys (3 tethered live hoppers, 3 pieces of white fibre board and 3 pieces of black fibre board) evenly spaced in a circular cage, lighted from above. The number of times that hoppers settled in each 1/3rd of the cage and within 2 hopper lengths (aggregated with) each decoy were counted and compared with the chance expectations. Both measures of behaviour gave similar results. Hoppers crowded from hatching or for only 4 days aggregated in over 50% of cases and always preferred the live hopper decoy. Hoppers reared isolated, on the other hand, aggregated in less than 45 % of cases and always preferred the black inanimate decoys. When formerly crowded hoppers were isolated they still showed a preference (although a reduced one) for tethered hopper decoys. It appears that a fundamental change in the behaviour of Locusta hoppers towards objects in the environment takes place: other live hoppers replace conspicuous. inanimate objects. It is hoped to report later on an analysis of the interactions between hoppers during social aggregations.

Affiliations: 1: (Anti-Locust Research Centre, London and Department of Zoology, University College, London

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