Behavioural mechanisms of nutrient balancing in Locusta migratoria nymphs

This study addresses the short-term behavioural mechanisms that underlie nutrient balancing by a herbivorous insect. Fifth instar Locusta migratoria nymphs were given one of four pairings of artificial foods, with each food containing different levels of protein and carbohydrate. The foods were chosen such that the treatment groups would have to ingest very different ratios of the foods to attain similar nutrient intakes. Over 6 days all four groups achieved a very close convergence of nutrient intakes, thus providing strong evidence that they simultaneously regulate both the quantity and balance of protein- and carbohydrate eaten, that is they show defence of an 'intake target'. Measurements of intake combined with detailed behavioural observations made over a 16-h period on the fifth day were used to investigate the behavioural mechanism by which insects attained this target. These revealed that the ratio of intake of foods was achieved by eating different numbers of meals on the two foods and not by eating different meal sizes. Differential meal numbers were, in turn, the result of switching between the two foods after different length runs of meals rather than any change in intervals between meals. The mean trajectory used to reach the intake target involved switching foods after 3.7 h and 4.6 meals. The functional consequences of such switching behaviour are considered. (C) 1995 The Association for the Study of Animal Behaviour