Figure 2.2 Geometric approach to nutrient intake in fifth-instar nymphs of Locusta migratoria fed a wide range of artificial diets.

Note: Nutrient consumption is shown as a bivariate plot of protein and carbohydrate consumption. Crosses indicate the intake target reached in experiments with various choices of foods (the circle is a separate estimate of the intake target). Asterisks indicate the growth target reached in no-choice experiments on different diets. Boxes at the end of the rails give the proportions of carbohydrate to protein. The intake target is close to a 1 :1 ratio.

Source: Simpson and Raubenheimer (1993). Philosophical Transactions of the Royal Society of London B. 342, 381-402, The Royal Society Publications.

excess of one nutrient or insufficient amounts of the other. But if the insect is allowed to choose between two foods containing different nutrient ratios (i.e. defining different rails), it will be able to attain any target lying within the nutrient space between the rails. This can be demonstrated by offering various combinations of paired diets.

Alternatively, nutrient balance can be achieved post-ingestively by removing nutrients which are in excess of metabolic requirements: this enables an insect to move across nutrient space from the intake target to the growth target (Fig. 2.2). This is also important in cases when the unbalanced foods are not complementary and it is impossible to reach the intake target. Post-ingestive aspects of nutrition can be examined by constructing utilization plots of nutrient output versus intake; a change in slope indicates the point above which ingested nutrient is not utilized (Fig. 2.3). Bicoordinate utilization plots

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