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To address the relationship between the body size and physiology of the larval tobacco hornworm, Manduca sexta, we examined its metabolic rate, food conversion indices and protein expression across the caterpillars instars. In order to understand the relationship between body size and metabolic rate, we examined the inter-individual variation in this relationship in 20 caterpillars. We found dramatic variation in the mass exponent between the different individuals ranging from a mass exponent of 0.68 to 0.95 (mean = 0.82 +/- 0.07 SD). The mass exponent was 0.83 when determined by combining the data from all 20 caterpillars. Based upon the amount and energy of the food eaten, feces produced, and weight gain, the effect of body size on food conversion indices were calculated daily for 4th and 5th instar M. sexta. Frass from late 5th instar caterpillars had 8% less energy per unit weight when measured by bomb calorimetry than frass from 4th instar and early 5th instar caterpillars. Preliminary data also suggest a difference in energy extraction efficiency from the 4th to 5th instar. To begin understanding these phenomena at a molecular level, we also investigated the expression level of an ion transport protein, masBSC (a cation-chloride cotransporter), across the late instars. Preliminary western blot data suggest that protein concentration of masBSC increases from the 4th to the 5th instars. (Supported by the Kenyon Summer Science Scholar Fellowships.)