Enzyme activities and mitochondrial substrate oxidation were measured in the posterior midguts of tobacco hornworms at different larval stages. A comparison of third, fourth, and fifth instars demonstrated no differences in citrate synthase activity (per gram of midgut), indicating that maximal aerobic capacity is unchanged during larval growth. The midguts of fifth instars had the highest activity of 3-hydroxyacyl-CoA dehydrogenase and carnitine acetyltransferase. Furthermore, mitochondria isolated from the midguts of fifth instar larvae had higher rates of palmitoyl carnitine oxidation when compared to mitochondria isolated from the fourth instar larvae. These results indicate that there is an increased preference for fatty acid oxidation in the midguts of fifth instar larvae. The presence of succinyl-CoA ketotransferase and acetoacetylCoA thiolase as well as the mitochondrial oxidation of acetoacetate and b-hydroxybutyrate indicate that ketone bodies can serve as substrates for the midgut. The activities of malic enzyme and glucose-6-phosphate dehydrogenase were significantly higher in fifth instars than in fourth instars, indicating an increased capacity for lipid synthesis. o 1994 ~i~e y -~i s s , Inc.The growth rate of larval lepidoptera far exceeds that of young mammals and birds (Reynolds, '90). One caterpillar, the tobacco hornworm, increases in mass nearly 10,000-fold in approximately 16 days and over 80% of this growth occurs in the fifth and final instar (Goodman et al., '85; Reynolds, '90). ' RI achieve this phenomenal growth rate, the tobacco hornworm has a high rate of food consumption. For example, a fifth instar tobacco hornworm eats 1.36 times its weight per day (Reynolds, '90). Food is digested and absorbed by the midgut, which fills much of the body cavity. As the larva grows so must the midgut. Although the larval mass increases continuously during the intermolt period, the midgut increases in size only during larval molting (Baldwin and Hakim, '91).During the molt, new cells are added to the epithelium so by the end of the molt the midgut has increased its cell number by $-fold. Over the course of four larval molts the midgut would increase in mass 256-fold. Because neither the size nor the type of epithelial cells changes during growth of the midgut, the surface area of the midgut cells also increases by 256-fold. This 256-fold increase in midgut surface area is far less than the 10,000-fold increase in mass observed for the whole larva and, therefore, older larvae have smaller midgut surface area to body mass ratio than younger larvae. The absolute growth rate of the caterpillar (g.hourl.anima1-'), however, is the greatest in the last instar. The larvae gain 6 mg 0 1994 WILEY-LISS, INC.in the first 48 hr of the first instar, whereas fifth instar larvae gain 4.3 g over the same time period (Goodman et al., '85). This represents a 717-fold increase in growth rate with only a 256-fold increase in midgut surface area, indicating that the growth rate per midgut surface area is larger in...

Ion transport and metabolism in the posterior midgut before, during and after the molt to the fifth instar of the tobacco hornworm Manduca sexta were investigated. In situ measurements reveal that the transepithelial potential difference of the posterior midgut falls during the molting process. This finding was confirmed by in vitro experiments in which it was demonstrated that both the transepithelial potential and the short-circuit current are lower in molting fourth instars compared with feeding fourth instars. The short-circuit current increases after ecdysis, with a maximal rate being achieved approximately 4 h after the molt. Resumption of feeding after the molt is not necessary to initiate this increase in active ion transport. The metabolic organization of the tissue also changes during the molting process. The maximal activities of glycolytic enzymes and 3-hydroxyacyl-CoA dehydrogenase, an enzyme of lipid ss-oxidation, decrease during the molting process and increase after ecdysis. Although citrate synthase activity, an index of maximal aerobic capacity, decreases during the molt and increases again after ecdysis, tissue respiration is the same in feeding fourth instars and molting larvae. This result indicates that a greater percentage of maximal aerobic capacity is used during molting and that energy may be diverted to cell proliferation and differentiation and away from the support of active ion transport at this time.