Isometric growth of larval insect midgut predicts that the ratio of midgut surface area to body mass decreases as larvae grow. Gut tissue and gut content masses were measured in first through fifth instar Manduca sexta larvae. Wet mass of gut tissue increased in relationship to body mass with a scaling exponent of 0.85 compared to an exponent of 1.33 for gut content mass, suggesting that surface area becomes increasingly limiting in larger larvae. To test the hypothesis that compensation for the decrease in relative surface area of the midgut occurs by increased expression of membrane proteins, we compared midgut mRNA expression in fourth and fifth instar. Surveyed genes encoded apical membrane proteins with diverse functions, including the potassium amino acid transporter KAAT1, ion channel CAATCH1, aminopeptidase msAPN3, V-type H-ATPase E subunit, and cation chloride cotransporter masBSC. KAAT1 was expressed 300- to 1500-fold higher in middle and posterior midgut compared to anterior midgut. Expression of msAPN3 was approximately 200-fold higher in posterior midgut than middle midgut. Expression of KAAT1 was 2.3- to 3.1-fold higher in fifth compared to fourth-instar larvae, and masBSC expression was 1.3- to 1.9-fold higher in fifth-instar larvae. Expression of msAPN3 and V-ATPase, but not KAAT1, decreased as body mass increased within the fifth instar. Although the increased expression of KAAT1 and masBSC in fifth-instar larvae supports the hypothesis of increased membrane protein expression in larger larvae, results from the other genes do not support this hypothesis.

Metabolic rate of tobacco hawkmoth (Manduca sexta) larvae increases with body mass to the approximately 0.90 power, greater than the 0.67 and 0.75 scaling exponents predicted by isometric increase of surface area and fractal geometry of exchange networks. We evaluated whether increased midgut folding and/or nutrient transporter expression in larger larvae contribute to this discrepancy. In 3rd through 5th instar larvae, midgut wet mass scaled with an estimated exponent of 0.85. Exponents for anterior, middle, and posterior midgut were 0.96, 0.70 and 0.85, respectively. Composite micrographs of transverse sections revealed progressively increased folding of the midgut perimeter in 1st through 5th instar larvae. These results suggest that relative midgut surface area declines less than predicted by theoretical models as larvae grow. Also, expression of mRNA encoding a potassium amino acid transporter (KAAT1) increased 2 to 3‐fold in 5th instar compared to 4th instar larvae. To assess the consequences of altering the expression of KAAT1, we used RNA interference to decrease midgut KAAT1 expression. Delivery of a 453bp dsRNA encoding KAAT1 through direct injection into the hemolymph resulted in an approximately 6‐fold reduction in KAAT1 mRNA expression, while oral delivery in a sucrose droplet did not reduce KAAT1 expression. Knockdown of KAAT1 mRNA expression did not affect larval growth. (NSF‐DMS0827208)