The respiratory system of growing caterpillars is challenged in two distinct ways as they develop from hatchlings to fifth instars preparing for pupation. First, across instars, body sizes and tracheal lengths increase substantially. Second, within each instar, animal mass can more than double while major tracheal respiratory system structures, such as spiracles and large tracheae, are fixed in size until molting. To test whether these growth processes result in a decrease in O2 delivery capacity relative to tissue oxygen needs, we exposed feeding Manduca sexta larvae of various ages to decreasing levels of atmospheric O2 and measured their metabolic rate and ability to feed. We found that near the beginning of all instars, M. sexta were able to maintain gas exchange and feed down to approximately 5 kPa O2, indicating that these insects are able to match tracheal O2 delivery to increased metabolic rates across instars. However, gas exchange and feeding of caterpillars nearing the molt were limited at much higher O2 levels (up to 15 kPa O2), suggesting that caterpillars have limited capacities to increase tracheal O2 delivery as O2 consumption rates increase within instars. It seems possible that the safety margin for O2 delivery may disappear completely in the last hours before ecdysis, providing an ultimate if not proximate explanation for the necessity of molting.