A Luna moth caterpillar spends its life eating the leaves of a host tree and grows to more than ten times its original length in about six weeks before it pupates. The adult which emerges some time later has no functioning mouthparts and only survives for about a week.
What possible reason could explain this seemingly self-destructive strategy?
After metamorphosis, an adult gains two important features: flying and mating. It specializes to its comparative advantage: dispersing its offspring. A moth that finds a new source of food succeeds not by eating, a waste of its expensive adult features, but by making new caterpillars which can eat many times more efficiently. The moth's purpose, after all, is to reproduce, and it cannot afford to do anything that interferes with that mission.
The moth follows a very simple program: eat a fixed amount, then search for new opportunities. While rigid (as it can only do one or the other, and it has to determine the right strategy ahead of time with limited information), this strategy, properly calibrated to the moth's environment, apparently is successful.
Various species which undergo metamorphosis have been shown to change the body size at which they transform in response to environmental cues. I am assuming this is not the case; the strategy does not require it.