What causes the caterpillar to grow so fast?

An important part of a caterpillar's survival strategy is to grow rapidly because at this stage of their life-cycle many species are extremely vulnerable to predation and to have the best chance of survival they need to pupate within a short space of time. However, there are species that remain caterpillars for a prolonged length of time, some overwintering and in the case of the Goat moth caterpillar remaining inside the trunk of a tree for several years.