Regulation of body size during larval development in the German cockroach, Blattella germanica

It may be advantageous for insects to attain a certain standard size when they become adult. Recent studies have demonstrated that in some species of insects, a threshold size must be reached in the last instar in order that the adult will attain the standard size. It has been revealed also, however, that there may be another type of regulation of body size during larval development in the German cockroach. Head width and other characteristics were measured in every larval instar up to the adult. Body size is distributed clearly into a small group and a large group in the 5th instar, and even in the 4th instar. This suggests that a larva has already determined the number of following moults by the late 3rd instar. Adult size is almost the same in 5- and 6-instar types. Among individuals which require the same number of instars to reach the adult stage, the coefficient of variation in body size increases until the 3rd instar then decreases to the adult stage. Smaller individuals in a particular instar after the 3rd tend to grow more than larger ones during that instar. These results suggest that a larva regulates its body size after the 3rd instar in order to attain the ‘norm’ for adult size.