All small animals that have hard skin and six legs get bigger by getting rid of their old skin and making a new skin hard. Young of these animals just have parts for feeding and growing, but older animals need parts for having sex and making babies -- and sometimes don't need parts for feeding any more.
Some animals grow up by changing little by little but others change themselves completely at one point. When this happens, almost all of their inside parts actually die and a whole new animal grows again from just a few tiny bits of their inside parts. When the parts die the stuff they were made out of is used by the parts still alive to grow all of the new inside parts.
Often the new animal looks very different from its young form and may have body parts like wings and sex parts that the younger form didn't have. Often these new animals don't live very long -- just long enough to have sex and then lay eggs or have babies -- and then die.