Behind Every Great Butterfly is a Great Caterpillar
Every butterfly you see is technically a senior citizen. It has already lived most of its life, mainly in a form that is very different from the butterfly. Butterflies lay eggs, like other insects. From each egg hatches a caterpillar, the larval stage of a butterfly. Its sole purpose in life is to find its host plant and consume as much of it as it possibly can!
Caterpillars are obviously much different than butterflies. Wingless, they get around on three pairs of jointed, or "true" legs, and five pairs of prolegs. Caterpillars are equipped with special chewing mouthparts, silk producing glands, and possibly defensive structures, such as hairs or spines. As the caterpillar eats it grows, but its growth is limited by the size of its unstretchable skin. To get around this limitation, it molts - sheds its own skin. Each caterpillar will molt several times during its life; the stage in between each molt is called an instar. As you might expect, the second instar is larger than the first, the third is larger than the second, and so on.
The very last molt produces, not another instar, but a pupa. Butterfly pupae have a smooth outer covering, called the chrysalis. Some butterfly pupae are additionally covered with strands of silk forming a cocoon. Within the pupa, metamorphosis takes place. This is the process where the entire body of the insect is literally rearranged to produce an adult butterfly. When complete, the butterfly emerges from the chrysalis by crawling out of a split in the pupa's skin, and prepares its wings for flight. Then it's off to find food and love!