Monarch Caterpillar

A Monarch is a caterpillar for about 2 weeks. Then it grows its pupal skin and begins to metamorphose. During metamorphosis, a great deal of the caterpillar's body dies (it is actually digested by its own digestive enzymes) and becomes liquid. There are a few key bits of tissue left, however, in different areas of the body which are not digested, called "imaginal buds" or "histoblasts", and these contain the information needed to rebuild the digested "soup" into a butterfly body.
Gordon's 1st Lepidoptera Page - (Butterflies)