Right from the first moment, when the egg is laid by the Monarch Butterfly female, the DNA map with instructions on everything they need to know and do throughout their life is built into the tiny egg that is less than 1mm in size.
When the caterpillar has developed to it's emerging size, inside the egg, it eats a hole in the shell and climbs out.
Then it turns and eats the egg shell - its first nourishment.
After that it begins to eat the flesh of the milkweed leaf that the egg was laid on.
From that point it continues to eat milkweed leaves, as it sheds its skin right through all the five instar sizes, until it forms its chrysalis.