The larval form of the monarch, or caterpillar as it is more commonly called, lives by eating various forms of milkweed plant. The come from an egg that is about the size of a pinhead that is laid on the underside of a milkweed leaf. The small caterpillars begin by eating the leaf that the egg was attached to. As they eat they grow and develop, shedding their skin numerous times. Each one of these developmental periods is called an instar. The sap from the milkweed is toxic to many insects and birds which makes the monarch rather unpalatable to most predators. The plant that this caterpillar is chewing on is commonly called butterfly weed and it is a species of milkweed.