green leaf on brown wooden table

Monarch caterpillar, a few days old, first- or second-instar. The eggs are laid on the underside of a Common Milkweed leaf. When the caterpillar hatches, it starts by eating its egg case, then eats the fuzz on the underside of the leaf, then eats down into the flesh of the leaf. The ruler's markings are in inches. This baby was about 0.2" or 5mm long.