Female monarch butterflies lay 300-500 eggs over 2-5 weeks of egg laying. Monarch females usually lay a single egg on the underside of a milkweed leaf, near the top of the plant. As females lay their eggs, they secrete a small amount of glue to attach the eggs directly to the plant. Eggs are only about the size of a pinhead or pencil tip and are off-white or yellow, characterized by longitudinal ridges that run from the tip to the base. The eggs hatch in 4-5 days after they are laid, depending on the temperature.
The newly hatched larva feeds voraciously on the milkweed, accumulating bitter chemicals from the host plant that help protect the monarch from predation by birds. Over the next few weeks, the caterpillar grows from 1/16th of an inch to about 2 inches in length, increasing its weight by a factor of 2,700. To accommodate this rapid growth, the caterpillar must shed its distinctively striped skin several times before it is ready for the next stage of its development.
A mature caterpillar usually leaves the milkweed to seek out a bare branch or similar sturdy surface. It attaches itself by spinning a silk anchor known as the cremaster from which it hangs upside down in the shape of the letter “j”. After settling down, the caterpillar sheds its skin a final time, revealing a beautiful green chrysalis decorated with delicate gold spots.