Some species have a pupal stage that lasts for two years.
Butterflies, moths, beetles, flies and bees have complete metamorphosis. Flying comes in handy.
Inside of the pupa, the caterpillar is rapidly changing. Patience is required in order to find them because they are small.
It also likes the warm weather. Butterfly eggs are usually laid on the leaves of plants, so if you are actively searching for these very tiny eggs, you will have to take some time and examine quite a few leaves in order to find some. During this time, a hardened case forms around the pupa to protect it from predators and extreme weather conditions.
References Ballard, K. And inside, the tissue, limbs and organs of the caterpillar transform.
It will stay like that for a little while so that it can rest from all the work of coming out of the chrysalis. The coolest thing about butterfly eggs, especially monarch butterfly eggs, is that if you look close enough you can actually see the tiny caterpillar growing inside of it.