|MadSci Network: Zoology|
I searched through all sorts of books trying to figure out what kind of moth or butterfly your cocoon was going to hatch out into. Frustratingly, I couldn't find anything. When I have no hope of figuring something out on my own, I often call up my wise friend Nancy, who has a PhD in entomology, and an incredible amount of knowledge about how life works. She seems to be able to answer almost anything I ask her. Sure enough, Nancy had a pretty good idea about what your cocoon was!
She said that the insect is a moth, not a butterfly. More specifically, she believes it is the moth named Callosamia promethea. Here is a picture of these pretty moths.
The moth will probably remain inside the cocoon for the rest of the winter. Only when it's warm outside (or if it's brought inside where it's warm) will the moth finally emerge from the cocoon. There are a couple of dangers that the hidden moth must survive while in the cocoon. One danger is an attack by parasites. You can tell if the cocoon has parasites in it by examining the surface of the cocoon. If there are holes in it, then it is likely to be parasitized. In that case the moth will never emerge as an adult.
A second danger is that a bird will eat it. If the post you nailed the cocoon on is in an exposed area, this might increase the chances that birds will eat it as a tasty snack. You might want to move the cocoon onto a tree where it would be more camouflaged.
You could bring the cocoon inside to hatch, but this would mean that it would not survive the winter outside. If you decide to bring it inside and hatch it early, then you should be sure to place it in a large container containing branches for the moth to climb up on and hang up-side- down from. The moth must hang up-side-down after it emerges from the cocoon to straighten out its wings and let them harden appropriately.
If you do hatch out the cocoon inside, either now or in the spring, when it's more likely to survive, see this other MAD Scientist essay I wrote to another person interested in cocoons that Nancy helped me out with.
If you have any more questions, please don't hesitate to ask!
Try the links in the MadSci Library for more information on Zoology.