|MadSci Network: Zoology|
How lucky you are to watch a caterpillar molting! Molting (also called shedding, or if you really want to get fancy, ecdysis, is the process many insects go through when they appear to be shucking off their exoskeleton, or outer covering. Caterpillars shed this 'dead' part with feet and all several times before going into the cocoon or chrysalis phase of metamorphosis (the feet aren't attached, just the outside-- imagine the part that falls off as an entire sheet around the whole caterpillar-- as if it were covered in Saran Wrap or something and it peeled off).
Each stage of the caterpillar's life between shedding events is called an 'instar.' Larval (or caterpillar) development is divided into several instars. Some swallowtails have five, other types of Lepidoptera (butterflies and moths) have different numbers of instars. This process doesn't hurt the caterpillars, but they may get kind of sluggish right before molting. You may have noticed that during parts of their development, caterpillars race around and eat like crazy, and at other times, they seem very slow and almost sick. This is normal, but it is hard to tell when caterpillars really are sick. Yours sounds healthy.
As for what kind of caterpillar you have, I really can't say. Without knowing where you live, I can't narrow down my choices to Northeastern US or Southwestern, and so on. However, here are some great web sites which may help.
This site will help you determine what kind of caterpillar you have. This way, if you live in Minnesota, you won't be fooled into thinking you have a caterpillar only found on Hawaii. Remember, caterpillars can also be moth larvae, so don't be too frustrated if you don't find yours on butterfly sites!
Another site that might help you identify which kind of caterpillar you found.
http://www3.pei.sympatico.ca/oehlkew/ A moth page, just in case.
http://www.chebucto.ns.ca/Environment/NHR/lepidoptera.html more on butterflies and moths
Enjoy metamorphosis! It's one of my favorite things in the world, and never ceases to amaze me.
Try the links in the MadSci Library for more information on Zoology.