Thanks for the ID and caution about the stinging. I had no idea they could sting

I thought my wife would be repulsed by the swarming mass of caterpillars when I took her out to show her the willow but before we went outside I had shown her a photo of the butterfly they would become. When she saw the willow she wanted to know if we could take a caterpillar and raise it in captivity so that we could watch it become a butterfly. Can this be done with this species? How would we go about raising it and how long would it take?

click on my name to find it in my profile. A small one wouldn't take more than a couple of weeks, a large one close to pupation much less. Handle the foliage it's on, not the caterpillar, to avoid the stinging hairs, and you should be fine.

Thank you for your help Hannah. Our rearing cage is built and now has its first inhabitant. It will be exciting if this works for us. From your description, it sounds like lots of things can go wrong.