Thank you.

So are they poisonous because they feed on nightshade, or are they safe to handle?

I found it interesting that the adults feed on moonflower nectar. I'm actually growing that right now.

I believe they feed on nightshade so that they won't taste good when something tries to eat them.

The adults are very pretty moths, also very large. I hope you get some to come to your flowers. Once the caterpillars are big enough (these ones look like they'll pupate soon) they'll drop into the soil and burrow down to pupate, then should emerge as adults in a few weeks. You could always put them in some sort of container with food and dirt and watch the process.

Here's a really interesting website from UW Madison with tons of info:

http://manduca.entomology.wisc.edu/about/lifecycle.html