This is what we have been preoccupied with the past few weeks. It started on a whim with one plant, but Mrs Monarch came back several times to lay which necessitated subsequent trips for additional swan plants (which is pretty much the only plant they eat).
If you ever want to learn anything new, just dive straight in. I've learnt so much by doing this than any book read beforehand could offer (though we have this adorable vintage book to keep us on track).
The caterpillars live on the terrace, which is a very convenient (not to mention safe and dry) spot for checking up on them.
I read somewhere that pumpkin is an acceptable substitute at a push, but only when the caterpillars are at their largest. Though I've since read that pumpkin lacks the chemical that is crucial for their metamorphosis so I've whipped it off them.
It's been quite something watching these critters develop from egg to chrysalis. Their lifecycle really is both mystery and miracle. Ten chrysalides and counting. I'll miss them when they go.