as far as i know caterpillars of any kind do not eat eachother, are you sure they arent just getting out at night? i had that problem a couple of times with mine, i had to hunt them down the next day. maybe theres a way for them to get out, check your habitat for any possible escape routes.

Thanks! I wondered about that, but I keep them in jars with lids. When they get bigger I put screens on the top. I have also never seen them on the sides of the plastic jars, only on leaves. Odd....

I had this issue just recently with my black swallowtail caterpillars, and it turned out there was a spider hiding in the cage that was picking them off one by one.

Some kinds of caterpillars do eat each other occasionally.

If caterpillars have no food for what they consider too long, they will turn on each other. Depends on the species how bad it will get and how quickly it happens. Cecropias will usually just nibble on somebody's tubercles. Tobacco hornworms will finish off whomever is the smallest.

And then there are the zebra swallowtails who will eat each other whether there are a zillion leaves or not.

My experience is monarchs of similar size going after each other is very rare but I've gathered that it is more likely if you have crowded conditions.