Wandering where to pupate?

Snail over at A Snail's Eye View has written about Danaus plexippus, the Monarch or Wanderer butterfly here. What better opportunity to post a photo of the lovely larva! It's sitting on the seed pod of one of the species' host plants, Gomphocarpus, the swan plant, and appears to be in some sort of stand-off with a hemipteran of some description.

It's not a species I'm overly familiar with and in fact I think this is one of the only times I've ever seen the caterpillar.