Fortunately this monarch caterpillar does not eat basil or mint plants. I'd noticed that the mint plant was looking a bit moth eaten, a closer inspection didn't reveal anything untoward. The next day the mint plant was nearly decimated, and this time I could see the (by now large) caterpillars. After I squashed them all, I thought I had better check the basil plants, and sure enough, there were some holes in the leaves, but I could see the caterpillar (more squashing followed).
It was interesting to note that the caterpillars that had been feeding on the mint did not smell of mint when squashed, however the basil eating caterpillars did smell of basil when squashed. (although I would not want them in my pesto, or at least I wouldn't want to know about it - could be a good protein addition to the pesto though)