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For what it's worth, quite a few caterpillars of common species are cannibalistic. But it remains to be determined whether they do so in order to get some "meat" in their diet, or because it eliminates the competition -- too many caterpillars on the same hostplant at the same time can strip it entirely, causing all to starve. Eating others of your species whenever you run across them is a workable way to thin out the "herd" so that there's enough hostplant left to go around.

Our back yard is a certified National Wildlife Foundation habitat for butterflies/moths (not as hard to get as it might sound), and we raise caterpillars to help boost local populations (most species are on the decline because of habitat destruction). We've released over a hundred adult butterflies so far this year, and have about another hundred currently in the "pipeline". Keeping some of them from eating each other is part of the process. The Polydamas Swallowtails are especially bad in that regard -- we've lost over a dozen that way so far.