The rest of the story is that this caterpillar is immune to the powerful toxins found in legumes, and indeed uses these poisons to defend itself against attacks by predators. If this reminds you of some other famous cases think about monarchs and milkweeds, and zebra butterflies and passion-vines. So here is another case of a larval insect stage making itself toxic by eating a poisonous plant. Consider what this means- that the ability to tolerate the toxins must have evolved over a very long period, and then coloration advertising the toxicity (aposematic or warning coloration) developed. Then predators must recognize the significance of the warning coloration, perhaps specifically or generically. Some predators such as lynx spiders apparently can handle the toxins and eat the caterpillars (see this article). Now the adult is distinctively colored (see photo from web) but I do not know if it is protected also.
So if we needed reminding that the natural world is amazingly complex and interesting, here is another case. The deeper we look the more we see, and the more impressive the web of life becomes. So get out there and groove on nature but don't eat the caterpillars!
-Bill Dunson, Englewood, FL