Ive, we haven't talked about gut purging in a while and I think that may be the black stuff your child decided was "yucky and cool". Kids say the most awesome things, by the way. I thought my caterpillar was dying the first time I saw it happen. You see, when a Swallowtail (as far as I know, only Swallowtails do this) is ready to make a chrysalis it goes through a process. It will find a place to settle down, usually somewhere it can stand up vertically, but not always. It will stay there for several hours, not eating, but pooping. Then, after a couple of hours comes "the gut purge". This is when the caterpillar flushes everything out of its digestive system. Cool fact - even some of its internal organs that are no longer needed are expelled! Some caterpillars take on a more translucent look after the gut purge because of this. Then, the caterpillar enters a "wandering phase" for several hours, looking for that perfect place to make a chrysalis. If you have a lot of caterpillars in one cage sometimes you have to move the wandering one to another cage because they will crawl over everyone and everything and generally make a nuisance of themselves. Then, they find that perfect spot and settle down. Then, they build the sling, as your one caterpillar did. Once they've built the sling, it takes about 24 hours for them to make the chrysalis. So if you saw them build the sling in early afternoon, chances are they will build their chrysalis in early afternoon of the next day. Non-Swallowtail caterpillars can do this part of the process a little quicker, in most cases. But all Swallowtails gut purge and it is just one of the many non-glamorous parts of the job I love explaining to the guests at the museum who think what I do is "so magical". Actually, it's science!
And for a little more science...while in the chrysalis, the caterpillar essentially breaks down into a liquid! This is one reason they expel those internal organs in the gut purge - they don't take anything not needed with them into the chrysalis. Their whole body reforms and it's only the last couple of days before they emerge than you can even make out the butterfly shape. When they come out, they pump fluid into their wings but will also expel the meconium (waste products) they have been storing while in the chrysalis. Metamorphosis is crazy cool! Except for when my Sulphur excreted some meconium on my t-shirt at the museum the other day. I sprayed it with Oxy Clean; it should be fine.
As an aside, I've watched caterpillars for so long, I can tell when they're getting ready to gut purge. Sometimes I move them to their own cage or tupperware to do it so they don't get the whole cage (and the food) all messy. You see, their frass (poop) actually starts to look a little different in consistency; it's hard to explain if you haven't seen it. It's actually a little sad I know this much about caterpillar poo. But at least I have a heads-up for when I need to move someone.
