Saddled Prominent Caterpillar

Interesting! Here's another link to a similar behavior by a caterpillar: http://www.projectnoah.org/spottings/20986064

John is correct, the "ant" is a parasitic wasp in the family Ichneumonidae. Here is a short snippet of some research done on the parasites of H. guttivitta: http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=8548423 The large black spots on the side of the caterpillar are due to parasites. This is a shot in the dark, but perhaps the parasite of H. guttivitta is similar to the behavior of Coleomegilla maculata and Dinocampus coccinellae which is well exhibited in Karen's spotting: http://www.projectnoah.org/spottings/31333074 Did the caterpillar seem physically attached to the caterpillar as if it couldn't move itself from it?

Great spotting Rieko, and fantastic info from John La Salle.

RiekoS. Very nice series - and I am not exactly sure what type of behaviour is going on here, but it seems pretty cool. First, not sure what the sac is - but it is not a caterpillar egg sac because the caterpillars are not mature (reproductive) adults and thus aren't capable of laying eggs. That leaves three possibilities: 1) the sac contains parasites of the caterpillar which have emerged and spun up cocoons underneath the caterpillar, and left the moribund caterpillar weak and near death (I have seen this many times before, but never with the cocoons enclosed in the sac-like structure you see here), 2) the caterpillar just happens to have wandered onto the egg sac of some other creature such as a spider egg sac, 3) some other weird biology that I have no idea about. As to the "ant" in the pictures - it is not an ant, it is a parasitic wasp. If you look closely at the video you will see that it is sticking its ovipositor (a thin egg-laying tube) into the sac - probably in an attempt to lay eggs on whatever is in the sac. I am not sure what this is, but I suspect it is a wingless Ichneumonidae. Just as an example, here is a link to a wingless ichneumonid that parasitises spider eggs as well as the cocoons of other parasitic wasps http://bugtracks.wordpress.com/2011/07/13/spider-parasitoids/ As a suggestion, just toss the sac into a jar with a bit of paper towel to prevent condensation and see what comes out.