Not for the squeamish!
If you're squeamish, you probably don't want to read about what's going on here.
This Hog Sphinx Moth caterpillar, seen here on Smartweed, has been parasitized by Cotesia congregata, a braconid wasp that lays its eggs in the caterpillar. (Many thanks to evanioidea for the species ID.) They are now pupating in the white cocoons hanging from its back. The caterpillar was still moving, but just barely.
There's no particular reason I should feel more sympathy for the caterpillar than for the wasp. In fact, sphinx moth caterpillars such as tobacco hornworm & tomato hornworm are agricultural pests, & such wasps are used as biological controls. In fact, this photo wasn't taken on a farm, but there are farm plots within a mile or so -- this is part of Cook College, Rutgers' ag school.) But it just seems like such a horrible death, eaten alive from the inside out. I'm a biologist & not particularly squeamish, but I find this photo both awfully cool and awfully disturbing.
There's some even cooler biology here. It turns out the wasp is aided by a virus (see en.wikipedia.org/wiki/Cotesia_congregata). The virus suppresses the caterpillar immune system, affects behavior by modulating the secretion of neuropeptides, and prevents metamorphosis. It is encoded in the wasp's nuclear DNA and expressed in specialized cells in its oviduct so that virions are injected into the host along with the egg. Being encoded in the wasp DNA, the virus has one of the largest genomes known of any virus (568 kb) and includes introns. It belongs to a genus called bracoviruses, which are found more broadly in braconid wasps. Similar viruses are found in the braconid's cousin, ichneumonids, though the two sets of viruses appear to have evolved independently.