In a "Nature Is Seriously Nasty" update, here's the latest of my garden's Hornworms:
The little white things are the cocoons from parasitic wasps. Eggs laid under the hornworm's skin, they eat their way out and build their cocoons before hatching out as new hornworm-hunting wasps. You're meant to leave the hornworms alone to go through this cycle as the wasps are beneficial, so we've had two like this on a tomato plant for 4 or 5 days.
Last night they both vanished, I'm assuming the whole gross thing became a bird's dinner. All that was left was a single spot of dark green hornworm gloop.