This Tobacco Hornworm has been colonized by one of it's natural predators, the Braconid Wasp. If you find a Caterpillar in this state, please don't kill it. It is still being used as a food source of the larva of this beneficial, non-stinging (for people anyway) wasp. The white little protrusions on the caterpillar's back are the cocoons in which the larvae are pupating into wasps. Their tiny eggs hatch on the caterpillar's skin & the larva burrow into the caterpillar, eating it's muscle tissue, leaving it's heart & organs intact, so that it can remain alive long enough for all of the wasp larvae to finish their larval stage & set up cocoons on it's back.
Once they pupate into mature Braconid Wasps, they fly off to mate & lay eggs on other Tobacco Hornworms, there by controlling the population of many a tomato growers' bane.