Caterpillars have a tough life and the ways in which they die are often unpleasant to say the least. Many are parasitized, often by Braconid wasps or Tachinid flies. Here, a Braconid wasp has laid eggs on/in the caterpillar. The larvae have emerged from their living host and spun cocoons. The caterpillar was still alive, amazingly, but only just. This is the caterpillar of the hummingbird sphinx moth, a familiar sight as it is dayflying. Here is a photo of the moth: https://pbase.com/laroseforest/image/125091780