This is poor Jeffrey IV (as named by Little Miss). He is one of the caterpillars she 'rescued' from our kale plants. Shortly after she had created a nice little home for them, he moved to the side of the container and started sprouting grubs. Little Miss did a bit of research and deduced that he was possessed by parasitic wasps.
Warning: The video is pretty graphic!
A bodily invasion
The tiny larva lurks inside the flesh of the caterpillar, soaking up its host's nutrients and drinking its blood. The wasp larva must keep its host alive, so it avoids damaging the vital organs.
The caterpillar swells as it eats, not knowing what lies ahead.
When the larva is ready to break out, it releases chemicals that paralyse the caterpillar. With its host stuck, the larva uses specialised, saw-like teeth to eat its way through the thick skin.
Remarkably, the wounded caterpillar does not always die. Some species even watch over the newly free larvae until they spin their own cocoons, ready to become adult wasps.
It's too late for poor Jeffrey IV, but it is very interesting to watch the process.