C Elliott's picture of ragwort and caterpillars catapulted my memory back to my school-days when we used to race these stripy jobs up the classroom windows. So I couldn't resist snapping this shot on a walk up in Mortimer's Forest. Unfortunately there was no window handy to set up a race.
As Chris pointed out the plant is highly toxic, causing liver damage. The poison is so powerful that the plants should not be handled, because the toxins can be absorbed through the skin. Given the caterpillars are pretty much made from the plant perhaps I should have treated them with a touch more respect as a schoolboy :)