Caterpillars of the Cinnabar Moth, loads of them, feeding on Ragwort at the edge of a building site.  I knew the plant was poisonous to horses*** but hadn't realised that the bright stripes of this caterpillar warn predators that they're going to taste awful.  Even better, their bodies store the poison and allow them protection through chrysalis and into moth.

***unfortunately this means that a lot of Ragwort is pulled up, depriving the Cinnabar Moth of places to lay their eggs.