Parasitic worms paint warning colours on their hosts using glowing bacteria

A robin flying over a field sees a juicy caterpillar on a leaf. It dives in for a closer look but it notices something strange: this larva is bright red and glowing slightly. Red means danger – this caterpillar is probably toxic and is best avoided. The robin leaves; the caterpillar apparently lives. But this particular caterpillar is already dead, and its corpse has been protected by two unseen and unlikely partners. Its warning colours are not its own – they were painted on by parasites.