Keep off!

Lurid advertising tells potential predators that these caterpillars are not a  sensible meal choice.
They are the larvae of the cinnabar moth and they derive their unpalatability from their toxic food plant, ragwort.

They are employing the strategy known as aposematism (meaning 'off sign' in ancient Greek) - instead of using subtle camouflage to blend in with their background they stand out in vivid contrast. 

This puzzled Darwin   because his theory of sexual selection whereby  females choose their mates based on how attractive they are,  could not apply to caterpillars since they are immature and hence not sexually active.
He put the question to his fellow, but lesser known,  evolutionary theorist Alfred Russel Wallace who suggested that if some caterpillars  "...are protected by a disagreeable taste or odour, it would be a positive advantage to them never to be mistaken for any of the palatable catterpillars [sic], because a slight wound such as would be caused by a peck of a bird’s bill almost always I believe kills a growing catterpillar. Any gaudy & conspicuous colour therefore, that would plainly distinguish them from the brown & green eatable catterpillars, would enable birds to recognise them easily as at a kind not fit for food, & thus they would escape seizure which is as bad as being eaten."

The jury's still out on whether birds' aversion to stripey caterpillars is innate or learnt. (Cuckoos, ever bucking the trend, do eat cinnabar moth caterpillars.)
And some species, including frogs, snakes and reptiles, mimic the warning colouration even though they are not toxic.

I've been looking out for cinnabar moths (which are also brightly coloured but red and black) and their caterpillars all summer and these are the first I've found. Like yesterday's plant, ragwort creates a dilemma for environmentalists. Although grazing animals avoid it when it's growing it can harm and even kill them when it's dry in hay. Many farmers and conservation professionals get rid of it as a matter of course and the cinnabar moth is in decline as a result.

PS  Talpa's blip today is on the same subject but he has the moth too!