Pale Tussock Moth larva.

What an interesting-looking caterpillar I found today in Deep Hayes Country Park, actually I found two. It turns out to be the larva of a Pale Tussock Moth, Calliteara pudibunda  http://www.ukmoths.org.uk/species/calliteara-pudibunda which feed on the leaves of many common trees and shrubs so of course they are not often seen until they come down to ground-level to pupate (though they also pupate up in the tree canopy too). Entomologists think the hairs may be: a defence mechanism against predators; for example they may deter birds from eating them (though Cuckoos specialise on eating hairy moth caterpillars), or prevent parasitic wasps from laying their eggs into their bodies (because the hairs are longer that the wasp's ovipositor). Other ideas are that may even be intended to make to caterpillar look bigger - hence too big to eat, or maybe even look too scary to eat! Some caterpillars have hairs that irritate or sting like nettles https://en.wikipedia.org/wiki/Caterpillar#Defenses. Whatever is the case - I think they are beautiful!