We came across these creepy little fellas on a walk at Puttenham, feeding on an oak leaf. Bizarrely, whilst one leaf was completely covered by them, there were no more on the tree that we could see. If they sensed movement close by, they would raise their heads and tails in a rather threatening manner and wave them blindly about.
Initially I thought they might be mullien moth caterpillars, but those aren’t hairy. When I looked them up I thought they were probably large white butterfly caterpillars, but I’ve never heard of those eating oak leaves (too busy eating brassicas). I found out that they are actually the caterpillars of the buff tip moth, Phalera bucephala, which feed on the leaves of deciduous trees. They start off feeding communally, like this, but then disperse as they progress through their instar stages.
Still, not nearly as scary as the oak processionary moth, an invader from mainland Europe which defoliates oak (and other deciduous) trees and poses a threat to health via its toxic, irritant hairs. Rather glad I didn’t meet any of those.