They've grown huge and are eating a lot, so must soon be ready to pupate. They're funny things that hang under a branch when they're not eating, with their front legs held tight under their bodies so they look like they're praying or clasping their hands together.
These are eating blackthorn and beech and when they pupate they'll spin a big silk cocoon, then emerge later in the summer.
The second type of caterpillars are Emperor Moths, which are large (up to 10cm wingspan) native moths. They're lovely and look like they're studded with golden jewels with long black hairs coming out of them. They're also eating a lot of blackthorn!