Red Admiral caterpillar, Heeswijk-Dinther, Netherlands
Interestingly, despite this species being highly common and often photographed, this is the first photo of its caterpillar. This one is stitching leafs together on nettle, the host plant they rely on for everything.
Reference used for ID (in dutch):
The Red Admiral is a well-known colourful butterfly, found in temperate Europe, Asia and North America. The Red Admiral has a 45–50 mm wing span. The species is resident only in warmer areas, but migrates north in spring, and sometimes again in autumn.