The spiny elm caterpillar turns into a mourning cloak butterfly (nymphalis antiopa) called Trauermantel in German.
It is a protected and endangered species in Austria and Switzerland. It's environment is usually at elevations above 900 meters.
I discovered this guy and his buddies while on a hike in the Oetztal Valley in Austria on my vacation.
It had just stopped raining and he still had the water droplets on his back as he was busy eating the leaves of a Salix cinerea (willow) bush.
The spiny elm caterpillar has eight red prolegs and is covered with white hairs and black spines. When fully grown, the mourning cloak caterpillars are about two inches in length.
Good thing I always have my macro extension ring with me in my camera bag - these close ups wouldn't have been possible with out it.
For more information: https://en.wikipedia.org/wiki/Nymphalis_antiopa