These two caterpillars look far too exotic to be native to The Netherlands, yet they are.
This first one is the caterpillar of a big, but surprisingly modest-looking moth called pale tussock (Calliteara pudibunda):
Olympus XZ-1, 28mm, ISO100, f5.6, 1/25s
The same species from above:
Olympus Stylus 1s, 42mm, ISO160, f3.2, 1/160s
This next one does turn into a spectacular butterfly. It is the caterpillar of the common yellow swallowtail (Papilio machaon), and the one in the photo was about 7.5cm (3") long:
Olympus XZ-1, 28mm, ISO100, f5.6, 1/80s
Unfortunately, I don't have a photo of the caterpillar, but you can have a look at it on WikiPedia.