What is this beetle and what are the strange antennae for?
From the pattern on the elytra (hard upper wings), it looks like you might have a beautiful male Polyphylla fullo. Compare your photos with the P. fullo in this illustration, or photos on the Wikipedia page.

edit to fully respond to the comments:

I suspect you're right that the antennae plates in your picture are just tightly closed up, giving a different appearance. Here's a photo I took of a related species where you can see both appearances of the antennae.

And I think that Alan Boyd is correct about the antennae being used as chemoreceptors to locate females. The wikipedia page on the Ten-Lined June Bug also mentions both of these points.