what is stripping my rhubarb leaves?

Leaves look like they have been ripped off the stalks just leaving a few tougher veins sticking out of the top of the stalk

No sign of slime or droppings

It is in  our fruit cage so it is not a passer by grabbing a handful on the way past

 Never seen this before

Any ideas please?

It was Snails that ate mine, and left them looking exactly as you described.
I finally caught them by getting up at 3am.

WoW! They must be huge snails  :blink:

No, Just blinkin loads of em...

Curculio seems to actually attack the stems by laying eggs, causing a jelly-like sap to leak out and decay to occur.  That doesn't seem on all fours with the original description, although I have seen that damage on plants and not known what caused it!