Here is a rather poor photo of a Walnut orb weaver, shot on an iPhone and focussed badly I'm afraid. But it does just about convey the incredible flatness of this example, which I can confirm was alive and to my knowledge hadn't simply been crushed. It seemed to be only a few millimetres thick and notably flatter than others I have seen. Maybe it just hasn't eaten in a while.

A bit of reading suggests that they have muscles in their abdomen that can flatten it at will (the muscles being attached to the dimples on the top of the abdomen) though I didn't find a very clear description of this ability. I wonder if that patch of silk beneath it, caught in the glare of the LED flash, is an egg sac?

Here is a much better picture that I took of one a couple of years back, which I blogged about at the time. You can see that it does have a rather flat abdomen, that being a defining feature. I also ran into loads of them in a pedestrian tunnel once where they really played up to their sinister appearance.