Remember my animated GIF of a spider splitting open? I said I'd get back to you once I'd found out what was going on. I found out then I er forgot to get back to you...
...it's its heart beating!
I got this bit of info:
'Looks suspiciously like it's heart beating to me. The vascular system runs up inside the abdomen just there, so it would make sense.
It also looks very much like Meta segmentata'
Thanks to Phil on that site!
(Cyclops: Link here: http://www.photomacrography.net/forum/viewtopic.php?t=7952&highlight=iainp+abdomen )