Flat-backed millipedes

My local - now disused - Victorian cemetery is like an old woodland. There are mature trees with woodpecker holes, tree stumps, dead wood, thick leaf litter. Dead wood is a habitat full of life even in winter. Slugs and snails, springtails, millipedes and centipedes, woodlice, mites, little beetles and their larvae, tiny spiders, and fly larvae: they might be a bit torpid, but still active. In the cemetery, I often come across flat-backed millipedes, Polydesmus on dead wood, often in pairs. They are relatively large and - true to their name -, have a flat, sculptured back. There are five similar species of this genus in the UK that can be only told apart by examining the male gonopods, an 8th pair of modified legs to transfer sperm, or the females' genitalia (which opens behind the second pair of legs), features that can only seen properly when dissected. Males have sturdier legs and appear to have a gap behind the 7th leg (where their gonopods are), where in females the gap appears behind the second pair. The most common species is P. angustus.
 A couple of weeks ago, I surprised this pair of embracing flat-backed millipedes under a piece of rotting stump, maybe mating or just checking each other out? In another photo I can see that the head of the millipede on top is holding onto the rear end of the other one.

Pair of Polydesmus 3rd March 2013

And a few days ago I found a pair of smaller ones, one of them is walking at the top of the post.