I've spotted some arthropods I've never seen before. They look, to me, almost identical to freshwater shrimps (Gammarus?), but are shiny black-brown and live in semi-dry leaf mold! Not sure what section of Bug forum to start this in (centipedes?)
I can't find any reference to them in books or online. At first I thought they were exceptionally large springtails - you see them when you rake through leaf litter, hopping around when you disturb them. Their hops are powerful, you can feel them hit your hand. The largest is about 5mm (the width of a CD jewel case) although I caught a glimpse of one that may have been a mm or so bigger. They come in various sizes, I've not seen anything that looks like a nymph or juvenile form, just identical smaller ones.
I found them ONLY in a back garden in Hampstead, London - I've been gardening since a kid and now work professionally all over London, working close to the ground, so I've looked out for them - but never seen them anywhere else. I caught a few and let them loose in a damp patch of my garden and they seemed to have set up a little colony because they were there a year later (have since moved).
I'd love to know what they are - once again, they look so much like freshwater shrimps (sideways comma-shaped, moving on their sides with kicks of two long legs) but they lived nowhere near water. I'm hoping they're an uncommon native and not some introduction with possible problems (although the resident blackbird loved them).