A short flight of steps at the end of our garden leads down to the stream. These steps have fallen into disrepair and, prior to interruption by "the white stuff", I was busily reconstructing them. The old structures consisted in part of timber - now much rotten - and, as I removed the damp and crumbling pieces of wood, dozens of woodlice scurried away, seeking shelter. Virtually all were specimens of Porcellio scaber, whose name may be uncharitably translated as "scabby piglet". (Our word "pork" comes from the same root.) This species is generally grey but a few amber coloured specimens are usually present. The back is covered in tiny rough "scabs".
The woodlice weren't seeking shelter from birds. They are not often eaten by birds as each segment of the body hides a pair of glands which secrete a foul fluid, effectively repelling most would-be predators. The woodlice were in fact seeking darkness and damp conditions; being crustaceans woodlice can quickly get dessicated and so die. Another familiar species, Armadillidium vulgare, is able to partly prevent the danger of drying out by rolling into a ball. Some species within this genus can survive in near-desert conditions. Armadillidium has been given an enormous number of common names: pillbug (they were sometimes covered in honey and given to the sick), roly-poly, slater, parson's pigs, tinnyhogs, wood clocks, grammar sows, and so on.
Our pocket park also contains Platyarthrus hoffmannseggi. This is a blind, albino woodlouse which is found in the nests of ants, usually - as in the pocket park - the Yellow Meadow Ant. The ants do not seek to drive these interlopers away so it is assumed that they perform a service, possibly eating, and so removing, faeces. I initially thought that this species was rather rare but, when a few years ago I mapped the woodlice of Northamptonshire (well, somebody had to do it!) I found it to be common and widespread.
The plates which form the back of a woodlouse are impregnated with lime, so they are often absent from acid environments such as heathland and moorland.
(My son. on reading this blog, asked, "What about Oniscus asellus?" Well, it is certainly present, both in my garden and in the Pocket park. It is a little larger than Porcellio scaber and has a rather glossy appearance. OK Jeremy?)