Hi Paul,
It seems you are looking for the big ones, not the minute soil-dwellers. They are indeed around all year, but in winter they'll "burrow in" a bit deeper. That is, either deeper into the decaying wood, or deeper into the ground (under deeper rocks etc).
At the moment they shouldn't be too hard to find though, but not
every feasible spot is a guaranteed hit. Just this afternoon I had to kill some time waiting around a hospital (visiting) and looked around in a cluster of fairly big rocks (20-30kg category) half buried in the ground. In the whole cluster of about 20 rocks only two rendered a "Jackpot" of some 100-300
Porcellio scaber scrambling for cover (they
do like to huddle-up together
)
Other favourite spots would be under large patches of quite loose bark on fairly rotten trees (or rather their corpses), or bark and tree-stumps half buried in the forest soil (or under flat boards or stone tiles on moist soils). On the moister spots you're a tad more likely to find groups of
Oniscus asellus on the somewhat dryer areas more
Porcellio scaber, or maybe the "Pill-bug"
Armadillidium vulgare, but there is a fair amount of overlap in acceptable moistness conditions for these (so you'll often find mixed groups).
The other "most abundant" species is
Philoscia muscorum but they don't group together quite as much - you'd be lucky to find a group of 10 or so and then you'd have to be quick to catch them as they certainly are even more "nervous" (and a hell of a lot quicker) than the others. On the other hand - this would be the time of year to enhance your chances - the cold
does make all of them a good deal slower to respond/run
Finding many relatively (or even very) small animals is also a bit of a "time of year" thing. Most youngsters born last summer/fall go into the winter still fairly small and hardly grow at all during winter (very reduced metabolism and growth rate) they should just about pick up on the growing track again round about now. In late spring, early summer most will be decent "half sized adults" or so and the new ones will not be born yet (or too minute to really notice), so the whole population will look "bigger" on average. The "huge" ones are more scarce as they will need very favourable environmental conditions to grow a lot and survive a few years (mostly 2-3 years, but some species up to 10 or so) to reach maximum size.
One lazy-mans trick that I've read about but never tested would be to distribute a number of hollowed-out potatoes in your garden overnight and see what's in/under them in the morning. I strongly suspect that to only work in warmer season though as they certainly wander about less at night when it's cold.
Hope it helps, Arp