Slaters, woodlouse, pill bugs; do they have a semi-permeable membrane?

So, I'm doing a Bio investigation. I can't seem to find/collect enough information to justify why slaters etc. lose water in low humidities and gain at high humidities. I know it has something to do with osmosis/diffusion, but can someone give me a detailed answer? I'm not even sure if they have semi-permeable membrane or not, but I know they have an exoskeleton.