Last Sunday morning's slug search, in addition to an incidental salamander, yielded 3 individuals of the philomycid slug Megapallifera mutabilis that I was after. Here is one of them in situ.
This slug was on damp soil under a pile of leaves among the roots of a beech tree. I removed it, cleaned off the debris that was stuck to it, weighed it and then returned it to its hideout. Its weight, along with those of the other 2 slugs, is now part of the data for an ongoing project that is trying to understand the annual generation cycle of this species.
While resting, these slugs—all slugs—try to maintain bodily contact with a wet surface whenever they can. Foraging out in the open often results in water loss via evaporation thru the skin and as slime. The drier the atmosphere and the surface a slug is on, the larger the water loss. Therefore, to regain the lost water slugs hide in wet tree holes, on damp soil under rocks or plant debris or similar spots.
These generalizations, also valid for snails, were the subject of a recent paper of mine.