Empty land snail shells are prime real estate for many invertebrates that are small enough to fit in them. Several species of bees, wasps, spiders, ants and pseudoscorpions are known to build nests or hide in snail shells. Last December I was lucky to witness a previously unrecorded insect-snail shell association. In this case the insect was an unidentified species of thrips (Thysanoptera) and the empty shells was the common North American snail Zonitoides arboreus.
A tentative familial assignment for the said thrips was given as Phlaeothripidae (tube-tailed thrips) after I posted these pictures on BugGuide.net.
You can read the details in this paper that just came out in the current issue of Triton.