I was amazed walking into the kitchen a few minutes ago when I came across this massive slug, stretched to over 10 cm long on the floor. A Limacus maculatus, it was covered on tiny mites, clustered near the bottom of the slug's body (click the photo to enlarge). Slugs and snails often carry mites of the genus Riccardoella but I have never noticed so many in a slug.
UPDATE. Thank you to Richard Comont and Ryan Clark for pointing me to the right species of mites. I have now amended the post.