On my list of things I must do when I get spare time* is the entry 'look at Riccardoella'.
Riccardoella is a genus of mites that lives in the lungs of slugs and snails, where they slurp up mucus and nibble on skin. There's an extraordinarily good picture of these critters on the Canon digital photographers' forum. (Thanks to Dark Orange for the link.)
As far as we know, the mites only occur on introduced slugs and snails in Australia. They don't seem to have made the jump to our local animals. Furthermore, there are two species—R. limacum, which lives on snails and R. oudemansi, which is only found on slugs. Why is there a difference? After all, slugs are simply snails without shells. What's keeping the mite species apart? Is it something to do with the mucus or the behaviour? Maybe there's really only one species that's variable in form? Or could there be a flock of very similar species that live on one or a few types of snail and/or slug?
And is it time for my medication?
*I may have plenty of breathing space to do all this when I'm sacked for insubordination.