... or something like it anyway!
Leopard slugs, like other terrestrial slugs & snails, are hermaphrodites. They produce both eggs & sperm, but must exchange sperm with another slug in order to fertilise their eggs. (This reproductive strategy means that an amorous snail doesn't have to find a partner of the opposite sex, it needs only to meet another snail. Of the same species, of course.) Actual copulation is preceded by a range of somewhat slimy courtship & precopulatory displays - in garden snails this involves (among other things) piercing one's partner with crystalline darts... Sounds painful, I know, but this part of the ritual apparently enhances uptake of the piercer's sperm by its partner.
Anyway, leopard slugs begin their courtship 'dances' (well, squirms) up in the trees, but at some point descend on a mucousy thread to hang suspended in mid-air, while continuing with their courtship. (As I wrote this, Lewis Carroll's Jabberwocky sprang to mind: Twas brillig, and the slithy toves/ did gyre & gimble in the wabe... Doesn't a 'slithy tove' sound a bit like a squirming slug to you??? No?)
And at some point they have to manage the sperm transfer itself. Each slug (& the anatomy's the same for snails) extends a long, sinuous penis from a positon on the side of its body, just back from the head. These swell as blood is forced into the sinuses within them - & also turn blue. This, again, is from the blood; the oxygen-carrier in molluscan (& crustacean) blood is the protein haemocyanin, & it gives the blood a bluish tinge.
And now, via PZ (who as usual finds all these things first), you can watch a pair of leopard slugs manage all this in glorious technicolour: (I agree with him that 'terrifying' is hardly the right adjective - 'awesome' or 'fascinating' might be closer to the mark. Or, maybe, just 'Wow!!')
(You can find a whole collection of wonderful insect-focused video clips on the BBC website.)