Following on from the private lives of snails, I bring you: slugs! (The first part of this post is largely a repost of something I wrote back in 2009.)
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 or slug doesn't have to find a partner of the opposite sex, it needs only to meet another snail. Or slug. 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?)
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.
Things get even stranger - and probably downright uncomfortable, for banana slugs, which are also hermaphrodites. For their love lives involve something called apophallation - my new word for the week, but I'll be hard pressed to slip it into casual conversation.
This is because apophallation happens when, well, when things don't go entirely as planned. Occasionally, after transferring sperm to its partner, one of the mating banana slugs finds that its penis has become stuck. When this happens, the owner of the offending organ tries hard to remove it, but if this doesn't work, then the penis is bitten off - and apparently eaten, by the slug that received its sperm. In describing this, Reise & Hutchinson (2002) note that if the behaviour is adaptive, it's probably the recipient rather than the amputee that benefits, by way of an additional meal.
That is, if it's adaptive at all - apophallation seems to be fairly rare, & a tale that's become embroidered in the telling, if the results of a google search are to be believed (rather like the apocryphal story about what female preying mantises do to their parthers after mating).
H.Reise & J.M.C.Hutchinson (2002) Penis-biting slugs: wild claims and confusions. Trends in Ecology & Evolution 17(4): 163. doi:10.1016/S0169-5347(02)02453-9