As I was walking on the wet grass of my lawn this morning, I noticed two slugs kind of entertwined and wrapped around this gelatinous milky mass. Naturally I assumed they were having sex. I did some research, and this is what I found. And no, I'm NOT obsessed with Mollusk intercourse.
First of all, slugs are hermaphrodites, they have both male and female sex organs and can function sexually as either. That also means that a slug can mate with, and fertilize itself. They rarely do that, however, if another slug is available to mate with.
The next thing that I found interesting about slug sex is the size of the wanker on these guys. An average slug penis is between half and three quarters of the length of the slug's body. In fact, penis size is reflected in the scientific name of one banana slug species: dolichophallus -- Latin for "long penis."
Now for foreplay...When a slug sees another slug who looks like he/she might be interested in sex, a long courtship ritual begins, consisting of the two slugs circling each other, nipping at one another and eating each other's slime, sideswiping each other with their tails, AND waving their penises in the air. If this works, and the two get along, they then intertwine and stimulate each other for several more hours.
The now thoroughly excited slug pair then moves on to the sex act itself. Depending on the species of slug, the two may now deposit sperm-sacks on each other's back, suspend themselves with webs of sticky mucus and turn their sex organs inside out, or just go for plain old mutual penetration. Incidentally, the female sex organ for some slugs is on the head...which leads me to speculate on all kinds of bad and inappropriate puns.
Sex is done and it's time for the slugs to separate....but wait!!!! Something seems to have gone wrong. The amply endowed slugs covered in extremely tacky mucus often have problems disengaging from each other. The pair will wiggle and writhe for a long time, attempting to break free, but eventually they may have to resort to apophallation. That means one slug has to bite off the penis of the other.
The newly female slug will not be able to regrow her male member, and will be forced to only offer eggs for the rest of her life.
http://www.academicpress.com/inscight/09062001/grapha.htm
http://ifaq.wap.org/sex/sexandgardenslug.html
http://www.cadvision.com/davideo/slugs.html
http://www.postmodern.com/~jka/slime.html