What we have here are a couple of slugs...
These large slugs are common in my yard, but I have seen them mating only this one time, and all I had handy was a video camera. So I carefully and not too quickly panned along the length of the pair, then put together this decent snapshot by stitching together a set of 3 video frames.
Aside from bringing out a surprising beauty of these normally sort of "blah" animals, the mating process of slugs will shake your preconceptions (groan!) of how these things work.
These slugs have an elaborate ritual involving much crawling around each other in circles, followed by twining together and hanging from a cord of slime, and culminated by flaring together of large genitalia that are extruded from underneath the mantle on one side of the body. The slugs of this species are hermaphroditic, but must exchange sperm in a symmetric act.
Other species do things a bit differently, including some that even convert males to females by a process called "apophallation" -- biting off the penis -- or so it says in one of the articles referenced below.
For more images and information on the biology, try some Google searches on "mating slugs". A particularly good page is at http://www.arnobrosi.com/slugbio.html. A set of excellent pictures can be found at http://oregonstate.edu/Dept/nurspest/Li ... rtship.htm. Other good general information can be found at http://www.xerces.org/Wings/spring2003.htm.
This is another shot from my archives, about 5 years old now. It is amazing how the camera technology has matured in that time!
--Rik
Also a great photograph as well, being stitched from a vidio you have done an excellent job. Would not have know if you had not said so. Thanks for the "slick" photograph here Rik, excellent work! 