These are a couple of Arion vulgaris mating. I photographed them in Antwerp, Belgium last July during the World Malacological Congress. A group of us came upon these slugs and hundreds of others around midnight one night as we were walking back from a pub to our dorms thru a wooded path. It was almost pitch black, so I turned on my flashlight. Suddenly, we realized that there were slugs everywhere. You have to be with people who study these animals to appreciate the excitement such a moment can bring: Look at all these slugs! Here is a mating pair!
The slugs in the genus Arion, although they are hermaphrodites, do not have penises. The mating processes of A. hortensis and A. subfuscus, related species, were described by H. E. Quick2. During mating each slug everts its lower (distal) genitalia, including the atrium1, the epiphallus and the oviduct. The everted organs apparently intertwine and exchange spermatophores. The white organs visible between the 2 slugs in the picture below are probably their everted atria.
During the week I was in Antwerp the identifications, or rather, the names of these slugs, which were quite common on the Campus Groenenborger of the University of Antwerp where we were staying, changed 3 times. First, I thought they were Arion rufus, then I was told that they were A. lusitanicus, and finally, the name was corrected to A. vulgaris3.
1The common tube into which the epiphallus and the oviduct open and which forms the entrance, or the exit, to the outside.
2Quick, H.E. 1946. The mating process in Arion hortensis and in Arion subfuscus. J. Conchol. 22:178-182.
3Appreciations to Ira Richling for the final name.