In the ever-changing world of taxonomy, it is rare for an animal to be still called by the scientific name it received more than 200 years ago. The common slug Limax maximus is one such rarity: it was named by Linnaeus in 1758.
The well-known and often photographed mating of L. maximus takes place while a couple of slugs are suspending themselves using their mucus from an elevated point, commonly a tree branch or a wall. One of the most often cited descriptions of this process, and the one that gives the impression of being the oldest, is the one by Adams (1898). According to Hyman (1967), however, an earlier account was that of Ferussac & Deshayes (1819), which I have not had a chance to see yet. Recently, I discovered an even older description of the mating of L. maximus, but I will leave that story to a future post. A more recent account is that of Langlois (1965). There is also a series of photographs of mating L. maximus taken by someone named Lynwood M. Chace that were published in the magazine Discovery in 1952. The photographs were accompanied by a description of the mating, but it is not clear who actually wrote the text and it doesn't provide any new information that wasn't already known at that time.
Limax maximus seems to mate mostly in the evening. Briefly, the process is as follows: 2 slugs climb up a vertical surface, one leading the other; then they start circling each other; then they intertwine and suspend themselves by a thick strand of mucus; their penises evert and they too intertwine forming a bell-shaped structure at their lower end; the exchange of spermatophores presumably takes place within the bell; following this, the slugs abruptly separate and move away.
Below is one of Adam's drawing showing 2 mating slugs with their everted penises and the bell-shaped structure at their tips.
Here are 2 of Adam's more detailed drawings showing the morphology of the tips of the intertwined penises of the mating slugs.
Last August I was lucky to photograph the entire sequence of the mating of a couple of L. maximus in my backyard. Those photographs will be in part 2.
Adams, L.E. 1898. Journal of Conchology 9:92
Férussac, D. de & Deshayes, G.-P. 1819. Histoire naturelle générale et particulière des mollusques terrestres et fluviatiles, etc. Tome 1: 8 + 184 pp [not seen by me; cited in Hyman (1967).]
Hyman, L.H. (1967). The Invertebrates, vol. 6, Mollusca 1. McGraw-Hill. New York.
THOMAS H. LANGLOIS. 1965. The conjugal behavior of the introduced european giant garden slug, Limax maximus l., as observed on south bass island, lake erie. OHIO JOURNAL OF SCIENCE 65:298. pdf