Slug sex

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After three days of slim pickings we were beginning to think the moth season was over. Then three large Underwing moths [Catocala family] arrived at the Moth tree last night--two Once-Married Underwings [C. unijuga] and one Sweetheart Underwing [C. Amatrix] The two attending mothers-rhymes-with-authors, Nick Wagerik and I, greeted them with delight and relief, [Reprieve!]. On the way home we stopped off at the north end of Cedar Hill to check for Limax maximus action. I'm talking about slugs.

Our lucky day: we saw the entire slug-mating drama, from intial pursuit through determined circling, through bungee-cord hanging and the flowering of the beautiful, opalescent -blue fan-shaped blossom at consummation. It is one of the most breathtaking sights either of us has ever seen during our years of nature observing. Small wonder that Charles Darwin once wrote:

“Anyone who has a chance to observe the love-making of slugs cannot doubt that these hermaphrodites use seduction and allure in their movements as they prepare for and accomplish their double embrace."
[Book IX, Descent of Man]

Hermaphrodites? Yes. Defined as snails without shells, slugs each come equipped with fully functional male and female reproductive organs. They can , in fact, mate with their own selves—a neat trick if you can do it. Yet they generally seek another slug to mate with, undoubtedly for purposes of genetic diversity. Even then their behavior is unorthodox to the extreme.

In their complex mating rituals each individual slug actually functions as both sexes simultaneously. It’s not easy to understand how this works --the human reproductive system seems ridiculously simple in comparison. Here’s how Drs. N. W. Runham and P. J. Hunter, the authors of that authoritative slug text –“Terrestrial Slugs” describe what happens just before and during the slug sex act:

’“For thirty to ninety minutes the couple follow each other in a tight circle caressing with their tentacles. Large amounts of mucus are secreted, and this forms a mucus string as the animals lower themselves from their support, at the same time entwining their bodies. The mucus string may extend to 45 cm [17.72 inches] The penis sacs are everted. At first these sacs are club-shaped but then a terminal fan forms. The two penial masses intertwine forming a tight spiral; the upper coils of the spiral then expand to form an umbrella shape which will eventually become lobed. Transfer of the sperm mass takes place while the penial masses are so extended [ they may reach 10 cm [3.94 inches]in length.”

Please note that the word “penial” seems to be exclusive to Drs. Runham and Hunter’s text. It is not in my big dictionary. Its meaning, however, is clear from the context.

Both of the pair provide sperm; both also provide eggs for fertilization. After sexual congress each of the pair lays 3 to 50 fertilized eggs beneath a piece of debris, in a crevice or in a hole in the ground. They can mate and produce fertilized eggs quite a few times. The source for this hard-to-obtain bit of information is The Western Society of Malacologists’ Field Guide to the Slug, the only book of its kind in existence.

PS I had my camera at the ready and meant to take photographs of the stages described above. But the process was just beginning at the moment we spotted the pair, and I was unable to unglue my eyes from the event for even a moment. The hour or so it took from beginning to end passed as if it were minutes. The photo at the top, therefore, is from the Internet.