I start this article by saying that if anyone knows for sure what kind of behaviour the two specimens of slug are carrying out, please share it in the comments.
As you know from previous articles, on my travels around the Basque Country I usually spend several days walking through the Basque forests, feeling Nature and exploring its native flora and fauna.
When I was walking for some hours I noticed for first time in my life some beech leaves with bumps on its surface that I didn't know what exactly they were at that very moment.
Now I know that those lumps are called "galls" and are made by an hybrid of mosquito and fly (Mikiola fagi) that lay their eggs on the leaves and their larvae feed within the tree/plant leaf tissue.
Even though inside these galls there is, usually, a living bug feeding on it, interestingly, it's not harmful to the general well-being of the plant or tree (as a general rule).
After walking another couple of hours I reached an area of the forest covered with dry leaves, which was the place where I met the main protagonists of this article.
At first I thought they were carrying out some mating ritual or even reproducing, but after reading more about slugs, I am not so sure.
Slugs are hermaphrodites (they can fertilize themselves and also use either of their two sexes with another specimen to reproduce, according to National Geographic), but if you look closely, neither of them have their sexual/reproductive organs outside (they can be seen from naked eye).
Perhaps inside that ball is their new progeny but, according to what I have read, slugs are born buried in holes so I still have the doubt about what they were doing there.
Video recorded with Samsung Galaxy S20 Ultra 5G camera.
Photographs taken with my Samsung Galaxy S20 Ultra 5G camera.