So all these animals, having left the sea, solved the problems of moving around and breathing air in their own differing ways. But there was another difficulty, mating. In the sea, animals need only release their eggs and sperm and the water mixed the two together. On dry land that couldn’t happen, even for the most moisture-loving of creatures. An individual slug carries both male and female organs. But even then, that was of no help. Each had to both give and receive. Somehow or other, pairs of individuals had to get together and the ways the have evolved in which to do so are quite extraordinary. Indeed, some of them are almost beyond imagining.

The leopard slug, you might think, has the simplest of habits. Maybe, but not when it comes to mating. When an individual is looking for a partner it give its trail of slime a special taste that advertises the fact. Another, if it feels the same way, will detect the invitation and start to follow. The pursuer, to confirm that it’s there and it’s ready to mate, gives the pursued a nibble. The leader heads upwards. An overhand is what’s neeeded. The underside of a branch will do very nicely. The two start to circle one another more and more closely until they entwine. For an hour or so they continue to wind themselves around one another. Then, suddenly, the pair releases their hold on the branch and start to slide downwards on a rope of mucus.

Now, in midair, they move to the next stage in their pairing. Each everts its male organ from just behind its head. These grow longer and longer. Then they, too, begin to entwine. They fan out to form a translucent, flower-like globe. And now, at last, sperm passes from one slug to another. The transfer is complete. Each has been fertilized.

Finally, their strange, balletic relationship comes to an end… with a bump.