We are brought up to despise most invertebrates, especially the slimy ones. But we should warm to slugs if only for their sex lives: creatures whose antics outdo anything thought up by Messalina, Zeus and all those carved and writhing figures on the Konark Temple. As gardeners wage war on slugs with beer and with eggshells, so slugs pursue their exotic and passionate lives. And yes, they do slime. Two sorts of slime: watery stuff, and thick, sticky stuff. They get about by gliding gracefully along this self-created carpet. It’s hard for humans to get excited about mucus – though it is life and death to the slugs – so let’s move on to sex.

Slugs are hermaphrodites. Both halves of a pair have penises, both halves present sperm to the partner, and both halves go off and lay eggs. Slugs have the best of both worlds. But they are not just wham-bammers. They believe in courtship. They are devoted, to the point of mania, to the concept of foreplay. It can go on for hours, circling, nibbling and lunging at each other. Sometimes they will savour each other’s mucus, perhaps to get genetic information, perhaps as a light sustaining snack. Anointed in mucus, they engage in a slimy and sensual ballet. Some species will do this suspended from long ropes of mucus: acrobatic, gravity-defiant, and no doubt as thrilling as doing it on a trapeze. The pace is slow, the rhythm sensuous, as if each nuance is relished. And some species have the most colossal penises: half as long as their own bodies. Some slugs have copulatory rituals in which the pair dance about each other, each partner waving a giant penis overhead.

The act continues with a mutual entering and a prolonged and slimy embrace. But then, how to break it off? The phrase, I fear, is no metaphor. With some species, a long and corkscrew shaped penis doesn’t always emerge too easily. In these circumstances – gentlemen are invited to cross their legs at this point – one slug will chew off the penis of the other. Sometimes both slugs will perform this feat. It is called apophallation. The slug, hermaphrodite no longer, goes away. Alas, he can’t grow another penis. So she carries on as a female forever after.

A backbone isn’t essential to an interesting life.

From Ten Millions Aliens: a Journey Through the Entire Animal Kingdom, just published by Short Books