A leopard slug's huge blue-tinted penis and upside-down reproductive acrobatics make for a colourful sex life.

Twisting their slimy bodies together, the large slugs dangle upside down from a glittering rope of mucus, slowly rotating. Then, out of the molluscs’ heads emerge large, blue, tube-like growths that wrap and writhe around each other.

If you come across this weird sight, you'd be forgiven for thinking the slug's wriggling blue protrusions were some sort of parasite, or even emerging young.

But what you’d actually be witnessing is a mating ritual of a pair of amorous leopard slugs, and the large blue tubes that grow out of the right side of the slugs’ heads are their immense penises.

Terrestrial slugs are hermaphrodites – so by wrapping their penises around one another, leopard slugs fertilise each other’s eggs.

Welcome to the strange, sticky and sensational world of slug sex.

“Leopard slugs and their relatives are unusual among slugs in that they hang upside-down to mate,” says Dr Ben Rowson, an expert in terrestrial molluscs at the National Museum Wales.

The bizarre mating method is featured in the latest series of Nature’s Weirdest Events on BBC Two. In the programme Dr Rowson says: “Not many people would sign up to watch slugs mating, but once you’ve actually seen it… it is beautiful, it takes a long time and it’s kind of hypnotic and elegant.”

Leopard slugs (Limax maximus) can grow up to 20cm in length and are recognisable by their distinctive, leopard-like spots and colouring. Their mating rituals most often occur after dark.

Sexual encounters in the slugs are relatively rare: they can fertilise their own eggs, so some individuals never mate. And those that do need only to mate once in their lives.

But because the slugs are hermaphrodites, they can partner with any other leopard slug they meet.

Amorous acrobatics

To mate, the slugs hang upside down from a string of mucus they have secreted because they need the help of gravity to extend their large penises from openings in their heads.

These appendages, which are the length of the slugs’ entire bodies, are often coloured blueish by the slugs’ body fluids.

Afterwards leopard slugs may lay up to 200 eggs, from which tiny, pale white babies emerge.

Their large penises “may be an adaptation to ensure sperm delivery”, explains Dr Rowson. “The most evolutionary successful slugs… are likely to be those that fertilise the largest possible number of eggs.

“To do this, they would have to ensure that as well as having their own eggs fertilised, they deliver their own sperm to other slugs… longer penises may be more effective at doing this.”

Although leopard slugs’ penises are proportionally large, some related slug species in southern Europe can grow their penises three of four times the length of their bodies, says Dr Rowson.

Slugs’ mating systems are diverse and colourful: some slugs do not have penises but stick together with precision. And among some African species recently discovered by Dr Rowson, things get even more bizarre: some have spiny or thorny genitalia, or arrow-like love darts “that make the mating of leopard slugs look tame”, he says.

“One Tanzanian species, Upembella nonae, even inserts a barbed-wire like spermatophore into its partner when mating.”

So as sensational as leopard slugs’ sex lives may be, the slimy lovers may find themselves outcompeted by fellow molluscs when it comes to mystifying mating systems.

Follow BBC Earth on Facebook, Twitter and Instagram.

Discover more about slugs with National Museum Wales