They arrive at sundown, as if by standing invitation. The moths come to the same woodland clearing each night in a flapping crowd, and then begins the frenzy of flirting and mating. With their dance fights, orgies, homoerotic behavior and even the occasional "aerial strike," the gold swift moth may have the most unpredictable sex life of any insect—which is to say, the most human-like.

No one knows the erotic habits of Phymatopus hecta better than John Turner. An emeritus biology professor at the University of Leeds in the United Kingdom, he's studied insect evolution for more than five decades. But he stumbled into the gold swift's world during his off-hours.

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Turner has a small vacation cottage in the Scottish Highlands. One summer evening there in 2008, while he was scrubbing the dishes, he happened to look out the window and see a gold swift orgy in full swing.

"I'd known about this behavior all my life," Turner says, but until now he'd only gotten brief glimpses.

Most moths do their wooing in the style of The Bachelor. The females perch somewhere, give off pheromones, and wait for a male to choose them. Gold swifts flip this script: males gather and display themselves for the females. You could say it's more like The Bachelorette; bologists, though, call it a "lek." Males of some bird, bat, antelope, wasp, and fish species gather to show off in the same way, but it's very unusual in moths.

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Although scientists had known about gold swift lekking for about a century, no one had studied it in detail. "Maybe because it occurred in the twilight, in odd corners of woods, Turner says, "and hardly anyone had taken the trouble to go back and look twice." Now the professor had an opportunity right outside his window to learn secrets no other human knew.

Turner spent his next seven years of vacations following the moths. He discovered a sexual repertoire that was endlessly surprising. "Every night they seemed to think up something new," he says.

The lek is only the beginning. While male moths work hard to lure females, females can also display for males. This kind of mutual attraction is nearly unheard of in moths. Both sexes may choose to perch and wait for a mate, or flirt in midair, performing dizzying solo dances or duets. They fan pheromones at each other to tempt with scent, but they also flaunt their looks.

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Mixing and matching so many different tricks "makes their courtship behavior very complicated," Turner says. His paper on the subject reads like a Kama Sutra for fuzzy winged insects.

If there really were a mating manual for Phymatopus hecta, these would be among its entries:

Pendulation

Each night's lek kicks off with males flying above the plants in a "pendulating" flight pattern. They seem to swing back and forth hypnotically in midair.

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Scent Brushes

Males send out pheromones from fluffy yellow appendages on their back legs that look like curtain fringe. They fold these retractable brushes away at the end of each night.

Fanning

Females waft their own pheromones toward male suitors by flapping their wings like a courtesan's fan.

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Tenting and Spread-Eagling

Perching males may hang with their wings folded in a tent shape or held stiffly out from their bodies in what Turner calls the "spread-eagle display." Both poses show off their handsome pattern of brown flecked with white.

Dance Fights

When two pendulating males get too close, they may fight. It's more of a dance-off than a brawl. They either pendulate at each other head-on or hover a few centimeters apart, quivering, until one of them flees out of intimidation.

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Mating Frenzy

A female who prefers the traditional moth mating style can simply perch and fan herself. Shortly thereafter, she may find herself swarmed by a dozen eager males at once.

Aerial Strike

In 1994, another researcher reported seeing a female gold swift dive-bomb a male in flight and take him to the ground for copulation. Turner never witnessed this.

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Partner Dancing

Before they settle down to mate, the female and male sometimes pendulate together in a midair duet.

Homoeroticism

Very occasionally, males try to mate with each other. (It doesn't work.) Turner says this behavior had him puzzled for years. But now he's pretty sure that males just get confused by all the female pheromones floating around and try to copulate with the closest thing.

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Copulation

The act itself is tricky—there's no vanilla sex position for gold swifts. First, the female clings to a leaf by her front legs and dangles vertically. The male clambers onto her belly. Then he swivels to hang upside-down from the female, his body "held only by the grip of the genitalia."

You should probably not try it at home, but this position helps keep mating moths safe. If a hungry wasp or spider comes close, the female simply lets go of her perch, and both lovers fall to safety in the brush below.

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In fact, the clearing by Turner's cottage may have become a popular lekking area because the plants that grow there have so many good copulation perches.

"It felt not as if I had found an interesting project, so much as that the moths had found me," Turner says.

(Turner is a poetry translator as well as a biologist. He says there's not as much overlap as you might think between the world of poetry and the complex romance of the gold swift. But writing scientific papers, with their rigid rules about formatting and language, reminds him of the 19th-century French sonnets he translates.)

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The gold swift's sex life is likely the most complicated in the insect world, Turner says. In this way, we can think of the moths like ourselves.

"Even though they have only pin-sized brains," Turner says, the gold swift and its courtship habits "remind one of the considerable flexibility shown by humans." He adds, "In other ways, of course, they are quite different."

For example, the females only mate once in their lifetimes, fertilizing all their eggs in one go. And they don't go on dinner dates. In fact, they don't eat at all—their adult bodies are only made for mating.

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"And, although it would be quite hard to prove it," Turner says, "you can bet they don't fantasize."

Elizabeth Preston is a science writer in Massachusetts. Her blog, Inkfish, is published by Discover. Follow her on Twitter @Inkfish.

Illustration by Jim Cooke