This picture of two mating insects comes with an unexpected twist. The one on top is a female, and she has a penis. The one on the bottom is a male, and he has the equivalent of a vagina. During sexual bouts that can last for 40 to 70 hours, she penetrates him and uses her genitals not to deliver sperm, but to collect it.

The 3-millimetre-long species is called Neotrogla curvata, and it lives in Brazilian caves. There are four Neotrogla species and the females all have penises. They belong to a group of insects called barkflies or barklice—closely related to lice and true bugs, and only distantly related to actual flies.

Caver Rodrigo Ferreira discovered these insects several years ago, and sent them to Charles Lienhard, a barklice specialist, for further study. He gave them their name, and also noted their unusual reversed genitals. Working with Lienhard and Ferreira, Kazunori Yoshizawa has now shown that the female’s sex organ doesn’t just look like a penis—it acts like one too.

For comparison, the female spotted hyena also has what looks like a penis. Actually, it’s a severely enlarged clitoris, or ‘pseudopenis’, which can grow up to 7 inches long. She doesn’t use it to penetrate a male; in fact, the male must penetrate her pseudopenis with his actual one. Later, she gives birth through it.

Neotrogla’s set-up is very different. The female has a penis-like protrusion called a gynosome, which is erectile and curved. The male has no such organ; he has an internal chamber instead. When she penetrates him during sex, he delivers sperm into a duct in her gynosome, which leads to a storage organ. He still ejaculates, but he does so inside his own body, not hers.

View Images Cross-section showing Neotrogla sex. Credit: Yoshizawa et al, 2014.

Neotrogla sex can last for days, so it’s important for the duo to stabilise themselves. The female does it by inflating the base of her gynosome inside the male. It’s covered in patches of tiny spines, which help to anchor her in place for her sexual marathon. You can find similar spines on the penises of many male animals where they provide extra stimulation during sex (as in cats, mice and chimps) or inflict horrendous wounds on the females (as in the seed beetle).

In Neotrogla, the spines are such good anchors that it’s impossible to separate a mating pair without killing the male. As Yoshizawa writes, “Pulling apart coupled specimens (N. curvata; n = 1) led to separation of the male abdomen from the thorax without breaking the genital coupling.” In other words: We tried yanking one pair apart; it didn’t work and the male kinda broke.

“This is a fascinating piece of natural history,” says Michael Jennions from Australian National University. “Evolutionary biologists have long been fascinated with why male genitalia, including penises, are so variable among species. Far less attention is given to female genitalia, in part due to the technical difficulty of measuring a cavity rather than a protrusion.”

View Images The penis (gynosome) of the female Neotrogla. Credit: Yoshizawa et al, 2014.

Neotrogla’s set-up is unlike any other in the animal kingdom. There are some mites and beetles in which females have a protruding organ that collects sperm, but none of these have special adaptations for anchoring themselves inside males. The female seahorse uses a protruding structure to place eggs inside the male’s pouch—it’s the male who gets pregnant and nurtures the eggs—but that’s an egg-laying device, not a penis. And, as I mentioned, the spotted hyena has an enlarged pseudopenis, but it’s still the male that does the penetrating.

What’s more, the Neotrogla penis is a completely new structure with a complex set of accompanying membranes, muscles and ducts. Its evolution wasn’t a simple case of elongating an existing body part like the clitoris of a hyena. It has no equivalent. “Evolution of such novelties is exceptionally rare, maybe

comparable with the origin of insect wings,” says Yoshizawa.

The male vagina, meanwhile, has evolved in tandem so that it has just the right shape to accommodate the female’s penis. It’s easy to understand why such structures are extremely rare—reversing the usual pattern of genitals is not easily done. So why did it happen in Neotrogla, and only Neotrogla?

Yoshizawa suspects that the answer lies in the insects’ environment and lifestyle. They live in extremely dry caves and their main food—bat droppings and bat carcasses—are hard to come by. Starvation is always just round the corner, but females can fend it off by mating. That’s because the males package their sperm into packets called spermatophores, which are also loaded with nutrients. During sex, females get a meal along with sperm.

This flips the usual male-female dynamic. Males typically benefit from mating as much as possible, so that they can father the most offspring. They compete for mates, while females, who bear the burden of actually raising offspring, are choosy about their partners. But in Neotrogla, it’s the females who crave multiple mates, so they can get their jaws on as many spermatophores as possible. They’re the ones who compete, and the males are the choosier sex.

Yoshizawa thinks that this reversed sexual conflict led to the evolution of reversed genitals. The female’s gynosome allows her to anchor herself to her mate, even if he is reluctant, so that she can grab as many spermatophores as possible. “It is very likely that the entire mating process is actively controlled by females to obtain more seminal gifts from males, whereas males are rather passive,” he says. This might also explain why Neotrogla sex lasts for so long.

“This is possibly correct, but we know there are other species where females benefit from similar gifts but do not have a ‘penis’, such as several species of crickets, flour beetles and bean weevils,” says Jennions. “Unfortunately, when you only have one instance of a trait, it becomes difficult to attribute its evolution to a specific force, such as sexual conflict. Until we find more examples of female penises, the jury will remain out on the explanation.”

Meanwhile, Yoshizawa is busy testing his hypothesis. He’s doing a lot more work on the behaviour of the mating insects, the characteristics of the spermatophores, the growth of the gynosome, and pre-adaptations in other related species that allows Neotrogla to evolve a female species.

“This study shows that most things that we use to define males and females are not written in stone,” says Goran Arnqvist from Uppsala University. “They are reversible and evolvable results of the relative costs and benefits of mating. In terms of traits that separate males and females, there is little left but the type of gametes [sperm or eggs] they have.”

UPDATE: A response to io9’s piece. (Here’s a direct link to this bit)

At io9, Annalee Newitz has written an interesting piece criticising much of the coverage of this story, including this post, and specifically the use of the term “female penis”. I disagree with many of her points and stand by the use of the term.

But first, to clarify, I absolutely agree with Newitz that cheap dick jokes are doing the topic a disservice, which is why you won’t find any here. The tone is as deadpan as I can muster—the only sniggering is reserved for the part of the study where one mating pair gets pulled apart and the male is accidentally bisected.

As to the other parts of Newitz’s critique, she repeatedly says that “female penis” is an inaccurate term that is “anthropomorphizing” Neotrogla’s anatomy—one should call the organ a “gynosome” (which I also do). I don’t agree that gynosome is accurate, while penis is not. As Diane Kelly, who studies penises points out: “As a technical term, a penis is a reproductive structure that transfers gametes from one member of a mating pair to another.” Which is exactly what is happening here.

Newitz points to differences. “When was the last time you found a penis that grew spines, absorbed nutrients, remained erect for 75 hours, or allowed its owner to get pregnant?” Actually spines are pretty common; long sexual bouts are pretty common; and the gynosome doesn’t absorb nutrients—it collects sperm packets that contain nutrients, which the animal then eats in the normal way. The key difference is that rather than delivering sperm, it collects it—as I stated right up top. And the only reason we think of penises as sending sex cells in that direction is that we never knew any other set-up could occur. Now we do, which either forces us to introduce a new term and demand that it be used, or to expand the bounds of our old term. I prefer the latter. I’m generally a lumper, rather than a splitter.

The gynosome is very much like a penis in both form and function. The authors highlight the differences by giving it its own specific name. But they also acknowledge its similarities to what we typically think of as penises by describing the organ as such, both in the title of their paper—“Female Penis, Male Vagina, and Their Correlated Evolution in a Cave Insect”—and throughout its text. They don’t get any special privilege because of their authorship, of course—but I’m pointing out that you can either look at this discovery through the lens of difference or similarity. And similarities are actually critical here because evolution crafts organs that are convergently similar—though different in the details—thanks to similar selection pressures.

In fact, there is a long tradition in anatomy of describing organs with almost metaphorical names. A snail’s foot is not remotely the same as a human’s foot, but they’re both muscular locomotive organs that are kinda on the bottom of the body. We call them both feet. An octopus radula is not a human tongue, but they’re both mobile things inside the mouth that perform feeding functions, so we call them both tongues. “Eye” gets used to refer to all manner of light-detecting organs regardless of huge differences in their anatomy, evolutionary history, physiology, because they all share the common theme of detecting light. And in a similar vein, a Neotrogla penis/gynosome is not the same as a human penis but they’re both used during penetrative sex for the transfer of gametes. Other penetrating sexual organs, like the aedagus (insect) and gonopodium (fish) are also colloquially known as penises.

So, do we make a special case for sex-related terms? Newitz would say yes, because of the cultural and social baggage that “female penis” carries, in a way that “snail foot” does not. This is the strongest part of the argument, and the part that gives me pause.

But Newitz also argues that the term “erases one of the most beautiful things about life, which is its awe-inspiring diversity”, and there I disagree. The post above specifically references that diversity—not just in Neotrogla but other animals like hyenas and seahorses, and goes into detail about sexual selection. It ends deliberately with a quote about how the split between males and females comes down to sex cells, and everything else is labile. If that’s not celebrating the diversity of life, I don’t know what is. I don’t think that referring to Neotrogla’s female sex organ as a penis whitewashes that diversity. If anything, it forces us to realise that one of the traits we often link to a penis–that it lives on a male–isn’t a necessary truth. The usage expands what we know, rather than erases.

Reference: Yoshizawa, Ferreira, Kamimura & Lienhard. 2014. Female Penis, Male Vagina, and Their Correlated Evolution in a Cave Insect. Current Biology http://dx.doi.org/10.1016/j.cub.2014.03.022