‘A Rare Treat’: Paleontologists Detect 50-million-year-old Fossil Bug Penis

Yes it was very small, but the insect found in Colorado is so extraordinarily preserved that we can still see the stripes on his little legs too

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The two halves of the fossil assassin bug from the Eocene. Note the pygophore on his last abdominal segment.
The two halves of the fossil assassin bug from the Eocene. Note the pygophore on his last abdominal segment. Credit: DANIEL R. SWANSON , SAM W. HEADS

Insects are legion, but fossil ones are rare and usually in terrible condition. Now a bug that lived 50 million years ago has been found in Colorado, in such an extraordinary state of preservation that we can still see the stripes on his little legs. And his genitalia.

His? Definitely so. The Eocene-era assassin bug and his male pride are reported by Daniel Swanson and Sam Heads of the University of Illinois, and colleagues, in the Papers of Paleontology.

Looking at the photograph, one’s eyes are drawn to the dark figure-8 seemingly attached to the insect’s middle leg – but that’s not the lad’s gonads, it’s a beetle. The genitals of the insect, sprawled forever more with his six legs akimbo, are at the end of the last body segment. What we see is the sac called the pygophore (aka pygofer), which derives from the Greek word pyg for “rump” and phore for “bearer.”

Ironically, if anything, according to this picture this genital capsule looks like a vulva. But it is not.

Think of the pygophore as a sort of chitinous codpiece. “It is a cup, hardened like the rest of the exoskeleton, that contains all of the typically softer, internal parts of the insect’s genitalia,” Swanson explains to Haaretz.  

Asked if lady assassin bugs have pygophores too, he relates that in “true bugs” (suborder Heteroptera), they do not. Only the male. Also, close inspection clearly shows the boy bug’s basal plate: a stirrup-shaped structure that supports the insect’s penis, the researchers say.

An assassin bug in action.Credit: Graham Wise

The fossil is so exquisite that it even shows the contours of the phallotheca, which is the internal pouch the penis occupies when the insect is not having sex. “The dark stuff inside [the pygophore – see photograph] is portions of the internal genitalia that have been differentially preserved,” Swanson notes.

“To see these fine structures in the internal genitalia is a rare treat,” he adds.

Undeniable, that. Said treat enabled Swanson and Heads to identify the insect as not only an unknown species, but an unknown genus (a genus is a group of species that arose from a common ancestor) in the broader family of assassin bugs, of which about 7,000 exist today.

Asked how sure they can be it’s an assassin bug, let alone an unknown species of assassin bug 50 million years after the event, Swanson replies: “When working with fossils, it’s difficult to be 100 percent certain about anything. But the evidence certainly supports the identification of an assassin bug. In fact, the genitalia that were fantastically preserved in this fossil strongly match in basic structure the genitalia of assassin bugs living today.”

Heads elaborates that indeed, after tens of millions of years, a species – however similar the fossil may look to extant ones – wouldn’t be the same. Fleas that infested dinosaurs and fleas that infest cats look similar but wouldn’t be the same species. They couldn’t be. “At higher organization levels (like family and genus), there are examples of animals that are basically unchanged from tens of millions of years ago. But we wouldn’t expect any species that lived millions of years ago to still be around today. Though a very slow process, evolution is still a dynamic one,” Heads points out. Also, this particular assassin bug hadn’t been found before in the fossil record.

The newly recognized assassin bug has been named Aphelicophontes danjuddi after the fossil collector Dan Judd, who donated his half of the specimen to science.

An assassin bug exhausting a tortoise beetle.Credit: gbohne

His half? The insect was discovered in 2006 when the rocky case housing it was split open, resulting in two fossils. Usually when this happens, one contains the actual fossil and the other its impression. But in this case each half actually contains half the insect – and yes, each half had half the pygophore.

“Our co-author Yinan Wang was in possession of one half of the specimen and learned later that a private collector (Mr. Dan Judd) had the other half. Mr. Judd was kind enough to donate the specimen to us so we had both parts for the study. We named the new species in his honor by way of thanks,” Heads told Haaretz.

Identification by genitalia

Assassin bugs look like a mash-up between a mosquito and a spider. The smallest extant ones are a wee half-centimeter and the biggest can be as long as 4 centimeters (about 1.6 inches) long. They are called assassin bugs because they are predators that suck, literally. Their bite can render you very uncomfortable, though not dead.

They exist in a wide range of shapes and colors. Most are dark, but some sport vivid hues. One, the masked hunter – an invasive assassin bug that spread from Central Europe to the Americas – camouflages itself as a ball of dust when in its youthful nymph stage. That’s good to know. Many eat other insects but some are vampiric, subsisting on blood sucked from birds, lizards, you – it’s all good. Some spread disease, including Chagas in the Americas.

Heads, a self-described expert on mineralized insect genitalia, explains why the discovery of the insect’s pygophore is such a momentous event and why it matters: the unique form of the genitalia can be key to determining species.

Opiliones harvestman.Credit: Bruce Marlin

Genital differentiation is not unique to insects, but they too would tend to be constrained in their mating habits. That prevents waste of precious sperm and can also help prevent the demon interbreeding, thanks to the shape of their bits. One famous example is the penis of some ducks, one of the more startling genital phenomena known to science. The male bird can’t mate with other avian species. Wouldn’t fit, which is probably just as well. Thankfully, its giant, sometimes-corkscrew-shaped penis remains hidden inside the abdomen until the duck is aroused. If you arouse a duck, you will know it.

The case of insect genitalia is harder to elucidate because they are smaller, and usually fossils don’t preserve information at such a resolution, let alone soft tissues (no insult intended).

“It’s almost unheard of for internal male genitalia to be preserved in carbonaceous compressions like ours,” Swanson says – though the team notes that fossil arthropod genitals have been found before. For example, in Scotland, paleontologists found a harvestman (a type of daddy longlegs) aged about 400 million years old whose genitalia were on display too. There are also fossil insects in amber dating to the Cretaceous, which began about 145 million years ago; if encased in the resin before they rot, they retain extraordinary detail.

One last thing. The dating of the Colorado assassin bug to 50 million years ago means the group is twice as old as had been thought. Arthropods go back hundreds of millions of years, possibly originating in the Ediacaran period half a billion years ago. The oldest known true insects go back about 400 million years, to the Devonian, if not more.

The new discovery sheds a ray of light on the evolution of the assassin bug family tree, Swanson explains. “The branch of the family tree to which the new fossil is thought to belong is currently estimated around 25 million years old. So it means that one branch of assassin bugs probably diversified earlier (or, is older) than we thought. It’s still an open question if this affects the overall age of assassin bugs as a whole, but probably not much, if any,” he says. Now we know.