LA's tar pits are a death trap. Except for this fly

Within the sprawling metropolis of Los Angeles, not far from West Hollywood and Beverly Hills, paleontologists are hard at work sorting through one of the richest collections of ice age fossils in the world.

Today, the La Brea Tar Pits, a public park and museum, lie between shopping centers and apartment buildings. But the sticky, black asphalt that fills the pits was oozing up from the ground long before people turned this land into a bustling city.

Across millennia, the tar pits captured over ten thousand mammals, creating a remarkably detailed record of the area’s natural history. But not every creature present in the asphalt is stuck in the past.

Any tourist who goes to watch scientists dig up the bones of saber-toothed cats and dire wolves should also keep an eye out for the plucky survivor whose ancestors likely watched those big mammals die: the petroleum fly.

There’s still a lot of mystery surrounding the petroleum fly, but one thing is clear: It has figured out how to make the most of a bad situation. Pools of asphalt are hell for most animals, but petroleum flies have turned them into a bountiful habitat.

“That’s their personal paradise,” says entomologist Martin Hauser, with the California Department of Food and Agriculture. “They have no competition in there because no one else can deal with it.”

Adult petroleum flies are fairly unassuming. They’re small and flecked like fruit flies. Though scientists don’t know exactly how, they are able to skate – and mate – on the asphalt pools. Their feet don’t get stuck, but if any other body part touches the sticky liquid, they’re out of luck.

An adult petroleum fly walks on top of the sticky tar pits without getting stuck.

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Josh Cassidy

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The translucent maggots, on the other hand, are truly in their element – they can fully submerge in the dark, viscous liquid.

“This is something that kills everything else,” says Kenneth Nickerson, a microbiologist at the University of Nebraska-Lincoln, who has studied petroleum flies.

Pools of natural asphalt form when petroleum from subterranean reservoirs seeps out of the ground. Most of the small molecules that make petroleum toxic to us quickly evaporate, but the asphalt that’s left behind is incredibly sticky.

Few animals that wade into a pool of asphalt manage to extricate themselves from it. In fact, the thick liquid keeps holding on even after an animal has succumbed to exhaustion or exposure, and its body has wasted away to bones. That’s why asphalt deposits around the world are particularly interesting to paleontologists.

“It just so happens that this extremophile organism lives in the medium that I dig fossils out of,” says Sean Campbell, a paleontologist with the Natural History Museums of Los Angeles County, which include the La Brea Tar Pits.

Scientists have also found petroleum flies in oil fields in Santa Paula and Ojai, south of Santa Barbara, as well as in seeps in Cuba and Trinidad, in the Caribbean.

By studying the flies in the La Brea Tar Pits, researchers are beginning to understand how their maggots are able to survive in this environment.

“They have multiple tricks,” says Hauser.

When Hauser examined petroleum fly maggots under a microscope, he noticed they’re a little more prickly than other fly larvae. He believes their rough skin keeps the asphalt away from their bodies.

Surprisingly, the maggots actually need a bit of asphalt on their backside to survive.

“They would dry out if they can’t get in contact with oil,” says Hauser.

As it turns out, the asphalt is also an essential moisturizer. While most insects are covered in a waxy layer that keeps moisture in, petroleum flies seem to lack this protection – likely because wax dissolves in asphalt.

As they swim in the asphalt, maggots breathe through snorkel-like tubes on their rear ends, ringed with hairs that keep them afloat.

“When the snorkel breaks the surface, the hairs just fold out,” says Hauser. “It’s a little bit like an umbrella.”

A crane fly gets stuck in the tar pits.

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Maggots feed on insects that have been caught in the asphalt. Dragonflies and other insects that spend their lives near ponds often mistake shiny pools of asphalt for water. When they try to skim the surface or land on it, the sticky substance pulls them down, into the maggots’ domain. Without snorkels like the ones petroleum fly larvae breathe through, those other insects quickly drown.

Petroleum fly larvae aren’t picky: They’ll turn any dying insect into a meal. When the maggots sense an insect sinking into their home, they wriggle over to it. Then, they scrape at its hard exoskeleton with their two black mouth hooks, probing for an exposed bit of soft tissue. When they find one, they make a hole and crawl inside the dying insect’s body.

A hungry petroleum fly larva inspects and eats an adult petroleum fly that got stuck in the tar pits.

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Josh Cassidy

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As they eat, petroleum fly maggots incidentally consume some asphalt. You can see it darkening their digestive tract through their translucent skin. It looks like their guts are filled with the black substance.

“That’s one of the big mysteries,” says Hauser. “How they can deal with this.”

Humans can handle eating a little bit of asphalt – medieval Persian doctors used to prescribe it for stomach ulcers. But the stuff would overwhelm our systems if we ate as much as a petroleum fly maggot. And even a little bit would expose us to carcinogens that short-lived insects don’t have to worry about.

Petroleum fly larvae eat asphalt regularly enough that scientists once thought they derived nourishment from it. Now, they know that’s not the case.

But it’s apparent that something happens to the asphalt as it passes through a maggot’s digestive tract. Between the mouth and the anus, the dark, viscous substance thins out and clears up. Whatever is responsible for that process could point to a better method for cleaning up oil spills.

That possibility piqued the interest of University of Nebraska-Lincoln microbiologist Nickerson. He had a hunch that bacteria inside the maggots might be breaking the asphalt down. So, he asked a paleontologist to collect some petroleum fly larvae from the La Brea Tar Pits and ship them to his lab in Lincoln.

First, Nickerson’s team identified the types of bacteria growing inside the maggots’ guts. Then, they tried growing the microorganisms in Petri dishes.

“Our goal at the time was to find microbes that would be good at degrading some of these complex hydrocarbons that you would find in the tar,” says Nickerson.

None of the bacteria that grew on Petri dishes proved capable of that feat. But Nickerson’s team found plenty of microbes that they couldn’t grow at the time, and he hopes more scientists will investigate them in the future.

Until then, the petroleum fly larva will continue to live in the present, swimming contentedly in its asphalt paradise.