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Flies Could Falsely Place Someone at a Crime Scene

The Australian sheep blowfly doesn't just eat nectar. It has a taste for a particular human body fluid—and it’s not blood.
The Australian sheep blowfly doesn’t just eat nectar. It has a taste for a particular human body fluid—and it’s not blood.

This might be the grossest science experiment I’ve ever written about—which is really saying something on a blog called Gory Details—but it’s also one of the most fascinating. It has to do with the taste a certain type of fly has for human bodily fluids.

Blowflies, in case you’re not familiar with them, are the flies of death. As I learned when rats died in my ceiling, these big shiny flies have an amazing ability to appear seemingly out of nowhere within moments of blood being spilled or at the slightest whiff of decay.

So, a lot of blowflies are sometimes found buzzing around a gory crime scene. That got forensic expert Annalisa Durdle wondering: With all those flies doing what flies do—flying around and pooping on stuff—could they be contaminating crime scenes?

“Interestingly, fly poo can also look very similar to blood spatter,” says Durdle, who studied forensic science at La Trobe University in Melbourne, Australia.

“Anyhow,” she e-mailed in response to my indelicate questions about her research, “it turns out that you can get full human DNA profiles from a single piece of fly poo. (I tend to refer to poo rather than vomit because in my experience flies tend to eat their vomit and most of what you have left is poo—although they do eat that too!)”

Clearly, blowflies are gross.

But could they falsely incriminate someone? To find out, Durdle needed to know what blowflies would really eat at a crime scene.  

So she did the experiment. Her team offered Australian sheep blowflies a crime scene buffet, with body fluids collected from volunteers—blood, saliva, and semen—plus other snacks that flies might find in a victim’s home: pet food, canned tuna, and even honey.

“You draw more flies with honey,” my mother always told me. But in this case, she was wrong.

What you draw more flies with, it turns out, is semen.

“It’s the crack cocaine of the fly world,” Durdle says. “They gorge on it; it makes them drunk (they stumble around, partly paralyzed—I’ve even seen one fly give up hope of cleaning itself properly and sit down on its bum!). Then they gorge some more and then it kills them. But they die happy!”

Video: Meet Annalisa Durdle, the coolest fly-poop scientist ever.

The flies liked pet food too, but weren’t much into blood, and they were really uninterested in saliva. Maybe they go for semen’s higher protein content—it contains more than 200 different proteins, at much higher levels than in blood. (Update: or maybe not. Protein levels vary, and Annalisa Durdle notes that “flies are like people—they don’t necessarily eat what is good for them!” Flies are attracted to various aromas, including sulphur-based ones, so it may be that semen is simply more alluring than other food sources.)

Another thing semen has plenty of: DNA.

Durdle tested flies’ poop after various meals. “If the flies had fed on semen or a combination with semen in it, then you got a full human DNA profile almost every time. With blood, it was maybe a third of the time and with saliva, never.”

“It was also interesting to find the flies generally preferred dry blood or semen to wet blood or semen,” Durdle says. “This could be important, because it means flies could continue to cause problems at a scene long after the biological material had dried.”

How big a deal is this? Durdle says, “You really need to look at the probabilities… the chances that a fly might feed on some poor guy’s semen (after he’s had some innocent quiet time to himself), and then fly into a crime scene and poo, potentially incriminating him.”

There’s also the chance that a forensic investigator could sample fly poop thinking it’s blood spatter, she says, and find DNA that’s not from the victim.

A fly might occasionally be helpful to the cause of criminal justice. If a fly eats bodily fluids from a crime scene and then flies away into another room and poops there, it might save a sample of DNA from the perpetrator’s attempts to clean up.

Flies aren’t the only potential problem for interpreting DNA. As the technology used in forensic labs has become more sensitive, there’s greater risk of picking up tiny bits of DNA transferred to a crime scene, forensic scientist Cynthia Cale argued last year in Nature.

In fact, Cale showed that one person can transfer another person’s DNA to a knife handle after two minutes of holding hands. (Next she says she’ll try shorter times, to see if even a brief encounter could transfer DNA.)  

The fly-poop research is interesting, Cale says. Blowflies would probably be more likely to transfer DNA within a crime scene rather than bringing it in from outside, but even that could confuse the reconstruction of a crime.

“I think the biggest impact might be when a defense lawyer uses it to raise doubt in the mind of a jury,” Durdle says.  


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Yes, Rats Can Swim Up Your Toilet. And It Gets Worse Than That.

They eat our food. They furnish their nests with our detritus. They chew through our sheet metal, our lead pipes and our concrete. They outsmart us at every turn. They are our shadow, our enemy, our next door neighbor.    —”Rat City!Spy magazine, 1988

“You have to think like the rat,” my new friend Gregg told me. At the time, we were pushing Gregg’s homemade rat detector through a small hole in my basement ceiling. He had bought an endoscope camera online—the kind a doctor uses to hunt for polyps in one’s nether regions—and attached it to a bent wire coat hanger. The camera’s images would be displayed on his laptop.

Gregg became obsessed with rats when they took over his girlfriend Anne’s house, across the street from mine. Having tracked and conquered her rats, he was eager to bring his rat-buster skills and tools to my infestation. Gregg showed up on a Sunday afternoon with the endoscope and a two-gallon bleach sprayer and explained my role: Simply turn the endoscope’s light up or down on his command as he threaded the coat hanger through ceilings and walls.

In the ceiling space above the basement bathroom, we hit the mother lode: towering piles of little black rat turds appeared on the laptop screen. “Here’s your nest,” Gregg proclaimed, our first small victory in what had been a long, losing battle. As I wrote in May, I had already suffered an invasion of live rats, followed by stinking dead rats and a Flymageddon of bottle flies and flesh flies that hatched out of their carcasses.

I had learned a few things about rats by this point: They are creatures of habit. They establish trackways through a house, following the same paths each day: in, out, to food, to nest. And they can, in fact, rise up from the sewers.

VIDEO: WATCH OUT! A rat’s super swimming ability and flexibility enable it to make its way easily from the city streets to your toilet. See how they do it.

This last point became central to my investigation. When my husband, Jay, cut out a section of the bathroom ceiling where Gregg’s endoscope had led us, we found that our rat nest was centered around an old sewer drain pipe that, unbeknownst to us, had been cut but never capped during the removal of an upstairs toilet. Dark oily smudges marked the rim where rats had climbed up from the sewers and dropped into my basement ceiling space.

Upon further research, I found that not only is it pretty easy for a rat to climb up a three-inch toilet drain pipe (most of the time there’s not even water in it), but I live in a part of D.C. with a combined sewer system, so the storm drains on the street and the pipes from the toilets run to the same place. A combined sewer is one big, happy, Rat Central Station. 

Having figured out how our rats were getting in, and assuming that any remaining rats would have been scared away by our noisy labors and hole-poking, Jay capped the pipe, and we congratulated ourselves on a mystery solved.

Maybe you read my last post, and you can see where this is going.

One With the Rats

Rats’ superpowers are near-mythical: They can swim for three days. They can fit through holes the size of a quarter. They’ve even been said to have no solid bones, just cartilage (definitely false, and I can’t confirm whether they can collapse their ribcages). I looked to science for the truth. But I was surprised by the dearth of studies on the Norway rat—the common city rat, Rattus norvegicus—in the wild (the wild in this case being any city on Earth). Despite our long human history with lab rats, we know very little about the lives of the rats in our homes.

In fact, as veterinary scientist Chelsea Himsworth told me, “We probably know more about the ecology of polar bears than we do about rats.” Himsworth is studying how rats spread disease in cities as part of the Vancouver Rat Project.

“The interesting thing about Norway rats is they don’t exist in the wild,” Himsworth said. Their migrations—through Asia, over continents and across oceans—are our migrations. They’ve been in contact with humans for so long that they not only live with us, they depend on us almost entirely for food.  

They don’t stray far from our homes. One of the most important findings of the Vancouver Rat Project has been that rats form highly stable family groups or colonies, block by block in a city. And when people break up rat families, say by indiscriminate trapping or poisoning, the remaining rats are forced to move—and that’s when they tend to spread disease.

Sewer Rats

I was, of course, trying not to be indiscriminate at all. I wanted to kill them all—the whole rat family.

I told this to Robert Corrigan, who was described to me as the “rat king of New York City.” He seems okay with the title. Corrigan has spent his career fighting rats up and down the Eastern Seaboard, which—with its dense population, waterways, and old pipes—is pretty much rat heaven.

Corrigan said he agreed with Gregg in part: To wipe out an infestation you have to think like a rat. “But I also think it’s not difficult to out-think a rat,” he said. Unlike many animals, a rat must have both food and water every single day to survive. No skipping meals.


“If it doesn’t have food and water, it goes into this kind of ‘crazy mode,'” Corrigan said. Rats have a very low tolerance for hunger—so to get rid of them simply ask where they’re getting food and eliminate the source.

But what about my rats?, I asked him. How were they getting food? Clearly they were coming up an old toilet pipe from the sewer, and there wasn’t any food in my basement ceiling.

That’s where it got a little ugly. I was right about the combined sewer system, Corrigan said; it does make it easier for rats to get into toilets. As if to make the point, the day after we capped our toilet pipe, a rat popped up in my next-door neighbor’s toilet.

Plus, toilet drainage turns out to be a boon for sewer rats. “Lots of food gets flushed,” Corrigan pointed out. (This remains hard for me to fathom, but I do recall a landlord once complaining about a tenant who always flushed chicken bones down the toilet.)

“Also, if push comes to shove, human feces and dog feces contain undigested food,” Corrigan said.

“They don’t turn up their nose at anything that floats by.”

Let’s pause on that for a moment. What Corrigan is saying is that the rats in my basement ceiling were climbing up and down a toilet pipe into the sewer every day, whereupon they ate and quite possibly dragged back up caches of food that may or may not have included human excrement.

“That’s repulsive to humans, but it’s called coprophagy, and it’s part of the reason rats are so successful,” he said. “They don’t turn up their nose at anything that floats by.” 

Not Again

So it was smart of us to cap the sewer pipe. But little did I know when we cut off the entrance and exit to the basement ceiling, that at least two more rats remained in the ceiling—or that only one would survive. Survivor Rat chewed its way out of the house, leaving in its wake a gnawed-off condensation tube spewing water into the basement ceiling. Loser Rat didn’t hold out long enough and died in unknown quarters, spawning a new flock of flesh flies.

When the big striped monsters began to emerge and cruise the basement skies, I pretty much lost it. I Can’t. Do. This. Again.

Caving to the chemical solution, I bought a bug-fogging bomb and waited until I thought most of the flies would be emerging from their pupal cases—when I’d have the best chance of killing them. (Check out this video of  house flies emerging.)

I approached a hole we’d cut in the ceiling where I’d observed flies emerging. Using salad tongs, I pinched the plastic cover and pulled it back an inch. A rain of black flies drip-dropped from the hole onto the floor, buzzing. They had emerged from their cases but couldn’t quite fly yet. Perfect. I yanked the cover the rest of the way off, jumped back as a mass of flies hit the ground, some taking wing, and hit the button on the fogger.

Then I dropped my tongs and ran.

Those are dead flies. Multiply by entire basement.  Photo by Erika Engelhaupt
Those are dead flies. Multiply by entire basement. Photo by Erika Engelhaupt

Here is what I came home to.

It wasn’t as bad as Flymageddon.