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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.  


7 thoughts on “Flies Could Falsely Place Someone at a Crime Scene

  1. Table 1 of Durdle et al.’s paper says semen is 5.5% protein by weight, which is less than blood’s 24.0%. How can that be reconciled with the statement about “semen’s higher protein content”? Is it a question of different kinds of proteins?

    1. Great question! I’ve added a note to the post based on following up with Annalisa Durdle. I was basing the higher protein content of semen versus blood on other research. However, hunting around further online, I see various studies giving different levels, and in some cases blood and semen appear to be similar in protein content. It seems it’s still a mystery what exactly the flies are so drawn to!

      average total protein concentration of semen: 5040 mg/100 mL

      Any experts out there want to weigh in?
      I’d also be curious what others think could explain flies’ strong preference for semen compared with blood. (No dirty jokes, please!) Sugar? Particular compounds?

  2. So hypothetically if human DNA survives the fly digestive tract, might it also survive the digestion of a fly by another predator like a spider?

    I’m sure the chances drop every time it passes through another digestive tract, but I’m impressed how much remains.

    1. @NCLaw I friend of mine is a molecular biologist.
      He tells me a human can pretty easily contaminate a crime scene without flies.

      He tells me it’s possible to clone DNA in a household kitchen. Take that DNA make a solution and spary it around your crime scene done.
      Now here’s the scary bit, he reckon he could basically write a recipe for me to do it.
      So it would be possible for police to learn how to do it from a forensics team (or a forensics team could just do it) or a criminal could learn to do it from someone (say a student who’s short on cash or just shady).

      Either way now that we know enough about DNA to easily manipulate it we need to really be aware of the possibilities if we’re ever in court as defendants, prosecutors, jurists or jurors.

      It’s no longer good enough to say some person’s DNA is found at the scene, there has to be a very good and provable narrative as to how it got there.

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